Re: Noise Problem. Both Comms Breaking Squelch

[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.