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.



One forgets
that they still have gain at a few tens of mhz. When it took off you
could hear it
in several of the radios. Darlington configurations have worse
stability problems.

I solved it by puttting about 100 ohms in the base right at the
transistor.

Tens of millihertz? Try again. And the base of the transistor at mid-gain
has more than 100 ohms of resistance in the control pot.




These circuits are designed by people not very skilled in the art.
They also suffer
the problem that if a bulb burns out shorted or there's an inadvertent
short on the
string of lights, the transistor fails. There is nothing to limit the
current.
That will usually take the pot too, especially if
it's near the high end of its range. The 100 ohm resistor will solve
that, too.

Unmitigated horsepoop. Bulbs don't burn out shorted. Bulbs burn out open.
If the transistor fails ( a million to one odds), the pot is open-circuited
and will not be damaged. You have absolutely no experience in the matter,
so why waste our time and bandwidth with your ignorance?





If it's not a darlington, the resistor will have to be smaller.

Don't apply for an engineering job at my company.



The cool way around all this is to design it with a P-FET power device
configured like an op-amp.


Why not an N-FET, or an NPN, or a PNP, all of which will solve the problem
elegantly.


Jim