which type potentiometer to use

[fitter108]

I am modifying a pyramid power supply to make it into a varible voltage and varible current supply. I have the schematic drawings for the changes and the ohm ratings but what I need help with is what type of pot to buy. I understand that a linear type- maybe- 10 turn- wire wound or carbon. Could someone help me with this question. I am not worried about price but want very stable setpoint. Also the voltage reg is a LM732   Thanks

[david77]

You mean LM723, right?

What I like to do is split the pot in one single turn and one 10 turn pot, so as to get a coarse and a fine
setting. Say you need a 5k pot, I'd use a 5k single turn and maybe a 500R 10 turn in series, some experimenting will be needed here.

I find it a PITA to turn and turn and turn to get to my desired voltage. On the other hand a single pot is
crap if you want a very exact setting, you move it a fraction of a mm and the voltage is in a completly
different range.

David
EDIT: Oh, and yes, linear is what you want.

[alm]

What I like to do is split the pot in one single turn and one 10 turn pot, so as to get a coarse and a fine
setting. Say you need a 5k pot, I'd use a 5k single turn and maybe a 500R 10 turn in series, some experimenting will be needed here.

I find it a PITA to turn and turn and turn to get to my desired voltage. On the other hand a single pot is
crap if you want a very exact setting, you move it a fraction of a mm and the voltage is in a completly
different range.

Agreed, although the fine adjustment can also be single turn, most commercial equipment works that way. It's a trade-off between speed and resolution (and stability, single turn pots tend to be at staying put after you set them according to Bob Pease).

[fitter108]

Yes that is a lm723.  The R value is 2.5k  for both current and volts and do you have a preferance between carbon - wirewound - ceramic.  Also do i wire the twin pots in parallel?

[david77]

I don't think it matters too much what type of pot you use. A simple carbon type will be
sufficient I'd say, but a better quality conductive plastic or ceramic type is always preferrable.
At work we stock these very simple pots, so I usually use them:
http://www.omeg.com/pc2bubr2.htm
They are alright for most things but high quality audio use.

If you use a 10 turn pot they are usually of higher quality anyway.

Not in parallel - in series!

David

[fitter108]

Thanks so much david77 I think i have figured out the series pots. I should be good to go