Understanding ppm specs for a potentiometer (lies, lies and more lies) - Page 1
Forum section: electronics / projects

author: SharpEars
author_rank: Unknown
timestamp: October 21, 2015, 05:16:31 pm
content: One of the most accurate linear multi-turn potentiometers (trimmers) made today is the Vishay 1285G. However, as I look through the datasheet, I see something that makes no sense:Vishay 1280G/1285G DatasheetUnder Features at the top right we see these spec figures:- ± 15 ppm/°C (model 1280G)- ± 5 ppm/°C (model 1285G)Impressive, unbelievable numbers, right? These specs seem to rival many low tempco precision resistors!But, then we see:- through the wiper ± 50 ppm/°CWhat the heck was the point of quoting 5 or 10 ppm/°C if that's 10-20% of the actual temperature shift you get through the wiper? Does the change in the accuracy of the total max resistance really matter that much? I mean who the heck uses a potentiometer without using the wiper??

author: f5r5e5d
author_rank: Frequent Contributor
timestamp: October 21, 2015, 05:22:24 pm
content: maybe sloppy withResistance vsRatio in the meaning of TCR?so likely they're saying don't use it as a Rheostat

author: SharpEars
author_rank: Unknown
timestamp: October 21, 2015, 05:25:05 pm
content: Quote from: f5r5e5d on October 21, 2015, 05:22:24 pmSo, you're saying that if you use it as a rheostat it's ± 50 ppm/°C, but if you use it as a ratio measurement, then it magically becomes ± 5 ppm/°C? I don't get it, you still have to use the wiper no matter what you do with it that includes its intended purpose - variability.

author: f5r5e5d
author_rank: Frequent Contributor
timestamp: October 21, 2015, 05:46:35 pm
content: the ratio would be determined by wiper position on the low TCResisance, and highly matching throughout the length of the bulk foil elementto model you need another R for the wiper contact resistance - but if you draw no current ( == Potentiometer mode ) then there's no V drop across the wiper contact R so wiper TCResistance doesn't affect the Potentiometer TCRatio

author: SharpEars
author_rank: Unknown
timestamp: October 21, 2015, 07:24:17 pm
content: Quote from: DiligentMinds.com on October 21, 2015, 06:46:51 pmAnd adjustable standard is exactly what I am aiming for. Thank you for the details.

author: SeanB
author_rank: Super Contributor
timestamp: October 21, 2015, 07:39:09 pm
content: That is why military avionics which has adjustments uses a pair of turrets riveted to the board, where you solder in the appropriate value resistor to do the adjustment. You can go select any resistor in the E96 range for each side, just by using a resistor bridge ( or two for a ratio) to get to a close value, then select the nearest value. Some were set using hand made and hand labelled Vishay glass film resistors, each with a serial number and the value on them to 5 digits.For the ground test board I got tired of desoldering them and doing the setup every time I wanted more than basic functionality test, so took a small piece of veroboard and used superglue to attach to the board, then put 6 trimmer pots on it and wired them up to the appropriate points so I could use it easily. That the case no longer could close was not an issue on the ground test unit, I think any pilot would refuse to fly when his pre flight check turns up a large bright orange box in place of the expected matt black one normally there.On helicopters the answer to all those issues is Vibrachock anti vibration mounts, and tab washers on every thing that is not held with either locktite, self locking nylock plates or a locking wire. otherwise it is only there for a short time till it falls off. There even the sheet metal fatigues from vibration, and even the hard points in the floor can vibrate out.

author: calexanian
author_rank: Super Contributor
timestamp: October 21, 2015, 08:00:11 pm
content: As said before. The tighter spec is for the total resistive element. The physical wiper contact ads its own dynamics and series impedance. The more important question is why would somebody use a multi turn trimmer alone in such a sensitive circuit. typically you would use precision resistors on either side of a much smaller trimmer to greatly reduce the tempco and drift effects of larger value trimmers. If your trimmer only inhabits 10% of your divider range you are also dividing your drifts and variations by a factor of 10 as well assuming you are using bang on fixed resistors.