Can’t find correct pot value - Page 1
Forum section: electronics / beginners

author: jeffjmr
author_rank: Regular Contributor
timestamp: March 16, 2022, 04:18:46 pm
content: I’d like some finer control over my benchtop PS current output. I replaced the voltage pot with a precision 10 turn unit and it is working great. I’d like to do the same on the current side.But my unit calls for a 3K pot and I can’t find a multi-turn pot in that value. But I can find a 5K pot.So, am I correct in assuming I can place a 7500 ohm resistor across pins 2 and 3 on the 5K pot to end up functionally at 0 to 3K?Thank,Jeff

author: TimFox
author_rank: Super Contributor
timestamp: March 16, 2022, 04:31:59 pm
content: Please show the entire potentiometer circuit.If there is no resistor in series with the existing pot, but the 3k resistance makes a difference to the current sense, then your solution of 7500 ohms in parallel with a 5000 ohm replacement pot will work well.If there is no resistor in series with the pot, and the 3k resistance is not important across the current sense resistor, then your parallel resistor is probably not needed.

author: Terry Bites
author_rank: Super Contributor
timestamp: March 16, 2022, 04:51:45 pm
content: buy a 3k pot?https://www.google.co.uk/search?q=3K+potentilometer&source=lmns&bih=999&biw=2048&hl=en&sa=X&ved=2ahUKEwjsoa-9i8v2AhV_i_0HHcVqDa8Q_AUoAHoECAEQAA

author: bob91343
author_rank: Super Contributor
timestamp: March 16, 2022, 04:54:57 pm
content: Those are not 10 turns.

author: Terry Bites
author_rank: Super Contributor
timestamp: March 16, 2022, 05:03:07 pm
content: did you check them all in one minute- fantastic

author: TimFox
author_rank: Super Contributor
timestamp: March 16, 2022, 05:09:07 pm
content: I checked the first page of results, all of which are single-turn, in less than a minute from your link, even with the typo.Similarly when adding "10-turn" to the search: Google goes to results foreither"3k" or "10 turn".

author: Vovk_Z
author_rank: Super Contributor
timestamp: March 16, 2022, 05:29:13 pm
content: I usually redesign a circuit to have a possibility to use 2k or 5k pots.

author: TimFox
author_rank: Super Contributor
timestamp: March 16, 2022, 05:32:40 pm
content: Similarly, when dealing with multi-turn trimpots, I find that the standard values are 10, 20, 50, 100, 200, 500, ... , 20k, 50k.

author: golden_labels
author_rank: Super Contributor
timestamp: March 16, 2022, 08:34:32 pm
content: Perhaps wait for OP to post the circuit? Depending on what’s the role and placement of the potentiometer, its exact value may be not important.While posting links to dynamically generated results, be aware that what others see may significantly differ from what you saw. That is in particular true for heavily profiled results like those from Goofle.

author: TimFox
author_rank: Super Contributor
timestamp: March 16, 2022, 09:07:53 pm
content: That's what I said in Reply #1, where I asked about the pot circuit.

author: golden_labels
author_rank: Super Contributor
timestamp: March 16, 2022, 11:23:46 pm
content: And that’s what I am referring to.

author: themadhippy
author_rank: Super Contributor
timestamp: March 17, 2022, 12:32:52 am
content: Buy a 3k pot and add a reduction drive

author: jeffjmr
author_rank: Regular Contributor
timestamp: March 17, 2022, 02:23:28 am
content: It doesn’t have to be a 10 turn. A 3 or 5 turn precision pot would still be an improvement over the 300 degree originals. I searched pretty well and could not find any multi-turn at 3K.It’s for my Leader LPS-152, a surprisingly stable PS on both voltage and current, but the 3k pot adjustments are just too coarse sometimes.There are three altogether, +1A and -1A on the 25v side and +5A on the 6v side, schematic sections for two are attached below, near the rectangular “Current” boxes. I have a 10 turn 10K on the 25v side, and a 5 turn 10k on the 6v side.Thanks all for your responses. I see no reason the 7500 ohm resistor in parallel shouldn’t work.Jeff

author: Ian.M
author_rank: Super Contributor
timestamp: March 17, 2022, 03:49:41 am
content: The current pot is picking off a voltage from a potential divider for the OPAMP, which has very high input impedance so the load on its wiper is negligible. Therefore your original assumption:Quote from: jeffjmr on March 16, 2022, 04:18:46 pmis correct, for this circuit.If there had been significant load on the wiper or if it had been wired as a two terminal rheostat, then a parallel resistor would have resulted in a highly undesirable change in the 'law' of the pot, from linear to something else (non-linear), determined by the circuit.

author: bob91343
author_rank: Super Contributor
timestamp: March 17, 2022, 06:09:39 am
content: The 150 Ohm and 62k resistors in series with the 3k pot can be increased or decreased proportionately to allow use of a standard pot. To use a 2k pot for instance, lower the two resistors by one-third to 100 Ohms and 39k Ohms and you should be good to go. To use a 5k pot, increase the two to 250 and 110k Ohms.

author: jeffjmr
author_rank: Regular Contributor
timestamp: March 17, 2022, 11:39:10 am
content: Quote from: bob91343 on March 17, 2022, 06:09:39 amI see how that would work. But I think it would be easier and less invasive to parallel one 7500 ohm onto the pot as opposed to changing out two on the PCB, provided I get the same result.But I appreciate your pointing that out. I did not study the schematic closely enough to think of that alternative solution.Jeff