Trimpot temperature coefficient and drift over time - Page 1
Forum section: electronics / beginners

author: NaxFM
author_rank: Regular Contributor
timestamp: August 15, 2022, 08:40:55 pm
content: Hi everyone, for a personal project I need to use a 25 turns cermet trimpot (the usual bourns ones) to fine tune an high precision circuit.I never worked with trimpots for high accuracy, and I was wondering about their stability and drift over time.The temperature coefficient of a (cheap) trimpot is terrible at the usual 100ppm/C, however the product will be ovenized and kept at a constant temperature.I'm worried that even at a constant temperature the drift over time would be too much. Do you happen to have any figures or numbers about the stability of trimpots.If I leave it on for a year at a constant temperature, how many ppm will I be off?I can't find any document related to this topic, maybe you know something that could help me.

author: mag_therm
author_rank: Frequent Contributor
timestamp: August 15, 2022, 10:59:12 pm
content: A look at the Bourns pcb range shows that the long skinny through hole RT12 milspec type has among the best enr.As you probably know, stability is often improved by putting trimpot in network with stable resistors above and below to minimize trim span and hence variations.What drift can you tolerate?

author: gbaddeley
author_rank: Regular Contributor
timestamp: August 16, 2022, 02:55:15 am
content: Can you design the circuit such that trimpot tempco has minimal effect? Eg pad it out with lower tempco resistors?

author: NaxFM
author_rank: Regular Contributor
timestamp: August 16, 2022, 06:09:22 am
content: Quote from: mag_therm on August 15, 2022, 10:59:12 pmLet's say that I "can" tolerate any drift because I can pair the trimmer with precision resistors to reduce the effects of drift.As in many cases, it's a matter of cost, I wanted to explore all the possibilities I had before going the route of an extremely reduced trim range which would give be best stability but also introduce the problem of not enough range.

author: NaxFM
author_rank: Regular Contributor
timestamp: August 16, 2022, 06:12:09 am
content: Quote from: gbaddeley on August 16, 2022, 02:55:15 amI'm already doing that, but the circuit should have a quite large trim range of 200-300mV, and I am already at the limit.

author: SeanB
author_rank: Super Contributor
timestamp: August 16, 2022, 06:52:03 am
content: Not possible to put in some padding resistors and shorting links, so you can have the trim range split up into 2 or 3 ranges, and then have a set of jumpers that you use to move the pot range into. Depending on the circuit you might even be able to do without the pot at all, and simply have a few preset ranges for offset adjustment. Otherwise if it is likely to be a rare job use the military approved method, which is to use 2 decade boxes, to select a set of fixed value resistors to use to set the offset.4 turret pins to solder them to on the PCB, and then select on test for the right resistors in the range.

author: Zero999
author_rank: Super Contributor
timestamp: August 16, 2022, 07:33:12 am
content: Can you design the circuit so the actual value is unimportant? The ratio of resistances from the wiper to either end should have virtually no temperature drift.Please post a schematic.

author: mag_therm
author_rank: Frequent Contributor
timestamp: August 16, 2022, 08:03:12 am
content: A description of pot testing including enr and crv.https://epci.eu/resistors-potentiometers-basic-principles/

author: Terry Bites
author_rank: Super Contributor
timestamp: August 16, 2022, 11:41:00 am
content: Can you show circuit and location of pot.There may be a better solution.

author: richard.cs
author_rank: Super Contributor
timestamp: August 16, 2022, 04:41:02 pm
content: A colleague of mine claims the multiturn ones are much less stable with temperature, based on testing with his previous employer. The datasheet accurately represents the stability of the resistive element, but thermal expansion of the mechanics apparently causes small jumps in resistance with temperature, the wiper physically moves. These jumps, so he tells me, are discrete steps, and have some hysteresis, so are hard to compensate for.I have not tested it myself, but it certainly seems plausible, and as a result I tend to use coarse/fine pots rather than multiturns in precision designs.

author: Terry Bites
author_rank: Super Contributor
timestamp: August 18, 2022, 07:19:06 pm
content: Pots will undermine precise analog circuit function. Resist the urge to tweak or the law of diminishing returns and frequent future twiddles will come to haunt you.If you can, get rid of the pots and fix the offset, gain, drift and other compensations etc in code.