Noise Problem. Both Comms Breaking Squelch

[Lancair IV-P Flyer]

Would anyone have an idea about the cause of this?

Problem:
Intermittent noise (sounds just like a squelch volume test) on both
communication radios. Lasts from 15 seconds to two minutes. Will
occur at any RPM including idle. Changing frequencies eliminates the
noise but the noise can and most likely will come back shortly on that
same frequency.

I have a back up alternator on this aircraft and when I have the main
alternator turned off to check the operation of the backup alternator,
the noise never seems to occur. Also, when the noise manifests, if I
turn off the alternator the noise always goes away. I have done this
several times on the ground and it has been 100%. The voltage
regulator has just been replaced and the noise is still present. The
alternator has been rebuilt three times in the last 30 hours of
operation. The alternator shop is tired of hearing from me. This
squelch break noise is new however in the last 15 hours of
operation.

The alternator has been having so much trouble because we have been
trying to locate an alternator field circuit breaker popping issue.
Typical scenario is the aircraft must fly for at least 40 minutes
before the breaker will pop. Usually, it pops in conjunction with a
small static crack in the headset and a voltage excursion of one to
two volts. This most often happens in level flight. This same
voltage excursion has been noted at least once when the main
alternator is turned off and running on the backup alternator.

All connections in the alternator/breaker/battery/ground circuit have
been cleaned, tightened and cable ends replaced when suspect. All to
no avail. This is a 24 volt single battery system. Running the 35
amp hydraulic pump to actuate flaps or gear will on occasion trigger a
field alternator breaker pop but many times it won't either.

Thanks in advance for any light you might be able to shine on this
problem.

[Nathan Young]

On Thu, 27 Mar 2008 15:27:52 -0700 (PDT), Lancair IV-P Flyer
> wrote:

>Would anyone have an idea about the cause of this?


Two ideas to check:

#1. Use a battery powered handheld radio to see if it breaks squelch
as well. This will tell you if the noise is RF based or coupled into
the DC power within the plane.

#2. Have you replaced the field circuit breaker? I don't know how it
would cause the squelch issue, but a bad breaker might contribute to
some of the odd electrical system issues you are experiencing.

I had a bad field breaker that worked about 99% of the time, but would
occasionally exhibit a resistance, which caused a voltage drop across
the breaker, which caused the voltage regulator to crank up the
voltage. After hitting turbulence, the breaker would jostle just
enough to lose the resistance, and then the over voltage regulator
would trip, and take the alternator off line. I went through 2
alternators, a VR, and an OVR before I decided to debug this problem
myself and eventually found the bad circuit breaker.

-Nathan

[Lancair IV-P Flyer]

On Mar 28, 5:26 am, Nathan Young > wrote:
> On Thu, 27 Mar 2008 15:27:52 -0700 (PDT), Lancair IV-P Flyer
>
> > wrote:
> >Would anyone have an idea about the cause of this?
>
> Two ideas to check:
>
> #1. Use a battery powered handheld radio to see if it breaks squelch
> as well. This will tell you if the noise is RF based or coupled into
> the DC power within the plane.
>
> #2. Have you replaced the field circuit breaker? I don't know how it
> would cause the squelch issue, but a bad breaker might contribute to
> some of the odd electrical system issues you are experiencing.
>
> I had a bad field breaker that worked about 99% of the time, but would
> occasionally exhibit a resistance, which caused a voltage drop across
> the breaker, which caused the voltage regulator to crank up the
> voltage. After hitting turbulence, the breaker would jostle just
> enough to lose the resistance, and then the over voltage regulator
> would trip, and take the alternator off line. I went through 2
> alternators, a VR, and an OVR before I decided to debug this problem
> myself and eventually found the bad circuit breaker.
>
> -Nathan

Nathan,

Thanks for the ideas. I will try the hand held radio test. I have
already replaced the field circuit breaker. There was no change in
the symptoms so I don't think that made a difference. I am wondering
if the breaking squelch is unrelated to voltage excursions and breaker
tripping events. Since I can usually get the squelch to break on the
ground, I am thinking of pulling breakers one at a time while the
squelch break event is in progress to see if there is an instrument
that may be causing that noise.

Thanks again,
Steve

[Lancair IV-P Flyer]

On Mar 28, 8:30 am, Lancair IV-P Flyer > wrote:
> On Mar 28, 5:26 am, Nathan Young > wrote:
>
>
>
> > On Thu, 27 Mar 2008 15:27:52 -0700 (PDT), Lancair IV-P Flyer
>
> > > wrote:
> > >Would anyone have an idea about the cause of this?
>
> > Two ideas to check:
>
> > #1. Use a battery powered handheld radio to see if it breaks squelch
> > as well. This will tell you if the noise is RF based or coupled into
> > the DC power within the plane.
>
> > #2. Have you replaced the field circuit breaker? I don't know how it
> > would cause the squelch issue, but a bad breaker might contribute to
> > some of the odd electrical system issues you are experiencing.
>
> > I had a bad field breaker that worked about 99% of the time, but would
> > occasionally exhibit a resistance, which caused a voltage drop across
> > the breaker, which caused the voltage regulator to crank up the
> > voltage. After hitting turbulence, the breaker would jostle just
> > enough to lose the resistance, and then the over voltage regulator
> > would trip, and take the alternator off line. I went through 2
> > alternators, a VR, and an OVR before I decided to debug this problem
> > myself and eventually found the bad circuit breaker.
>
> > -Nathan
>
> Nathan,
>
> Thanks for the ideas. I will try the hand held radio test. I have
> already replaced the field circuit breaker. There was no change in
> the symptoms so I don't think that made a difference. I am wondering
> if the breaking squelch is unrelated to voltage excursions and breaker
> tripping events. Since I can usually get the squelch to break on the
> ground, I am thinking of pulling breakers one at a time while the
> squelch break event is in progress to see if there is an instrument
> that may be causing that noise.
>
> Thanks again,
> Steve

The handheld radio had no static. However, it gave me an idea to
power up the radios without the engine running to isolate whether it
is alternator based. I can get the static without the engine
running. The alternator switch is off so the VR is not powered up
either so the static is coming from a powered instrument it would
seem. So, why does the static go away when the engine is running and
the alternator switch is turned off?

Steve

[[email protected]]

On Mar 27, 5:27*pm, Lancair IV-P Flyer > wrote:
> Would anyone have an idea about the cause of this?
>
> Problem:
> Intermittent noise (sounds just like a squelch volume test) on both
> communication radios. *Lasts from 15 seconds to two minutes. *Will
> occur at any RPM including idle. *Changing frequencies eliminates the
> noise but the noise can and most likely will come back shortly on that
> same frequency.
>
> I have a back up alternator on this aircraft and when I have the main
> alternator turned off to check the operation of the backup alternator,
> the noise never seems to occur. *Also, when the noise manifests, if I
> turn off the alternator the noise always goes away. *I have done this
> several times on the ground and it has been 100%. *The voltage
> regulator has just been replaced and the noise is still present. *The
> alternator has been rebuilt three times in the last 30 hours of
> operation. *The alternator shop is tired of hearing from me. *This
> squelch break noise is new however in the last 15 hours of
> operation.
>
> The alternator has been having so much trouble because we have been
> trying to locate an alternator field circuit breaker popping issue.
> Typical scenario is the aircraft must fly for at least 40 minutes
> before the breaker will pop. *Usually, it pops in conjunction with a
> small static crack in the headset and a voltage excursion of one to
> two volts. *This most often happens in level flight. *This same
> voltage excursion has been noted at least once when the main
> alternator is turned off and running on the backup alternator.
>
> All connections in the alternator/breaker/battery/ground circuit have
> been cleaned, tightened and cable ends replaced when suspect. *All to
> no avail. *This is a 24 volt single battery system. *Running the 35
> amp hydraulic pump to actuate flaps or gear will on occasion trigger a
> field alternator breaker pop but many times it won't either.
>
> Thanks in advance for any light you might be able to shine on this
> problem.

I had a similar thing happen with one of my Vikings once. I took it
over to a freind at CRP who owns an avionics shop. He put an
oscilloscole on my 12 volt bus and found an ac sine wave. Easy fix!
Repair the alternator. An alternator is an ac device. there is a diode
trio (set of 3 diodes) that rectify the ac to dc. If a diode goes bad
it will still provide voltage but will have an ac componant and noise.
An easy way to test it is to pull the alt field circuit breaker. If
this is your problem, you owe me a ride.

Dewey

[Lancair IV-P Flyer]

On Mar 29, 4:45 am, "
> wrote:
> On Mar 27, 5:27 pm, Lancair IV-P Flyer > wrote:
>
>
>
> > Would anyone have an idea about the cause of this?
>
> > Problem:
> > Intermittent noise (sounds just like a squelch volume test) on both
> > communication radios. Lasts from 15 seconds to two minutes. Will
> > occur at any RPM including idle. Changing frequencies eliminates the
> > noise but the noise can and most likely will come back shortly on that
> > same frequency.
>
> > I have a back up alternator on this aircraft and when I have the main
> > alternator turned off to check the operation of the backup alternator,
> > the noise never seems to occur. Also, when the noise manifests, if I
> > turn off the alternator the noise always goes away. I have done this
> > several times on the ground and it has been 100%. The voltage
> > regulator has just been replaced and the noise is still present. The
> > alternator has been rebuilt three times in the last 30 hours of
> > operation. The alternator shop is tired of hearing from me. This
> > squelch break noise is new however in the last 15 hours of
> > operation.
>
> > The alternator has been having so much trouble because we have been
> > trying to locate an alternator field circuit breaker popping issue.
> > Typical scenario is the aircraft must fly for at least 40 minutes
> > before the breaker will pop. Usually, it pops in conjunction with a
> > small static crack in the headset and a voltage excursion of one to
> > two volts. This most often happens in level flight. This same
> > voltage excursion has been noted at least once when the main
> > alternator is turned off and running on the backup alternator.
>
> > All connections in the alternator/breaker/battery/ground circuit have
> > been cleaned, tightened and cable ends replaced when suspect. All to
> > no avail. This is a 24 volt single battery system. Running the 35
> > amp hydraulic pump to actuate flaps or gear will on occasion trigger a
> > field alternator breaker pop but many times it won't either.
>
> > Thanks in advance for any light you might be able to shine on this
> > problem.
>
> I had a similar thing happen with one of my Vikings once. I took it
> over to a freind at CRP who owns an avionics shop. He put an
> oscilloscole on my 12 volt bus and found an ac sine wave. Easy fix!
> Repair the alternator. An alternator is an ac device. there is a diode
> trio (set of 3 diodes) that rectify the ac to dc. If a diode goes bad
> it will still provide voltage but will have an ac componant and noise.
> An easy way to test it is to pull the alt field circuit breaker. If
> this is your problem, you owe me a ride.
>
> Dewey

Dewey,

Thanks for your suggestion on the alternator. My only problem is I
can make the noise appear when the engine is not running (only battery
voltage). When the engine is running and the alternator is online, I
can get the noise to stop by turning off the alternator switch. So, I
thought the alternator was it until I did the test without the engine
running.

Thanks,
Steve

[Nathan Young]

On Fri, 28 Mar 2008 12:03:30 -0700 (PDT), Lancair IV-P Flyer
> wrote:

>
>The handheld radio had no static. However, it gave me an idea to
>power up the radios without the engine running to isolate whether it
>is alternator based. I can get the static without the engine
>running. The alternator switch is off so the VR is not powered up
>either so the static is coming from a powered instrument it would
>seem. So, why does the static go away when the engine is running and
>the alternator switch is turned off?

Hmmm. It is odd that the shutting off the alternator field current
seems to fix the static, but meanwhile you also get the static while
on the ground without the engine running.

I think your idea to pull breakers while on the ground without the
engine running is a good next step.

You mentioned a voltmeter in the plane. Does it oscillate or is it
fairly stable? I don't know if you have access to one, but it would
be really helpful to see an oscilloscope on the main bus when the
static condition occurs.

BTW - where are you based?

[Lancair IV-P Flyer]

On Mar 29, 7:57 am, Nathan Young > wrote:
> On Fri, 28 Mar 2008 12:03:30 -0700 (PDT), Lancair IV-P Flyer
>
> > wrote:
>
> >The handheld radio had no static. However, it gave me an idea to
> >power up the radios without the engine running to isolate whether it
> >is alternator based. I can get the static without the engine
> >running. The alternator switch is off so the VR is not powered up
> >either so the static is coming from a powered instrument it would
> >seem. So, why does the static go away when the engine is running and
> >the alternator switch is turned off?
>
> Hmmm. It is odd that the shutting off the alternator field current
> seems to fix the static, but meanwhile you also get the static while
> on the ground without the engine running.
>
> I think your idea to pull breakers while on the ground without the
> engine running is a good next step.
>
> You mentioned a voltmeter in the plane. Does it oscillate or is it
> fairly stable? I don't know if you have access to one, but it would
> be really helpful to see an oscilloscope on the main bus when the
> static condition occurs.
>
> BTW - where are you based?

Nathan,

I have two voltmeters on board. They both show very steady voltage
within 0.1 volt except when there is a voltage excursion which occurs
at least 40 minutes into a flight. The voltage excursions are usually
less than two volts but sometimes there is enough of a spike to trip
the alternator field circuit breaker. I replaced the circuit breaker
but no change in the problem. I now have a new voltage regulator but
have not flown the plane yet to see if that made a difference. I have
a couple of trips planned for this week and will get a good look at
things with the new VR.

The oscilloscope idea is a good one. I will have to see if I can find
access to one.

I am based at LWS (Lewiston, Idaho on the Lewis and Clark trail).

Steve

[MikeMl]

Lancair IV-P Flyer wrote:
> On Mar 29, 4:45 am, "
Steve,

I do not have any specific Lancair experience, however, I know about
Cessnas. The new glass-panel Cessnas have an alternator controller which
includes an overvoltage detector which incorporates an electronic
"crowbar" circuit. It purposely overloads and trips the Alternator Field
Breaker if a momentary bus voltage transient is detected. Could your
Lancair have a similar system?

On the radios unsquelching, is it possible that the radios are just
overly voltage sensitive? With the alternator on-line, the bus voltage
should be nominally 28.5V. With the alternator off-line, the bus voltage
will quickly sag to about 24V. Some radios do better than others at not
having their squelch threshold change when the input power changes that
much.

MikeM

[Lancair IV-P Flyer]

On Mar 31, 5:59 pm, MikeMl > wrote:
> Lancair IV-P Flyer wrote:
> > On Mar 29, 4:45 am, "
>
> Steve,
>
> I do not have any specific Lancair experience, however, I know about
> Cessnas. The new glass-panel Cessnas have an alternator controller which
> includes an overvoltage detector which incorporates an electronic
> "crowbar" circuit. It purposely overloads and trips the Alternator Field
> Breaker if a momentary bus voltage transient is detected. Could your
> Lancair have a similar system?
>
> On the radios unsquelching, is it possible that the radios are just
> overly voltage sensitive? With the alternator on-line, the bus voltage
> should be nominally 28.5V. With the alternator off-line, the bus voltage
> will quickly sag to about 24V. Some radios do better than others at not
> having their squelch threshold change when the input power changes that
> much.
>
> MikeM

Mike,

The solid state voltage regulators used in many experimental category
aircraft do in fact have "crowbar" type protection as well as over
voltage protection. As I understand the operation of the crowbar trip
from a simple over voltage trip is if the crowbar opens the circuit,
it can only be reset by turning off the alternator switch. Once that
occurs, you can reset the breaker, turn the alternator switch back on
and the alternator will be back in business. My breaker trips are
always resettable without resetting requirement of the alternator
switch. So, I think something is shorting to ground that shouldn't
be. I am thinking the main alternator may still be the culprit or
possibly the battery, which is an AGM type, may be shorting as it
heats up. I am going to try pulling the field breaker and sense
breaker of the backup alternator to see if the problems occur during
the isolation. If so, then running from the backup system and pulling
the breakers of the main system. If the failures only occur on the
main system I am going to replace the alternator with a completely
different unit. If both tests show voltage excursions, I am going to
replace the battery and see if that isn't the culprit. There is
really not much left to try.

Thanks for your help.
Steve

[MikeMl]

Lancair IV-P Flyer wrote:
> On Mar 31, 5:59 pm, MikeMl > wrote:
>> Lancair IV-P Flyer wrote:
>>> On Mar 29, 4:45 am, "
>> Steve,
>>
>> I do not have any specific Lancair experience, however, I know about
>> Cessnas. The new glass-panel Cessnas have an alternator controller which
>> includes an overvoltage detector which incorporates an electronic
>> "crowbar" circuit. It purposely overloads and trips the Alternator Field
>> Breaker if a momentary bus voltage transient is detected. Could your
>> Lancair have a similar system?
>>
>> On the radios unsquelching, is it possible that the radios are just
>> overly voltage sensitive? With the alternator on-line, the bus voltage
>> should be nominally 28.5V. With the alternator off-line, the bus voltage
>> will quickly sag to about 24V. Some radios do better than others at not
>> having their squelch threshold change when the input power changes that
>> much.
>>
>> MikeM
>
> Mike,
>
> The solid state voltage regulators used in many experimental category
> aircraft do in fact have "crowbar" type protection as well as over
> voltage protection. As I understand the operation of the crowbar trip
> from a simple over voltage trip is if the crowbar opens the circuit,
> it can only be reset by turning off the alternator switch. Once that
> occurs, you can reset the breaker, turn the alternator switch back on
> and the alternator will be back in business.

No, what you are describing is how the 1976 to 1986 steam-gauge Cessnas
worked. They have a latching relay in the OverVoltageProtection module
which has to be reset by momentarily turning off the ALT side of the
Master Switch.

The glass panel Cessnas (post 2003?) actually have an electronic crowbar
which artificially creates an overload current which blows the
Alternator Field Breaker when an overvoltage is detected. The Breaker
must be physically reset in order to bring the Alternator back on line.
A momentary spike lasting only milliseconds will trip the Field breaker;
a truly stupid system! This system has an AD against it; spurious
tripping, what else. Many new Cessna owners have had issues with this,
and have spent money replacing things like alternators when the root
cause was an OV circuit which was too sensitive to short duration
transients caused by things like an inductive gear pump motor turning off!

My breaker trips are
> always resettable without resetting requirement of the alternator
> switch. So, I think something is shorting to ground that shouldn't
> be. I am thinking the main alternator may still be the culprit or
> possibly the battery, which is an AGM type, may be shorting as it
> heats up. I am going to try pulling the field breaker and sense
> breaker of the backup alternator to see if the problems occur during
> the isolation. If so, then running from the backup system and pulling
> the breakers of the main system. If the failures only occur on the
> main system I am going to replace the alternator with a completely
> different unit. If both tests show voltage excursions, I am going to
> replace the battery and see if that isn't the culprit. There is
> really not much left to try.
>
> Thanks for your help.
> Steve

The only faults (other than the possibility we are discussing above)
that would cause the Field Breaker to trip (in order of likelyhood) is:
rubbed through insulation on wiring between the breaker and the
alternator field, a short between a wire shield and its center
conductor, a loose brush holder inside alternator, or a fault inside the
Voltage Regulator. You have already looked at wiring and replaced the
alternator.

Since your trips occur many minutes into a flight, ask yourself what
happens to the bus voltage as the alternator recharges the battery after
the startup? The higher the bus voltage climbs, the closer it gets to
the OVP's trip voltage, biasing it ever closer to its trip point. I'll
take a bet that you have an oversensitive OVP. The battery has nothing
to do with this; the VR should keep the voltage on the battery at no
higher than 28.8V.

Find out what voltage your OVP trips at. I would do the following ground
test: Isolate the OVP/VR from the Field Breaker (just open it if it is
the pullable kind). Connect a 3A, 28V regulated, current-limited
dual-metered adjustable Power Supply to the wire downstream from the
Field Breaker. Start with the supply set to about 24V with a
short-circuit current set to 3A; you should see about 1.5A flowing which
is going into the alternator field winding.

Slowly increase the supply voltage; at about 28.5V, you should see the
supply current suddenly drop to a few tens of mA. That is the cutout
voltage at which the VR regulates the bus voltage (by turning off the
field excitation). Slowly decrease the supply voltage until the field
current jumps up again; that is the cutin voltage, and might be a few
tens of mV lower than the cutout voltage (hysteresis).

Now increase the supply voltage above the cutout voltage and watch what
happens. Ideally, the OverVoltage trip should be at 32V or higher.
Observe what happens when the OVP fires. Does it act like a short
circuit to the supply? (i.e. did the supply go into current limiting and
stay there until you disconnect a wire?). What you are looking for is at
what voltage did the trip occur, and would the trip have blown the field
breaker?

MikeM (PhD EE, retired)
Skylane, Pacer.

[nrp]

> MikeM (PhD EE, retired)
> Skylane, Pacer.
I had a 172M that gave random overvoltage tripout problems too. First
we spun the alternator with an electric motor on the gorund & looked
at the output voltage with an oscilloscope. Sure enough it was
tripping out at well less than 16 V.

Maybe these units drift? We replaced it with new, but noted that
there is an exposed tweekpot on the OV sensor that probably adjusts
the trip point.

[Peter Clark]

On Tue, 01 Apr 2008 08:56:27 -0600, MikeMl > wrote:

>The glass panel Cessnas (post 2003?) actually have an electronic crowbar
>which artificially creates an overload current which blows the
>Alternator Field Breaker when an overvoltage is detected. The Breaker
>must be physically reset in order to bring the Alternator back on line.
>A momentary spike lasting only milliseconds will trip the Field breaker;
>a truly stupid system! This system has an AD against it; spurious
>tripping, what else. Many new Cessna owners have had issues with this,
>and have spent money replacing things like alternators when the root
>cause was an OV circuit which was too sensitive to short duration
>transients caused by things like an inductive gear pump motor turning off!

Do you have an AD number for that?

[MikeMl]

Peter Clark wrote:
> On Tue, 01 Apr 2008 08:56:27 -0600, MikeMl > wrote:

>
> Do you have an AD number for that?

I don't own a 182T. Now that you mention it, it was a Cessna Service
Bulletin that required replacement of the ACU. It was discussed
extensively on the Cessna Pilot's Assoc. forums.

[Peter Clark]

On Tue, 01 Apr 2008 15:47:49 -0600, MikeMl > wrote:

>Peter Clark wrote:
>> On Tue, 01 Apr 2008 08:56:27 -0600, MikeMl > wrote:
>
>>
>> Do you have an AD number for that?
>
>I don't own a 182T. Now that you mention it, it was a Cessna Service
>Bulletin that required replacement of the ACU. It was discussed
>extensively on the Cessna Pilot's Assoc. forums.

I'd like to know the SB number for that. The only SB I can find that
references alternators is SB06-24-03 which is an alternator
replacement for manufacturing defects in Kellly Areospace alternators
made from 8/95 to 8/04.

[Lancair IV-P Flyer]

On Mar 27, 2:27 pm, Lancair IV-P Flyer > wrote:
> Would anyone have an idea about the cause of this?
>
> Problem:
> Intermittent noise (sounds just like a squelch volume test) on both
> communication radios. Lasts from 15 seconds to two minutes. Will
> occur at any RPM including idle. Changing frequencies eliminates the
> noise but the noise can and most likely will come back shortly on that
> same frequency.
>
> I have a back up alternator on this aircraft and when I have the main
> alternator turned off to check the operation of the backup alternator,
> the noise never seems to occur. Also, when the noise manifests, if I
> turn off the alternator the noise always goes away. I have done this
> several times on the ground and it has been 100%. The voltage
> regulator has just been replaced and the noise is still present. The
> alternator has been rebuilt three times in the last 30 hours of
> operation. The alternator shop is tired of hearing from me. This
> squelch break noise is new however in the last 15 hours of
> operation.
>
> The alternator has been having so much trouble because we have been
> trying to locate an alternator field circuit breaker popping issue.
> Typical scenario is the aircraft must fly for at least 40 minutes
> before the breaker will pop. Usually, it pops in conjunction with a
> small static crack in the headset and a voltage excursion of one to
> two volts. This most often happens in level flight. This same
> voltage excursion has been noted at least once when the main
> alternator is turned off and running on the backup alternator.
>
> All connections in the alternator/breaker/battery/ground circuit have
> been cleaned, tightened and cable ends replaced when suspect. All to
> no avail. This is a 24 volt single battery system. Running the 35
> amp hydraulic pump to actuate flaps or gear will on occasion trigger a
> field alternator breaker pop but many times it won't either.
>
> Thanks in advance for any light you might be able to shine on this
> problem.

Mike,

The OV protection is part of the voltage regulator product. The one
we are using is an LR3C 24 volt from B&C Specialties. It is widely
used in the experimental market and has a bullet proof history of no
problems. Since we had tried everything else the company sent us a
replacement regulator to try just in case we had a problem. I flew
the airplane last week with the new regulator and saw no change in the
symptoms. So, I am pretty confident the OV protection is not causing
the problem.

Regarding the field wire integrity, I hooked up a multimeter in series
to the field breaker and looked at the amperage to the breaker during
a flight. I was hoping for a building amperage which would have
indicated resistance building then I could have begun searching for
what was building resistance. But the multimeter amperage reading was
dead solid at 1.5 amps which is quite a cushion from the 5 amp
rating. Something is causing a voltage spike. I just have to find
it.

I am grateful for your help please forward any additional ideas you
may have on this.

Steve

[MikeMl]

Lancair IV-P Flyer wrote:
> On Mar 27, 2:27 pm, Lancair IV-P Flyer > wrote:

> Mike,
>
> The OV protection is part of the voltage regulator product. The one
> we are using is an LR3C 24 volt from B&C Specialties. It is widely
> used in the experimental market and has a bullet proof history of no
> problems. Since we had tried everything else the company sent us a
> replacement regulator to try just in case we had a problem. I flew
> the airplane last week with the new regulator and saw no change in the
> symptoms. So, I am pretty confident the OV protection is not causing
> the problem.

And I am just as confident that it is!!!

According to this description:

http://www.bandcspecialty.com/QuickFacts_LR3C.pdf

the LR3C has exactly the type of crowbar circuit that I have been
describing! If it detects what it thinks is an "overvoltage" condition,
then it responds by firing its "protection crowbar", which instantly
overloads the aircraft's Field Breaker, causing it to overheat and trip,
which removes power from the LR3C, and therefore removes excitation from
the Alternator's field circuit, thereby taking the entire charging
system offline until the Field Breaker is reset.

I personally think this is a DUMB design that causes many more problems
than it prevents.

> Regarding the field wire integrity, I hooked up a multimeter in series
> to the field breaker and looked at the amperage to the breaker during
> a flight. I was hoping for a building amperage which would have
> indicated resistance building then I could have begun searching for
> what was building resistance. But the multimeter amperage reading was
> dead solid at 1.5 amps which is quite a cushion from the 5 amp
> rating.

This is normal behaviour. Think of the alternator as a current
amplifier. Its output current is nominally about 25 times its field
current. In other words, it takes about 1A of Field Current to produce
25A of output current. In steady flight, many minutes after engine start
after the battery has recharged, the average electrical load in the
aircraft is somewhere around 20 to 40A, so the alternator has to produce
20 to 40A, meaning its Field Current will be 0.8 to 1.5A. Almost all of
the current that you measured at the Field Breaker is flowing through
the Alternator Field to ground. The LR3C regulator effectively
"regulates" the Field Current so that the Alternator output just matches
the electrical load.

The current that is tripping the Field Breaker is a momentary overload
cause by the LR3's crowbar that lasts only a few 10s of msec. It would
take a "peak-capture&hold" type of meter to display it!

A minor nit. If there was a "building resistance" in the field circuit,
that would reduce the field current thereby reducing the likelyhood that
the Field Breaker would trip. It takes an unplanned shunt path (fault)
to ground to increase the field current. The crowbar inside the LR3 is a
"shunt" path to ground when it fires.

>Something is causing a voltage spike. I just have to find
> it.

Yes, that is the root cause of your problem; its just not where you have
been looking.

You have one of three problems:

1. The OverVoltage detection level of the LR3 is set too low (too close
to the actual bus voltage, assuming that is correct). Solution, raise
the LR3's Overvoltage Threshold. My preference for a realistic
Overvoltage Threshold is 31V.

2. The Bus voltage really is climbing to unsafe levels. For your AGM
battery, the bus voltage should never get above 28.5V. Solution: adjust
the regulated bus voltage to 28.5V or lower.

3. There is an inductive load somewhere in the aircraft (flap motor,
gear pump motor, autopilot servo, trim servo) which during its normal
cycling puts a short duration inductive electrical transient voltage
spike onto the main bus. The spike is of sufficient energy that the OVP
circuit sees it, and reacts to it by firing its crowbar. Solution: find
the source of the spike and suppress it at its source, or make the LR3
less sensitive to short duration spikes, either by raising its
Overvoltage detection threshold, or by "filtering" its sensing input to
prevent it from "seeing" the short-duration spikes.

> I am grateful for your help please forward any additional ideas you
> may have on this.

I outlined a method of testing the Regulation voltage and the
Overvoltage Threshold voltage of the VR/OVP in situ (using a lab supply)
in my other post. The only thing new is that the LR3 is a "linear"
regulator, so you will see the Field Current decrease linearly between
about 28V and 28.4V, rather than exhibiting a bistable on-off behaviour.

MikeM

[Lancair IV-P Flyer]

On Apr 2, 9:26 am, MikeMl > wrote:
> Lancair IV-P Flyer wrote:
> > On Mar 27, 2:27 pm, Lancair IV-P Flyer > wrote:
> > Mike,
>
> > The OV protection is part of the voltage regulator product. The one
> > we are using is an LR3C 24 volt from B&C Specialties. It is widely
> > used in the experimental market and has a bullet proof history of no
> > problems. Since we had tried everything else the company sent us a
> > replacement regulator to try just in case we had a problem. I flew
> > the airplane last week with the new regulator and saw no change in the
> > symptoms. So, I am pretty confident the OV protection is not causing
> > the problem.
>
> And I am just as confident that it is!!!
>
> According to this description:
>
> http://www.bandcspecialty.com/QuickFacts_LR3C.pdf
>
> the LR3C has exactly the type of crowbar circuit that I have been
> describing! If it detects what it thinks is an "overvoltage" condition,
> then it responds by firing its "protection crowbar", which instantly
> overloads the aircraft's Field Breaker, causing it to overheat and trip,
> which removes power from the LR3C, and therefore removes excitation from
> the Alternator's field circuit, thereby taking the entire charging
> system offline until the Field Breaker is reset.
>
> I personally think this is a DUMB design that causes many more problems
> than it prevents.
>
> > Regarding the field wire integrity, I hooked up a multimeter in series
> > to the field breaker and looked at the amperage to the breaker during
> > a flight. I was hoping for a building amperage which would have
> > indicated resistance building then I could have begun searching for
> > what was building resistance. But the multimeter amperage reading was
> > dead solid at 1.5 amps which is quite a cushion from the 5 amp
> > rating.
>
> This is normal behaviour. Think of the alternator as a current
> amplifier. Its output current is nominally about 25 times its field
> current. In other words, it takes about 1A of Field Current to produce
> 25A of output current. In steady flight, many minutes after engine start
> after the battery has recharged, the average electrical load in the
> aircraft is somewhere around 20 to 40A, so the alternator has to produce
> 20 to 40A, meaning its Field Current will be 0.8 to 1.5A. Almost all of
> the current that you measured at the Field Breaker is flowing through
> the Alternator Field to ground. The LR3C regulator effectively
> "regulates" the Field Current so that the Alternator output just matches
> the electrical load.
>
> The current that is tripping the Field Breaker is a momentary overload
> cause by the LR3's crowbar that lasts only a few 10s of msec. It would
> take a "peak-capture&hold" type of meter to display it!
>
> A minor nit. If there was a "building resistance" in the field circuit,
> that would reduce the field current thereby reducing the likelyhood that
> the Field Breaker would trip. It takes an unplanned shunt path (fault)
> to ground to increase the field current. The crowbar inside the LR3 is a
> "shunt" path to ground when it fires.
>
> >Something is causing a voltage spike. I just have to find
> > it.
>
> Yes, that is the root cause of your problem; its just not where you have
> been looking.
>
> You have one of three problems:
>
> 1. The OverVoltage detection level of the LR3 is set too low (too close
> to the actual bus voltage, assuming that is correct). Solution, raise
> the LR3's Overvoltage Threshold. My preference for a realistic
> Overvoltage Threshold is 31V.
>
> 2. The Bus voltage really is climbing to unsafe levels. For your AGM
> battery, the bus voltage should never get above 28.5V. Solution: adjust
> the regulated bus voltage to 28.5V or lower.
>
> 3. There is an inductive load somewhere in the aircraft (flap motor,
> gear pump motor, autopilot servo, trim servo) which during its normal
> cycling puts a short duration inductive electrical transient voltage
> spike onto the main bus. The spike is of sufficient energy that the OVP
> circuit sees it, and reacts to it by firing its crowbar. Solution: find
> the source of the spike and suppress it at its source, or make the LR3
> less sensitive to short duration spikes, either by raising its
> Overvoltage detection threshold, or by "filtering" its sensing input to
> prevent it from "seeing" the short-duration spikes.
>
> > I am grateful for your help please forward any additional ideas you
> > may have on this.
>
> I outlined a method of testing the Regulation voltage and the
> Overvoltage Threshold voltage of the VR/OVP in situ (using a lab supply)
> in my other post. The only thing new is that the LR3 is a "linear"
> regulator, so you will see the Field Current decrease linearly between
> about 28V and 28.4V, rather than exhibiting a bistable on-off behaviour.
>
> MikeM

Mike,

Your explanations are really helping me through this problem. Thank
you very much for your continued involvement.

Using your theories, I contacted the president of B&C Specialties
today and asked him what the crowbar trip point is set at as the LR3C
is a sealed box. The only user adjustable item is a pot screw for
adjusting the voltage the regulator maintains. Bill, the president,
said the crowbar trip point is set at 32 volts.

You are absolutely correct about the voltage set point ideal is at
28.5 volts. I got that number from speaking with the Concorde battery
tech. We have the regulator set so that at high RPM we are getting
28.5 and at idle rpm we get 28.2 volts. This has been the set point
for several months.

We have a JPI 930 installed that gives us voltage, load amperage and
other useful information. Each of those have a user defined alarm
limit. The voltage alarm I have set is at 29.5 volts. Any time I
hear the telltale "crack" of static in the headset, I look to the JPI
or the voltage readout on the Davtron timer and can see the voltage
excursion. It is usually less than one volt. Whenever the voltage
excursion goes above the alarm point I see an alarm post as the
display color goes to red, and the word "alarm" appears. Most of the
time when we have a trip event on the field breaker it occurs without
the voltage spiking high enough to trigger the alarm set point of 29.5
volts.

Regarding the idea that something in the aircraft is triggering a
transient voltage spike onto the bus, this is the theory we have been
working on for the most part since August. After we got the
alternator fixed all of our attention was looking for the origin of
the transient. The flap and gear are run by the hydraulic pump which
is the highest draw in aircraft. The circuit breaker is a 35 amp size
and when actuating the flap or gear one can see up to 35 amp increase
in the load on the load meter shown on the JPI 930. There is a light
on the panel that comes on anytime the hydraulic pump is actuating
which it does on its own to maintain specified pressure. I have
looked closely when we begin to hear static and see voltage excursions
which are precursors to a trip event to see if they are accompanied by
a cycling of the hydraulic pump. I have never seen that to be the case
except on landing operations when the flap and gear are actuated. In
those occasions, when the field breaker trips it is directly in
response to hydraulic pump actuation.

Mike, given this information, what do you suggest I do next to try and
isolate the cause?

Thanks again, you have a better grasp on this issue than anyone I have
talked to since I began working on it 7 months ago.

Steve

[nrp]

Another long shot is might the master switch or master relay be
intermittent? Normally I'd think any inductive kickbacks from motors
etc would be ballasted by the battery - as long as it stays connected
to the main bus. But if there is anything intermittent in the master
contactor system, any inductive load will kick into whatever
electronics are down-circuit from the intermittent.

A classic example of this is if a master contactor should stutter
(like from a weak battery) when the starter is engaged, the collapsing
field from the starter could generate enormous voltage spikes which
could wipe out any electronics that are also turned on. This is
probably why we are told to turn off radios etc before starting.

[Jim Stewart]

Peter wrote:
> MikeMl > wrote
>
>> If it detects what it thinks is an "overvoltage" condition,
>> then it responds by firing its "protection crowbar", which instantly
>> overloads the aircraft's Field Breaker, causing it to overheat and trip,
>> which removes power from the LR3C, and therefore removes excitation from
>> the Alternator's field circuit, thereby taking the entire charging
>> system offline until the Field Breaker is reset.
>>
>> I personally think this is a DUMB design that causes many more problems
>> than it prevents.
>
> I am an electronics engineer (35 years' design experience) and can't
> believe anybody would do something so stupid in an aeroplane.
>
> Overvoltage crowbars are used on switching power supplies which have
> instant acting short circuit protection features and whose output
> power is limited by the magnetic components anyway.
>
> But on an aeroplane you have a very powerful alternator and more to
> the point you have thermal circuit breakers which take a while to
> trip. They are not like the magnetic ones in one's house which trip
> really fast. The thermal ones have to heat up first.
>
> If one was going to do an overvoltage protector for an aeroplane, the
> way to do it is to put something in series with the alternator field
> winding (i.e. in series with the existing voltage regulator) which
> goes open circuit when the bus voltage reaches say 32V. That will kill
> the alternator output very fast.

I think the problem with that approach is that the
huge inductive surge of the field collapsing would
add to the already present overvoltage.

Crowbars are used in military and spacecraft design
where a positive zero-volts shutdown is needed.

[Ross]

nrp wrote:
> Another long shot is might the master switch or master relay be
> intermittent? Normally I'd think any inductive kickbacks from motors
> etc would be ballasted by the battery - as long as it stays connected
> to the main bus. But if there is anything intermittent in the master
> contactor system, any inductive load will kick into whatever
> electronics are down-circuit from the intermittent.
>
> A classic example of this is if a master contactor should stutter
> (like from a weak battery) when the starter is engaged, the collapsing
> field from the starter could generate enormous voltage spikes which
> could wipe out any electronics that are also turned on. This is
> probably why we are told to turn off radios etc before starting.

May not be applicable, but I have a '65 Cessna that has a single master
switch (DPST). It does connect the alternator and pull in the master
solenoid. I had ammeter swings and they would get so bad as to shut down
the system. I guess it was making the OVR trip. I found quite by
accident that it was the bad (design) master switch on the alternator
side. Replaced with a really good (designed) switch and I have had no
more problems.

--

Regards, Ross
C-172F 180HP
KSWI

[David Lesher]

Peter > writes:


>MikeMl > wrote

>>I personally think this is a DUMB design that causes many more problems
>>than it prevents.

>Overvoltage crowbars are used on switching power supplies which have
>instant acting short circuit protection features and whose output
>power is limited by the magnetic components anyway.
.....
>If one was going to do an overvoltage protector for an aeroplane, the
>way to do it is to put something in series with the alternator field
>winding (i.e. in series with the existing voltage regulator) which
>goes open circuit when the bus voltage reaches say 32V. That will kill
>the alternator output very fast.



Err, that field has an inductance of something on the order of
1H. Opening the excitation to it will do what, in the short
term? Crowbarring that field to ground & clearing the 5-10 amp field
breaker sounds like a good idea to me, given the price of avionics in a
aircraft, and the proprensity for people/Murphy to do bad things....

[I've seen car owners yank the battery cable off while the alternator is
going full tilt; "The battery only starts the car; the alternator runs
it..." The result was a 65V+ "load dump" hundreds of ms long into the
car...fried computers, dead stereos, you name it.]

Falsing is a problem with any protective system design. How fast is too
fast? How slow is too slow to save the consumers from overvoltage? It
sounds like the OVP is a shade oversensitive, but I also wonder about the
spikes that trip it. I'd first check grounds: the alternator ground, the
battery ground, the regulator ground, the engine-frame jumper.....


I have NO clue how this related to noise breaking the squelch....

[MikeMl]

David Lesher wrote:

> Err, that field has an inductance of something on the order of
> 1H. Opening the excitation to it will do what, in the short
> term? Crowbarring that field to ground & clearing the 5-10 amp field
> breaker sounds like a good idea to me, given the price of avionics in a
> aircraft, and the proprensity for people/Murphy to do bad things....

> [I've seen car owners yank the battery cable off while the alternator is
> going full tilt; "The battery only starts the car; the alternator runs
> it..." The result was a 65V+ "load dump" hundreds of ms long into the
> car...fried computers, dead stereos, you name it.]
>

I dont have an internal schematic of the LR3C to know exactly what the
crowbar does. Two possibilities:

One is that the crowbar is upstream of the regulator so it just blows
the Field Breaker, effectively just removing power from the
regulator/field, leaving whatever current is already flowing in the
field inductance to dissipate itself in its own coil resistance. There
is almost certainly a catch (snubber) diode across the field winding
oriented such that the field current decays with a time constant of RL,
where R is the coil resistance, and L is the coil inductance.

Two is that the crowbar is downstream of the regulator (directly across
the field winding). Throwing a dead short across the coil does not
change the time constant mentioned above. The current will still
continue to flow with a time constant of RL. The only way to "shut off"
the current faster is to instantaneously reverse the voltage applied to
the field winding, which a simple crowbar does not do.

Neither method (unless the LR3C is a lot more complicated than I think
it is) prevents a "load dump" as David Lesher describes it! This is why
I think it is a poor design. It is no better than a more conventional
OVP module such as used in Cessnas and Pipers which simply breaks the
connection between the Field Breaker and the input to the regulator.

Actually, I have been thinking about Steve's problem some more, and the
transient which is tripping the crowbar could well be coming from a
"load dump lite". He mentioned that his hydraulic pump is his biggest
single load, ~35A. If his other loads are about 20A, then with pump
running, the alternator is cranking out close to 55A. At the instant the
hydraulic pump turns off, the field current is like 2A. Due to the
high inductance of the field winding, it takes about a 1/4 second for
the field current to decay back down to the less than the 1A it takes to
produce an alternator output of 20A, during which the bus voltage spikes
up, held in check only by the impedance of the battery.

This could well be the event that triggers the OVP crowbar! I have seen
similar OVP triggering in a Cessna 210, where the hydraulic gear pump
motor cycles spontaneously as the pressure in the system leaks down.

I called B&C today to ask about this, and the only suggestion was to
connect pin 3 (sense input) of the LR3C as close to the battery as
possible. In other words, the battery is the spike filter of last
resort, so if there is a lot of wire (resistance) between pin 3 and the
battery, then the impedance along the wire (and in the master relay)
could allow pin 3 to see a higher voltage during the transient.

B&C does not make a 28V ACU without OVP. Pin 3 on the LR3C is used as
the sense input for both the OVP circuit and the regulator. B&C's owner
could not tell me what the effective input resistance looking into Pin 3
is. If it were high enough, you could put a series R, shunt C filter in
this wire to prevent the OVP part of the circuit from triggering when it
shouldn't. However, this could also effect the control loop dynamics of
the voltage regulator. I would try 100 Ohms in series with Pin 3 and
1000 uF (+ end to pin3 and - end to pin 7), time constant of 0.1 sec.
You might have to tweak the voltage setting after adding the filter.

The other possibility is to open the box, and put an appropriate filter
between pin 3 and the OVP circuit such that it doesn't effect what the
VR sees.

[David Lesher]

MikeMl > writes:


> > [I've seen car owners yank the battery cable off while the alternator is
> > going full tilt; "The battery only starts the car; the alternator runs
> > it..." The result was a 65V+ "load dump" hundreds of ms long into the
> > car...fried computers, dead stereos, you name it.]
> >

>I dont have an internal schematic of the LR3C to know exactly what the
>crowbar does. Two possibilities:

>One is that the crowbar is upstream of the regulator so it just blows
>the Field Breaker, effectively just removing power from the
>regulator/field, leaving whatever current is already flowing in the
>field inductance to dissipate itself in its own coil resistance. There
>is almost certainly a catch (snubber) diode across the field winding
>oriented such that the field current decays with a time constant of RL,
>where R is the coil resistance, and L is the coil inductance.

I'd hope.. but is there one??

>Two is that the crowbar is downstream of the regulator (directly across
>the field winding).

Doesn't this assume the regulator is high-side; with the bottom of the
field grounded? I thought the consensus was they were low-side, al-la GM;
i.e. the top end of the field gets fed with +13V from the aux diodes or
battery; and the regulator is in the leg to ground.

>Neither method (unless the LR3C is a lot more complicated than I think
>it is) prevents a "load dump" as David Lesher describes it! This is why
>I think it is a poor design. It is no better than a more conventional
>OVP module such as used in Cessnas and Pipers which simply breaks the
>connection between the Field Breaker and the input to the regulator.

The issue there is opening that will need a fast, high voltage device
that you can turn off. The SCR crowbar gets turned on to protect,
an easier task.

The issue with load dumps is not just the spike peak voltage; it's
how long they last...i.e. total energy. The no-battery stunt was so
harmful because the Xl of the field ensured it could not halt alternator
quickly. So it wasn't a few ms of spike, it was many hundreds of ms
worth...



>Actually, I have been thinking about Steve's problem some more, and the
>transient which is tripping the crowbar could well be coming from a
>"load dump lite".

I don't recall the OP's remarks; did he indicate the pump cycling
was tripping the breaker? Or was it an unknown?

I agree the pump cycling could be an issue; your savior is often the
low impedance of the battery eating those spikes. If it's not, the
spikes may have gotten bigger, or the Xc of the battery lower...

One spike solution is large MOV's across the spike creator. The gotcha
there is: MOV's actually are sacrificial, as small spikes erode them
over time; large ones will cause them to explode...

[i]
>I called B&C today to ask about this, and the only suggestion was to
>connect pin 3 (sense input) of the LR3C as close to the battery as
>possible. In other words, the battery is the spike filter of last
>resort, so if there is a lot of wire (resistance) between pin 3 and the
>battery, then the impedance along the wire (and in the master relay)
>could allow pin 3 to see a higher voltage during the transient.

The other reason to do so is because the regulator is there to protect &
serve the battery. You don't care all that much about the bus being a
half volt too high or low; you DO care about the battery being that, and
the bus does have drop. So you want to sense near/at the battery.

But that brings a dilemma. You must have the sense line draw some
current. Why? So the regulator is sure its sense line has not become
disconnected from the battery. {If it does come loose, the regulator
says "0 volts! More power to the field, Scotty" to compensate, and
the smell of burnt silicon soon fills the area. There were some early
transistorized auto regulators that did just that; long gone I hope!} But
then, even with but a few mils of draw, if it does that for 2 months in
the hangar....your battery is dead. So it must be switched, and that....


>The other possibility is to open the box, and put an appropriate filter
>between pin 3 and the OVP circuit such that it doesn't effect what the
>VR sees.

I wonder if an appropriate ferrite toroid on the sense lead could stop the
noise spikes from tripping the OVP without degrading regulation. I can't
recall what such toroids look like [i.e SPICE] when saturated with DC as
it would be...

[MikeMl]

David Lesher wrote:
> MikeMl > writes:
>> ...There
>> is almost certainly a catch (snubber) diode across the field winding
>> oriented such that the field current decays with a time constant of RL,
>> where R is the coil resistance, and L is the coil inductance.
>
> I'd hope.. but is there one??

Every transistorized regulator that I have ever looked inside of has had
a snubber diode (inside the regulator) connected across the field
winding. Both type B high-side and type A low-side regulators.

>> Two is that the crowbar is downstream of the regulator (directly across
>> the field winding).
>
> Doesn't this assume the regulator is high-side; with the bottom of the
> field grounded? I thought the consensus was they were low-side, al-la GM;
> i.e. the top end of the field gets fed with +13V from the aux diodes or
> battery; and the regulator is in the leg to ground.

According to B&C's website, the LR3C is a high-side, grounded-field type
of regulator.


>> Neither method (unless the LR3C is a lot more complicated than I think
>> it is) prevents a "load dump" as David Lesher describes it! This is why
>> I think it is a poor design. It is no better than a more conventional
>> OVP module such as used in Cessnas and Pipers which simply breaks the
>> connection between the Field Breaker and the input to the regulator.
>
> The issue there is opening that will need a fast, high voltage device
> that you can turn off. The SCR crowbar gets turned on to protect,
> an easier task.

Without a snubber diode, the open-circuited voltage across the field
winding can spike to thousands of volts. You might get the field current
to cease faster, but you will be replacing lots of regulator transistors.

>
> The issue with load dumps is not just the spike peak voltage; it's
> how long they last...i.e. total energy. The no-battery stunt was so
> harmful because the Xl of the field ensured it could not halt alternator
> quickly. So it wasn't a few ms of spike, it was many hundreds of ms
> worth...

My measurements indicate that it takes about 250msec for the field
current to decay during a load dump event.

> [i]
>> Actually, I have been thinking about Steve's problem some more, and the
>> transient which is tripping the crowbar could well be coming from a
>> "load dump lite".
>
> I don't recall the OP's remarks; did he indicate the pump cycling
> was tripping the breaker? Or was it an unknown?
>
> I agree the pump cycling could be an issue; your savior is often the
> low impedance of the battery eating those spikes. If it's not, the
> spikes may have gotten bigger, or the Xc of the battery lower...
>
> One spike solution is large MOV's across the spike creator. The gotcha
> there is: MOV's actually are sacrificial, as small spikes erode them
> over time; large ones will cause them to explode...
>
> [i]
>> I called B&C today to ask about this, and the only suggestion was to
>> connect pin 3 (sense input) of the LR3C as close to the battery as
>> possible. In other words, the battery is the spike filter of last
>> resort, so if there is a lot of wire (resistance) between pin 3 and the
>> battery, then the impedance along the wire (and in the master relay)
>> could allow pin 3 to see a higher voltage during the transient.
>
> The other reason to do so is because the regulator is there to protect &
> serve the battery. You don't care all that much about the bus being a
> half volt too high or low; you DO care about the battery being that, and
> the bus does have drop. So you want to sense near/at the battery.
>
> But that brings a dilemma. You must have the sense line draw some
> current. Why? So the regulator is sure its sense line has not become
> disconnected from the battery. {If it does come loose, the regulator
> says "0 volts! More power to the field, Scotty" to compensate, and
> the smell of burnt silicon soon fills the area. There were some early
> transistorized auto regulators that did just that; long gone I hope!} But
> then, even with but a few mils of draw, if it does that for 2 months in
> the hangar....your battery is dead. So it must be switched, and that....
>
>
>> The other possibility is to open the box, and put an appropriate filter
>> between pin 3 and the OVP circuit such that it doesn't effect what the
>> VR sees.
>
> I wonder if an appropriate ferrite toroid on the sense lead could stop the
> noise spikes from tripping the OVP without degrading regulation. I can't
> recall what such toroids look like when saturated with DC as
> it would be...

I thought about an external L-C filter, but that could screw-up the
voltage regulator's control loop dynamics. The RC filter seems safer.

[MikeMl]

Lancair IV-P Flyer wrote:
> On Apr 2, 9:26 am, MikeMl > wrote:
....
> Mike, given this information, what do you suggest I do next to try and
> isolate the cause?

Do the static test on the LR3C using the lab power supply I outlined in
the earlier post. You can do it with the LR3C either in or out of the
aircraft! If doing it on the bench, use a 28V 1A lamp as a substitute
for the alternator field.