variable resistor

[akis]

I want to examine the possibility of creating a variable resistor to retain the parameters of a normal, good quality fixed value resistor. The main quality is little or no change with temperature, current, voltage and frequency.

A potentiometer, I found, is too noisy and unstable, does not like DC through it and so on.

A simple transistor can act as a variable resistor between collector and emitter, unfortunately I do not know how to control it so its "resistance" stays the same regardless of Vce and Ic.

Any ideas? I skipped-read the thread about the LED controlled resistor thingy, and maybe this is a solution?

[richie00boy]

Check out stepped attenuators.

[akis]

Ah I see, a huge switch with dozens of resistors. Just like what I have on my ageing bench power supply.

[akis]

Forgot to say that is certainly a possibility and in that case I'd use just 4 settings, I would not need more. A rotary switch with 4 positions, and 4 resistors. Maybe 6 positions and 6 resistors.

But if I wanted something that will do the job on the PCB, not at the front panel so the signal does not have to travel over extra wires.

[richie00boy]

Digital pot chip then.

[sumaudioguy]

I think the potentiometer problems you experience are from cheap pots. Try a wire wound in the 500 ohm size.

Like this except PCB? 43C1500 Honeywell Potentiometers

or this? Digi-Key - SP140-1.0K-ND (Manufacturer - 140-0-0-102)

[akis]

These pots are very expensive, I can buy two Eminence Stonehenge 12" speakers for that price, and the speakers are the most crucial and expensive part.

My solution will be to use 4 or 6 resistors, placed very close to the transistor (1/4 "), so that the voltage across them stays close to the transistor. There will be 4 or 6 short leads/tracks to a rotary switch (1.5" track + 2-3" leads), and those will be connected to earth. The whole switch will be earthed. Hopefully this will limit noise and distortion.

[AndrewT]

the 12 position rotary switch is quite cheap.
It can be bought in 4 versions:
12w 1p
6w 2p
4w 3p
3w 4p

[akis]

Yes I will use a rotary switch, I already have a bunch in my toolbox. The comment about "pots are very expensive" refered to the previous post and some very very expensive wire-wound pots

[Conrad Hoffman]

What are you trying to do? Is this something that will be adjusted often like a volume control, or an internal function like bias that's just adjusted every now and then?

CH