Rheostat Audio Interference

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