Effect of temperatur on trimpots ? - Page 1
Forum section: electronics / projects

author: thinkfat
author_rank: Unknown
timestamp: November 04, 2020, 12:16:04 pm
content: Hi,In a somewhat temperature sensitive application I want to use a trimpot to set a bias voltage. I'm using a small Bourns 3313-J 50k SMD trimpot. Temperature coefficient of the resistance is spec'd as +/- 100ppm/°C. That's quite a bit.However, when using the trimpot as a voltage divider, wouldn't the voltage on the slider contact be constant?

author: Alti
author_rank: Frequent Contributor
timestamp: November 04, 2020, 01:46:41 pm
content: That depends on the definition of "constant".If it says +-100ppm, I think it applies to the whole resistive trace.How do you know this isn't +100ppm for upper half and -100ppm for lower half? You could gain some statistical data and figure out values but be realistic, you cannot expect the whole trace is Xppm +-0.00000001ppm.

author: thinkfat
author_rank: Unknown
timestamp: November 04, 2020, 02:24:16 pm
content: I'm expecting as the trimpot has a specified linearity that it will be fairly homogeneous. Thus the ratio between the two parts separated by the slider should be invariant towards temperature.Edit: the trimpot is connected to a voltage reference with 2ppm/°C.

author: mikerj
author_rank: Super Contributor
timestamp: November 04, 2020, 03:50:54 pm
content: Quote from: Alti on November 04, 2020, 01:46:41 pmHow could the exact same material have the opposite temperature coefficient?The tempco applies to the end-end resistance of the potentiometer but the ratio between two sections of the track should remain much closer than that.

author: Kleinstein
author_rank: Super Contributor
timestamp: November 04, 2020, 04:06:20 pm
content: The temperature coefficient for the ratio is usually considerably better than for the total resistance. However there are often no separate specs for this. So typical the TC for the set voltage will be better than 100 ppm, but no guarantee.

author: Gyro
author_rank: Super Contributor
timestamp: November 04, 2020, 04:43:58 pm
content: Quote from: thinkfat on November 04, 2020, 02:24:16 pmIf you're using it with a 2ppm/'C reference, then you should really be using it as a fine trim of a pair of very low TC resistors rather than on its own, so that its TC contribution is negligible. Ideally, you want its contribution to be a couple of orders of magnitude lower than the main divider resistors.

author: thinkfat
author_rank: Unknown
timestamp: November 04, 2020, 05:35:58 pm
content: Quote from: Gyro on November 04, 2020, 04:43:58 pmThat would also mean a very small voltage range. Not applicable, I'm afraid.

author: Gyro
author_rank: Super Contributor
timestamp: November 04, 2020, 05:40:39 pm
content: That's a shame. I don't think you could reasonably expect <2ppm/'C matching along the length of a 100ppm/'C pot.

author: Alti
author_rank: Frequent Contributor
timestamp: November 04, 2020, 05:42:05 pm
content: Quote from: mikerj on November 04, 2020, 03:50:54 pmNow we jump from vague definition of constance to vague definition of sameness.Exact sameness, to be exact.

author: thinkfat
author_rank: Unknown
timestamp: November 04, 2020, 07:48:54 pm
content: Quote from: Alti on November 04, 2020, 05:42:05 pmI don't want to consider exotic effects for now.Quote from: thinkfat on November 04, 2020, 02:24:16 pmI just realized that linearity plays no part in this. Even if the resistive material is not applied homogeneously this is inconsequential for the static case, so long as the TC of the material is homogeneous.In fact, the TC of the set voltage might be a lot better than with a fixed voltage divider made of individual, low-TC resistors.What I didn't consider yet is however the TC of the contact resistance. There is tiny bit of current flowing into the slider contact, mainly the leakage current of the opamp input it is connected to.

author: Kleinstein
author_rank: Super Contributor
timestamp: November 04, 2020, 09:45:01 pm
content: The material of the trimmer is nominal the same, but there can be still unintentional differences. So the real TC matching would not be perfect either. With resistor network the relative TC is something like 5 to 25 times better than the absolute TC - a would expect about a comparable ratio for the trimmer.There is another downside with some trimmers: especially the thick film trimmers and carbon ones can be quite noisy. With a significant voltage at the trimmer there will also be quite some low frequency noise.Chances are the trimmer may be more noisy than a 2 ppm/K reference.

author: Siwastaja
author_rank: Super Contributor
timestamp: November 05, 2020, 06:08:26 pm
content: Why do you need a lot of trimming range?Trimming would be normally used to trim out the original inaccuracies, in which case if the inaccuracy of the trim circuit itself is attenuated.For example, if the total inaccuracy of the original circuit without trim is max +/-100 ppm, then add trim for +/-100 ppm; even if the trim itself is only 1% accurate, this will affect the original circuit by +/- 1ppm.

author: thinkfat
author_rank: Unknown
timestamp: November 05, 2020, 08:21:41 pm
content: Quote from: Kleinstein on November 04, 2020, 09:45:01 pmI've read some papers about flicker noise in resistors. The reference voltage is only 2.5V. Might not be such a big problem after all. But I'll definitely review the feedback network of the amplifier in question.

author: thinkfat
author_rank: Unknown
timestamp: November 05, 2020, 08:24:13 pm
content: Quote from: Siwastaja on November 05, 2020, 06:08:26 pmI'm not trimming. I need to set a bias voltage (or offset, if you will) over a fairly large range. Absolute accuracy is not required. It just shouldn't change very much with temperature after it was set.

author: Kleinstein
author_rank: Super Contributor
timestamp: November 06, 2020, 07:21:22 pm
content: If the exact value does not matter, why use a trimmer and not a pair of selected resistors or maybe steps on a chain of equal resistors. These are usually more stable than a trimmer.

author: thinkfat
author_rank: Unknown
timestamp: November 08, 2020, 09:25:42 pm
content: Quote from: Kleinstein on November 06, 2020, 07:21:22 pmYes, that's not a bad idea. Taps on a resistor ladder would work, especially since I don't really need to hit a particular voltage precisely. I likely don't have the necessary real estate on the PCB, though. 3313-J are quite compact and still easy to adjust without breaking out the glasses

author: richard.cs
author_rank: Super Contributor
timestamp: November 08, 2020, 10:59:57 pm
content: 3313-J 50k is a single turn part so should be OK, but be very careful with the multi-turn types. The mechanics has dissimilar materials and can often be quite temperature sensitive resulting in the wiper jumping as the temperature changes.

author: bson
author_rank: Unknown
timestamp: November 09, 2020, 03:38:57 am
content: Quote from: thinkfat on November 04, 2020, 07:48:54 pmYes, I would expect as much, assuming a low current so there is no thermal difference across the pot.For a divider where the two resistors have the same TC,Vout/Vin = tc*R2/(tc*R1 + tc*R2) = tc*R2 / (tc*(R1 + R2)) = tc/tc * R2/(R1 + R2)I.e., as long as the material property is the same across the pot the TC factors out. Even if not exactly the same it still largely factors out.