Q: Does PTC swap (i.e. 'upgrade') in DMM = calibration issues? - Page 1
Forum section: electronics / testgear

author: em132
author_rank: Contributor
timestamp: June 12, 2025, 02:30:28 am
content: Quick hypothetical question: Would swapping a "wimpy" PTC withone with more chest hairsaffect a meter's calibration (assuming the resting resistances on both are similar)?Did a quick search online and on this forum and couldn't find a straightforward answer.TIA.

author: bdunham7
author_rank: Super Contributor
timestamp: June 12, 2025, 02:55:50 am
content: Quote from: em132 on June 12, 2025, 02:30:28 amYes. Now if you ask me "how much?" I'd say "it depends".For voltage readings, there should be essentially zero effect. For others such as resistance, capacitance, etc, there may be some differences. For some meters that use PTCs for Lo-Z operation it actually states in the manual that you need to let it cool down for a few minutes after tripping the PTC (applying higher voltages in Lo-Z mode) before the resistance or other ranges are within spec.

author: Fungus
author_rank: Super Contributor
timestamp: June 12, 2025, 03:50:21 am
content: Quote from: em132 on June 12, 2025, 02:30:28 amYes.eg. Try putting your meter in Ohms and connecting it to mains for a few seconds. Readings will be way off until the PTC cools down.

author: em132
author_rank: Contributor
timestamp: June 12, 2025, 05:19:04 am
content: @bdunham7LoZ not a consideration for this "hypothetical" instrument, so we can put that to rest.And let's assume the two PTCs are within 5% of each other in resistance (@ 25°C), and that I don't usually make a habit out of using my DMMs as space heaters, would the readings on stuff other than Volts really be that affected?

author: Kleinstein
author_rank: Super Contributor
timestamp: June 12, 2025, 05:55:24 am
content: There should be little direct effect of the PTC properties (resistance). There could be a little effect from thermal EMF due to PTC heat or at the PTC - that would normally be in the µV range. Here already a change in the thermals from a different case can make a difference. Also the capacitance reading could be directly effected, but it depends how the meter is build and which PTC to change.If the effect is relevant also depends on the meter grade: With a 7 digit meter it needs more care than what is visible with a 3 digit meter.However calibration is also about verifying that the meter actually works. Any change of a part, even if not directly effecting the readings has a small chance to cause damages (e.g. from ESD or solder droplettes, PCB flexing). So such action would void a formal calibration for a good reason.

author: bdunham7
author_rank: Super Contributor
timestamp: June 12, 2025, 10:02:28 am
content: Quote from: em132 on June 12, 2025, 05:19:04 amI wasn't saying this would only affect meters with Lo-Z function, I was pointing out that changes in the resistance of the PTC are known to affect readings in some instruments. IDK what the resistance of a typical 1.1k PTC would be after it is tripped and then cools for say 30 seconds, but I don't think it is going to be 10X higher or anything like that. Yet meters where the PTC has been heated can have errors that persist for several minutes. Then it becomes a question of exactly how much 5%--or whatever value you choose--matters. That depends on the specific design of the meter so I don't know if a general rule is helpful here. The type that uses a fixed precision test current and a secondary voltage sensing circuit may not be affected very much in resistance ranges. But not all meters work that way.

author: Fungus
author_rank: Super Contributor
timestamp: June 12, 2025, 10:15:36 am
content: It's gonna depend on the meter.You could try it and report back with meter model and findings. In the name of science.Better yet, use the meter carefully and you'll never need to worry about the PTC.

author: joeqsmith
author_rank: Super Contributor
timestamp: June 12, 2025, 01:11:38 pm
content: Quote from: em132 on June 12, 2025, 02:30:28 amIf you are modding a meter, I assume it is some POS and you would rather tamper with the meter than buy proper tools. I have no idea what your skill level is, but lets assume you are young and stupid with little skills. You may be making the meter less safe/robust than it was and never know it. What ever is driving you to suggest the change, I assume the meter is functional. How ever you are using it, you have not damaged it yet. How do you plan to evaluate the mods to know that you gained anything?I guess the upside is you can brag to your peers about the mods. Ignoring Darwin, prove it to yourself. Swap the PTC with a resistor of +/-50% and take some measurements. Do you see any difference?Just as a comparison, shown is a low end DMM. I added PTCs and series resistance similar to what you would find in a higher class meter. The meter was never realigned. I demonstrate taking a few measurements with it after the mods.