# Looking for alternative potentiometer on current control.

#### Vwolpeet

Joined Mar 18, 2017
8
I want to build this circuit manly for fun. But also to expand knowledge. I want to use multi turn pots for voltage/current controls. The problem being the current control pot is a 250k value, which run $80-$100. Can I bring this down without causing harm to the op-amp? Any help is great. Thank you.

#### dl324

Joined Mar 30, 2015
12,518
If you can give up resolution, you can substitute 200k or 500k single turn trimmers which cost less than $1 each in small quantity. Personally, I think using multi-turn pots for voltage control is a PITA. I'd much rather use two single turn trimmers in series; one for coarse control and one for fine. Thread Starter #### Vwolpeet Joined Mar 18, 2017 8 If you can give up resolution, you can substitute 200k or 500k single turn trimmers which cost less than$1 each in small quantity.

Personally, I think using multi-turn pots for voltage control is a PITA. I'd much rather use two single turn trimmers in series; one for coarse control and one for fine.
That's the problem I want more resolution with a multi turn. But the price point is to high.

#### dl324

Joined Mar 30, 2015
12,518
That's the problem I want more resolution with a multi turn. But the price point is to high.
If you think you need something, then you need to pay for it.

Since you're doing this for fun, where does the resolution requirement come from?

I have a power supply that uses a multi-turn pot to adjust voltage and current. Take it from experience. It is a PITA to change voltage.

Where is it specified that the pots are multi-turn?

#### Vwolpeet

Joined Mar 18, 2017
8
I was wondering if I can use a lower value pot. Keeping the current adjustment range. While not putting the opamp under stress or damage.

#### Vwolpeet

Joined Mar 18, 2017
8
If you think you need something, then you need to pay for it.

Since you're doing this for fun, where does the resolution requirement come from?

I have a power supply that uses a multi-turn pot to adjust voltage and current. Take it from experience. It is a PITA to change voltage.

Where is it specified that the pots are multi-turn?
It does not specify multi turn but after reading and some online tutorials saying multi turn are preferred for hitting more precise numbers .700 vs .701-.710 that's the only reason.

#### crutschow

Joined Mar 14, 2008
26,426
I was wondering if I can use a lower value pot. Keeping the current adjustment range. While not putting the opamp under stress or damage.
Yes, you can lower both pot values but other resistor values will need to be changed proportionally.

As dl324 stated, an alternate technique to get higher resolution is to use two single-turn pots in series with one being, say 10 times the resistance of the other, e.g. 250k and 25K.
Thus the larger one gives coarse control and the smaller gives fine control with resolution comparable to a 10-turn pot.
That is usually cheaper than one 10-turn pot.

#### Vwolpeet

Joined Mar 18, 2017
8
Yes, you can lower both pot values but other resistor values will need to be changed proportionally.

As dl324 stated, an alternate technique to get higher resolution is to use two single-turn pots in series with one being, say 10 times the resistance of the other, e.g. 250k and 25K.
Thus the larger one gives coarse control and the smaller gives fine control with resolution comparable to a 10-turn pot.
That is usually cheaper than one 10-turn pot.
Then that's what I'll go with. Thank you.

#### dl324

Joined Mar 30, 2015
12,518
It does not specify multi turn but after reading and some online tutorials saying multi turn are preferred for hitting more precise numbers .700 vs .701-.710 that's the only reason.
What application requires that kind of resolution? Do you have a voltmeter that has that resolution? Do you expect line/load regulation of that circuit, with ripple, to give an output voltage change of less than 1mV?

#### crutschow

Joined Mar 14, 2008
26,426
Do you have a voltmeter that has that resolution?
My freebie multimeter from Harbor Freight does.

#### dl324

Joined Mar 30, 2015
12,518
My freebie multimeter from Harbor Freight does.
But only on the 2V range and even though it has the resolution, it's unlikely to be calibrated to be accurate to 1mV.

#### crutschow

Joined Mar 14, 2008
26,426
But only on the 2V range and even though it has the resolution, it's unlikely to be calibrated to be accurate to 1mV.
Of course.
No meter has the same resolution on all ranges.

But his example was "for hitting more precise numbers .700 vs .701-.710" so I assume he is more concerned about resolution than accuracy.
Sometimes you want to be able to precisely change the voltage even if the accuracy is (typically) less than the precision.