[time-nuts] OXCO Issues -- Latest

Al Wolfe alw.k9si at gmail.com
Sun May 12 15:38:55 EDT 2013

Years ago we were taught that it was poor engineering practice to use pots 
to trim a DC value, especially if any appreciable current was to be drawn 
from the wiper. (Probably true for any kind of signal on a pot) It seems 
that current through the wiper would eventually erode away the material 
under the wiper. This is true especially for set-and-forget applications. If 
the wiper is used only as a voltage reference and very little current drawn 
then it is probably OK for most situations.

The above info is mainly for carbon and ceramic pots. Wire wound ones tend 
to be somewhat more forgiving but they tend to be step-variable between the 
wire turns. Often the sweet spot is between the steps and they aren't very 
useful for fine tuning purposes. Sometimes we would use two pots in series. 
One was set up as a rheostat and was around 5 percent of the ohmage of the 
main one. It was used for fine tuning but the problem of DC current through 
the wiper remains and is exacerbated.

Probably the best design is to use fixed resistors for most of the voltage 
divider circuit and the variable element be a small fraction of the total 
divider resistance.


> Well, I decided to make the "Y" incision and pulled the circuit board
> out of the oven.  (Photos later after I crop them.)
> Yes, the trimpot is part of the circuit associated with the thermistor.
> My plan is to check the resistors (surprisingly most are carbon film
> except those in the thermistor circuit), look at the circuit board under
> a magnifier, and stick it back in the foam for testing after making any
> repairs.
> Thanks again for the continuing comments and advice.
> Fred

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