Rheostat Audio Interference

RST Engineering wrote:

wrote in message
ups.com...

Trouble was, the thing was oscillating when at mid brightness
positions. Too much
capacitance on the output. A known problem with emitter followers.

Horsefeathers. Emitter followers have less than unity voltage gain and are
stable as rocks. And how does a resistive LED load become capacitive?
Difficult to imagine.

True for a real emitter follower, however if you have an N-stage
darlington with N=3, it can oscillate.

This was the subject of an IEEE Transactions article in the early 80's
and a source of great embarrassment for an engineer I worked with that
didn't read the article until after putting the design in production.

snip

--
Jim Pennino

Remove .spam.sux to reply.

Might want to read up.

If emitter followers are driving a load with a capacitive component, it
is reflected
to the input as a negative resistance. You can prove this with only
the simplest
hybrid pi model. It's a parasitic oscillation, not a loop oscillation
which couldn't
happen with gain 1 as you point out. You add enough series base
resistance
to swamp out the negative resistance, making it stable. There is
enough capacitance
in the wiring to excite the phenomena. This is why you see resistors
in the base
circuits of emitter followers all the time.

Darlingtons are much worse. You can show that with the hybird pi model
as well.
The oscillation will be near fT. That's a few mhz for 3055 type
devices.

Bulbs usually burn out open, true. What about some other short? The
circuit has NO short current protection other than the beta of the
transisitor. The base current becomes 1/beta of whatever short circuit
current flows. If the pot is set very near the max end of it's range,
dissapation will destroy the upper part of the resulting divider stick.

I have replaced enough of the panel mount edge adjust pots in Bonanzas
which have
this exact setup to know. Know what those cost?

The P-fet works great because you can get the output clear to the rail,
not the rail - 1 diode drop
as you are limited to with the 3055 approach or 2 drops in the
darlington approach.
It's inherently current limited by IDss as well. Draw it out: The
source goes to +14,
the drain to the lamps to ground. The control pot goes with the hot
end to +14, the
wiper to the gate, and the other end to the bulbs. So it also has a
small amount
of loop gain-- makes the adjustment very smooth. The huge gate-drain
capacitance
of the v-fet structure miller multiplied by the gain of the FET ensures
stability under
all conditions.

Maybe you should write it up for kit planes.

I won't be applying to RST anytime soon, but I did think you were
better than this.

Bill Hale

wrote in message
oups.com...


Might want to read up.

Have. Lots.


If emitter followers are driving a load with a capacitive component, it
is reflected
to the input as a negative resistance. This is why you see resistors
in the base
circuits of emitter followers all the time.

I really don't want to get into the whys and wherefores of parasitic
oscillation in this ng. Hell, the guy only wants his dimmer fixed. And
yes, a 100 ohm base decoupling resistor has been standard for me for the
last 40 years, mostly because I really can't control how well the collector
is bypassed for DC, audio, and RF simultaneously. It is difficult (not
impossible) to get parasitic oscillation where the collector is bypassed
from DC to daylight (or at least out to Ft).

The only place I *won't* use a base decoupling resistor is in a VHF power
amplifier where the input impedance is an order of magnitude lower than the
decoupling resistor.



Bulbs usually burn out open, true. What about some other short? The
circuit has NO short current protection other than the beta of the
transisitor. The base current becomes 1/beta of whatever short circuit
current flows. If the pot is set very near the max end of it's range,
dissapation will destroy the upper part of the resulting divider stick.

How do we know that there is no short circuit current protection? Did you
take it apart or do you have a schematic of this particular dimmer? We sure
could have saved a lot of wild ass guessing about the problem. He MIGHT
have it overloaded, but without knowing the particulars of this
installation, we are doing rectorandom guesses at the problem.



I have replaced enough of the panel mount edge adjust pots in Bonanzas
which have
this exact setup to know. Know what those cost?

No, and I have a hard time believing that Beech, the overdesigner of the
industry, put something out without short circuit protection of some sort.
However, if you have replaced them, then you are one up on me. No, I don't
know what they cost, but a simple current shutdown (with or without
foldback) is less than half a buck's worth of parts at the front end.


The P-fet works great because you can get the output clear to the rail,
not the rail - 1 diode drop
as you are limited to with the 3055 approach
Maybe you should write it up for kit planes.

You probably want to look at January, April, May, June, July 2001 Kitplanes
where I used N-channel, P-channel, NPN, and PNP transistors as the output
devices, explaining exactly what the tradeoffs were between each of the
devices. All in all, about ten designs.



I won't be applying to RST anytime soon,

And I thank you kindly.


but I did think you were
better than this.

When I'm in sci.electronics.design, I'm really quite careful about the
nuances of design. When I'm trying to get some poor guy's lamp dimmer to
work on RAH, RAO, or RAP, I'm a little less careful about being precisely
technically correct. For a dimmer that apparently worked correctly once
upon a time, poor design is about the last place I try and look. I'm not
above reengineering a crappy design, but if it really does have a parasitic
oscillation at the top end of the range, you'd have thought that in my last
45 years in this biz I'd have come across one, no?

Now, let's get back to fixing this sucker with what we DO know. Did the
fact that he can't key his radio transmitter or hear his radio receiver when
the unit was acting up mean anything to you? I doubt parasitic oscillation
keeps the transmitter key line from kicking the transmitter on. Use ALL the
clues, not just the one you are most comfortable with.

Jim

"RST Engineering" writes:



Unmitigated horsepoop. Bulbs don't burn out shorted. Bulbs burn out open.

I wish.... I had some hard experience with shorted ?317's I think
they were -- the 14v version of 327's. And lots of owners of
early-generation X10 modules discovered they would fail into full-on
when their 120vac lamp failed into full-off.

The clear solution for the OP was what the NASA LeRC 10x10 Supersonic
Wind Tunnel used for ""dimmers"" for the seven 40KHP drive motors
-- large glass tanks of salt water. The electrodes were cranked in
deeper to speed things up....

Oh, if you go that route, be sure and stick to positive G maneuvers
or you'll have a mess....
--
A host is a host from coast to
& no one will talk to a host that's close........[v].(301) 56-LINUX
Unless the host (that isn't close).........................pob 1433
is busy, hung or dead....................................20915-1433

Describe the failure mode that lets a bulb short out.

Jim



"David Lesher" wrote in message
...
"RST Engineering" writes:



Unmitigated horsepoop. Bulbs don't burn out shorted. Bulbs burn out
open.

I wish.... I had some hard experience with shorted ?317's I think
they were -- the 14v version of 327's. And lots of owners of
early-generation X10 modules discovered they would fail into full-on
when their 120vac lamp failed into full-off.

"RST Engineering" writes:

Describe the failure mode that lets a bulb short out.


I'm not an expert on same; just a victim. But from conversations
with someone at GE Nela Park decades ago; the filament breaks, and
can fall down from both gravity and err "sprong"ing when it lets go...

If the shortened filament end touches the OTHER post, it will draw
lots more current since it is shorter. It very soon burns out, but
in the meantime....

Or a length comes loose at both ends; it falls across the posts at the
bottom and.....


--
A host is a host from coast to
& no one will talk to a host that's close........[v].(301) 56-LINUX
Unless the host (that isn't close).........................pob 1433
is busy, hung or dead....................................20915-1433

OK, let me reiterate what you have told me. Please take your time to
respond so that we don't go chasing down blind alleys. THere are some
things you have said that are contradictory.

1. THe nav lights (you are talking about the ones on the wingtips -- red,
green, white in the back) are working just fine. What would this have to do
with the problem?

2. The internal instrument lights are working just fine. I'm presuming
what you are telling me is that if you connect a wire directly from the +12
supply to the instrument lights then they illuminate at the proper level.
You DID take the dimmer out of the circuit for this test, didn't you?

3. WHen you turn up the dimmer potentiometer so that the "instrument
lights" (do you mean the radio backlights or do you have some sort of
lighting system on the gyros?) then you get a certain amount of noise,
mainly in the headsets but some in the speaker as well.

4. You also said somewhere along the line that you cannot transmit when
this noise occurs, but that you CAN transmit using a handheld mic. However,
when you key up the transmitter, the radio lights dim.

5. You have a fixation on a "bad" transistor or a "bad" potentiometer.
Please let's not guess at solutions until we can prove something.

6. You said that you can't hear radio transmissions, yet in a subsequent
post you said that you CAN hear radio transmissions. Which is it?

7. I guess I'm a little unclear about the difference between "nav" and
"instrument" lighting. Can you elaborate?

8. I don't suppose there is a chance in hell that you have a schematic of
the dimmer?

9. What sort of test equipment can we presume as we toddle down the fixit
trail? A handheld AM broadcast band receiver is a hell of a good buzz
detector.


Jim


"Jonathan Goodish" wrote in message
...
This evening, I noticed a problem with the nav/instrument lighting and
panel intercom in my Cherokee that I haven't noticed before. The nav
lights and internal instrument lights (I have KX170Bs, KMA20 audio
panel, etc.) work just fine. The problem is that when I turn the
rheostat up so that the internal instrument lights come on, there is an
escalation in electrical noise in the intercom to the point that the
intercom cuts out (i.e. I can't communicate with any other person in the
airplane, can't hear radio transmissions, etc.) When I turn the
rheostat down to the point where the instrument lights are out, but the
navs are still on, there is no problem. I have no problem with the
panel light rheostat.

This problem wasn't always present, and I was caught off guard by it
tonight. I did swap the original non-shielded intercom wiring with
shielding wiring a couple months ago, and brought everything back to a
single-point ground, but I can't image that this has anything to do with
the rheostat issue.

Does anyone have any ideas?



Thanks,
JKG

It sounds like there are two likely possibilities: bad potentiometer or bad
power transistor. When I crank the pot up so that the internal radio lights
illuminate, I get an escalating buzz that quickly squelches the panel mount
intercom. Although I can hear the buzz in the headset (but not as easily in
the aircraft speaker), I can also hear radio broadcasts, but I can't
transmit and am unable the communicate via the intercom. When I turn the
lights back down, the problem goes away.

However, when I key the hand mic with the radio lights illuminated, the
radio lights dim significantly. I thought this was odd considering I don't
ever remember that happening in the past, and it certainly didn't happen
previously when the mic was keyed via the intercom.

A similar circuit that controls the panel lights produces no such effects.



Any ideas? Does this sound like a bad potentiometer, transistor, or

something else?

In article ,
"RST Engineering" wrote:
1. THe nav lights (you are talking about the ones on the wingtips -- red,
green, white in the back) are working just fine. What would this have to do
with the problem?

Nothing, other than the fact that they are activated by the same switch.
in the Cherokee.



2. The internal instrument lights are working just fine. I'm presuming
what you are telling me is that if you connect a wire directly from the +12
supply to the instrument lights then they illuminate at the proper level.
You DID take the dimmer out of the circuit for this test, didn't you?

No. With the dimmer in, the instrument lights (radio backlights)
illuminate and dim just fine.




3. WHen you turn up the dimmer potentiometer so that the "instrument
lights" (do you mean the radio backlights or do you have some sort of
lighting system on the gyros?) then you get a certain amount of noise,
mainly in the headsets but some in the speaker as well.

I have two circuits, panel lights and the radio backlights. The circuit
in question is the radio backlights.

You are correct, when I turn the pot so that the radio backlights
illuminate, I get noise that escalates in volume as I turn the lights
up. The lights appear to illuminate normally. When I get more than a
little current in circuit, the intercom squelches the mics on all
positions.



4. You also said somewhere along the line that you cannot transmit when
this noise occurs, but that you CAN transmit using a handheld mic. However,
when you key up the transmitter, the radio lights dim.

That is correct. The hand mic causes the radio backlights to go from
full bright to dim when it is keyed.



5. You have a fixation on a "bad" transistor or a "bad" potentiometer.
Please let's not guess at solutions until we can prove something.

I don't necessarily have a fixation on anything, otherwise I wouldn't be
asking for opinions. It's pretty obvious that something is wrong, but I
don't know what that "something" is at the moment.



6. You said that you can't hear radio transmissions, yet in a subsequent
post you said that you CAN hear radio transmissions. Which is it?

I can hear transmissions at all times, even when the buzzing is present
through the intercom when the radio backlights are at full brightness.



8. I don't suppose there is a chance in hell that you have a schematic of
the dimmer?

Yes, I do.



9. What sort of test equipment can we presume as we toddle down the fixit
trail? A handheld AM broadcast band receiver is a hell of a good buzz
detector.

I have an AM radio and a digital VOM and that's about it at the moment.




JKG

OK, one last question. How long has this exact radio stack (radio,
intercom, headset, etc.) been in the airplane, and have you ever witnessed
it working correctly? Has anything ELSE electrical in the airplane changed
between the time it worked correctly and the present time?

I'm going to go out on a bodacious limb and say that there is some vital
connection between your radio light dimmer and your PTT line. THere should
be absolutely no connection between the two.

Jim




"Jonathan Goodish" wrote in message
...
In article ,
"RST Engineering" wrote:

1. THe nav lights (you are talking about the ones on the wingtips --
red,
green, white in the back) are working just fine. What would this have to
do
with the problem?

Nothing, other than the fact that they are activated by the same switch.
in the Cherokee.

The same switch as what? Are you saying that by turning the radio lights
pot from off to just barely on (switch snap) also turns on the nav lights?





2. The internal instrument lights are working just fine. I'm presuming
what you are telling me is that if you connect a wire directly from the
+12
supply to the instrument lights then they illuminate at the proper level.
You DID take the dimmer out of the circuit for this test, didn't you?

No. With the dimmer in, the instrument lights (radio backlights)
illuminate and dim just fine.

OK, let's agree on some terminology so we don't keep running down the same
rabbit path. Does the SAME dimmer run the instrument (post) lights as the
radio lights? If not, let's have an INSTRUMENT LIGHT dimmer and a RADIO
LIGHT dimmer.





3. WHen you turn up the dimmer potentiometer so that the "instrument
lights" (do you mean the radio backlights or do you have some sort of
lighting system on the gyros?) then you get a certain amount of noise,
mainly in the headsets but some in the speaker as well.

I have two circuits, panel lights and the radio backlights. The circuit
in question is the radio backlights.

You are correct, when I turn the pot so that the radio backlights
illuminate, I get noise that escalates in volume as I turn the lights
up. The lights appear to illuminate normally. When I get more than a
little current in circuit, the intercom squelches the mics on all
positions.

Squelching the microphones turn them off. Are you saying that the radio
dimmer turns ALL the microphones off when the lights start to get bright?


6. You said that you can't hear radio transmissions, yet in a subsequent
post you said that you CAN hear radio transmissions. Which is it?

I can hear transmissions at all times, even when the buzzing is present
through the intercom when the radio backlights are at full brightness.



8. I don't suppose there is a chance in hell that you have a schematic
of
the dimmer?

Yes, I do.



9. What sort of test equipment can we presume as we toddle down the
fixit
trail? A handheld AM broadcast band receiver is a hell of a good buzz
detector.

I have an AM radio and a digital VOM and that's about it at the moment.

In article ,
"RST Engineering" wrote:
Nothing, other than the fact that they are activated by the same switch.
in the Cherokee.

The same switch as what? Are you saying that by turning the radio lights
pot from off to just barely on (switch snap) also turns on the nav lights?

Yes.


OK, let's agree on some terminology so we don't keep running down the same
rabbit path. Does the SAME dimmer run the instrument (post) lights as the
radio lights? If not, let's have an INSTRUMENT LIGHT dimmer and a RADIO
LIGHT dimmer.

No. We are talking about the radio light dimmer circuit.



You are correct, when I turn the pot so that the radio backlights
illuminate, I get noise that escalates in volume as I turn the lights
up. The lights appear to illuminate normally. When I get more than a
little current in circuit, the intercom squelches the mics on all
positions.

Squelching the microphones turn them off. Are you saying that the radio
dimmer turns ALL the microphones off when the lights start to get bright?

Yes.


OK, one last question. How long has this exact radio stack (radio,
intercom, headset, etc.) been in the airplane, and have you ever witnessed
it working correctly? Has anything ELSE electrical in the airplane changed
between the time it worked correctly and the present time?

It's all been in there for years. We used to fly quite a bit at night,
but haven't so much in recent years. We did the other night, and that's
when I noticed this problem. I don't remember it having been there
before. BUT ...

I didn't have much time tonight, but I did collect some more data. With
the master on, the avionics master off, and the radio light dimmer on,
the electrical noise is still in the headset even though the intercom is
not powered since the avionics master is off. The noise escalates as
the Century 1 autopilot gyro spins up... that's where the whine is
coming from. When I pull the connector out of the Century 1 to power it
down, the whine is instantly gone, but I can still hear scratches
through the headset when I move the radio light dimmer.

There is a terminal block under the panel to which it appears some of
the avionics is grounded, including my intercom. When I pulled the
intercom ground off of this terminal block and grounded it to the
airframe, even near to the block, the problem disappeared and I regained
normal intercom operation with the radio lights on full bright and no
noise in the intercom. I now wonder if the terminal block is grounded
to the airframe, or what is going on there. Would it be typical for a
terminal block to be anything OTHER than a ground? I didn't have
anything with me to measure voltage on this block or check for
continuity with ground.



JKG