Trimmer question

[James P]

When substituting resistors with trimmers is it a safe bet to use a higher value trimmer that is say 15-20% higher than the original, to allow for tweaking either side of the original value?

Do all 3 legs need to be used?  :oops:

Thanks a lot

James P

[Ge_Whiz]

Ideally, you want a trimmer about twice the value of the fixed resistor so that the trimmer is set about half-way. Unfortunately, trimmers come in only a limited range of values, so you don't usually get the choice. But higher than the fixed value, definitely.

You can get away with only using two legs. However, the trimmer could then fail open-circuit, preventing the circuit from working (or, conceivably though rarely, damaging the circuit). Also, it is said that using all three pins in the usual way helps with noise immunity.

[Peter Snowberg]

watch out about turning the trimmer down too far!

If I'm substituting a 10K for example, I'll usually put a 4.7K in series with a 10K trimmer. That gives 50% to 150% value adjustment and for most things 4.7K as a lowest setting isn't going to cause any blue smoke to be let out of the parts. I would always try to keep at least 1K in series with the pot if you're not sure about the circuit. 2.2K is better still. With 2.2K across 9 volts, the meximum current draw is only about 4 milliwats so no blue smoke.

[James P]

You can get away with only using two legs. However, the trimmer could then fail open-circuit, preventing the circuit from working (or, conceivably though rarely, damaging the circuit). Also, it is said that using all three pins in the usual way helps with noise immunity.

What happens to Jake the Peg then - the extra leg? Reference it to ground or loop back into the circuit?

watch out about turning the trimmer down too far!

If I'm substituting a 10K for example, I'll usually put a 4.7K in series with a 10K trimmer. That gives 50% to 150% value adjustment and for most things 4.7K as a lowest setting isn't going to cause any blue smoke to be let out of the parts. I would always try to keep at least 1K in series with the pot if you're not sure about the circuit. 2.2K is better still. With 2.2K across 9 volts, the meximum current draw is only about 4 milliwats so no blue smoke.

Thanks Peter, that's a good insurance against burning parts!!

James P

[Ge_Whiz]

Usual arrangement for trimmers - outer legs are wired in place of the fixed resistor connections, then the wiper (middle leg) is connected to one or other outer leg, doesn't matter which.

[tclixx]

So if  I wanted to put a trimmer pot in place of a 68k resistor and don't want it to go below 47k. I would put a 47k resistor in series with a 100k trimmer pot.

What about if i wanted to put a trimmer pot in place of a fixed capacitor that was .01uf and I don't want it to go below .01uf or higher than .025uf or .03uf. What size pot would you use.?