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Haru said:How do you connect a variable resistor?...It has 3 pins, I only want to use 2 pins
eblc1388 said:Hi Dr. EM,
Apart from the reasons that Audioguru and others had said, connecting the third pin would effectively put the wiper contact resistance in parallel with the unused portion of the trimmer thus helps to lower the contact resistance and improves circuit reliability even when your trimmer is new.
Nigel Goodwin said:The 'improvement' is going to be EXTREMELY negligible, and won't make the slightest bit of difference.
Nigel Goodwin said:This 'may' be an advantage?, or it could even be a disadvantage?, depending entirely on it's exact use, and how you would like it to fail in case of a preset failure.
Nigel Goodwin said:Commercially you see both methods used, presumably based on what they want to happen?.
eblc1388 said:Nigel Goodwin said:The 'improvement' is going to be EXTREMELY negligible, and won't make the slightest bit of difference.
Well I tested one trimmer in my circuit and there is a 2mV difference(out of 2V) across the trimmer with and without the third pin shorted to the wiper.
I don't know about the circuits you normally use but I do care about such improvement, especially when they are recommended by trimmer manufacturers and does not cost me an extra penny.
Nigel Goodwin said:This 'may' be an advantage?, or it could even be a disadvantage?, depending entirely on it's exact use, and how you would like it to fail in case of a preset failure.
An advantage to fail to open circuit? Strange argument.
Nigel Goodwin said:Commercially you see both methods used, presumably based on what they want to happen?.
I'm not surprised. Even the trimmer manufacturer remarked that 50% of users do not aware of the reasons of connecting up the third pin, that's why they specially mentioned it in their literature.
Nigel Goodwin said:That's a 0.1% difference! - do you thing that's not negligible?.
Nigel Goodwin said:For that matter, would it make any difference anyway?.
Nigel Goodwin said:I really see no improvement in the figures you mentioned, can YOU adjust a preset to 0.1%? - a pretty pointless exercise, particularly when the spec on the track is probably +/-20%!.
Nigel Goodwin said:Going completely O/C could make the circuit 'fail safe', or if not that, at least fail so obviously you know something is wrong.
Failing so it's just out of adjustment could cause damage, as the fault could then continue for extended periods.
Nigel Goodwin said:I've never said not to do it, only to be aware of the reasons for doing so, and not just doing so for no good reason.
eblc1388 said:Nigel Goodwin said:That's a 0.1% difference! - do you thing that's not negligible?.
To you yes, to me no.
Nigel Goodwin said:For that matter, would it make any difference anyway?.
For PIC and general applications, no. For metering and instrumentation, yes.
Nigel Goodwin said:I really see no improvement in the figures you mentioned, can YOU adjust a preset to 0.1%? - a pretty pointless exercise, particularly when the spec on the track is probably +/-20%!.
You have mixed up adjustability and device tolerance. These are two different things. An easy mistake for many people to make.