Correct way to hook up a potentiometer for low noise? - Page 1
Forum section: electronics / projects

author: RandallMcRee
author_rank: Frequent Contributor
timestamp: May 06, 2016, 09:53:38 pm
content: This is something I have not seen anywhere and have always been curious about.A potentiometer is a three-terminal device (we are just concerned with those--I know other types exist).Now my circuit calls for a variable resistor and a resistor is a two-terminal device. So typically I connect the wiper and either the CW or CCW terminal and call it a day. Job done?But...I see some schematics where the wiper is connected to the "other" terminal (the one I leave dangling). I can't see that electrically it would make a difference. Is there a reason for it? The only thing I can think of is that there might be lower noise in a sensitive circuit? Maybe there is a difference at the pot min/max?Can someone enlighten me?Thanks!Randy

author: Fungus
author_rank: Super Contributor
timestamp: May 06, 2016, 10:03:49 pm
content: The only thing I can think of is that the wiper can never be floating/open circuit.Might make a difference if the pot gets dirty+scratchy.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 06, 2016, 10:10:13 pm
content: It's standard practice to "rheostat-strap" a potentiometer, tying the unused end to the wiper, when used as a variable resistor. Thus, if the wiper becomes intermittent (possible for some wirewound types, or if it develops "scratchy" contacts), the resistor never becomes open circuit, but only opens to its maximum value.Tim

author: ajb
author_rank: Super Contributor
timestamp: May 06, 2016, 10:12:04 pm
content: Yes, a typical failure mode for a pot is for the wiper to go open circuit, either completely or intermittently as the pot gets dirty. If you only connect the wiper and one end of the pot to your circuit, then if the wiper loses contact then the whole pot is an open circuit as far as your circuit is concerned, which could cause that circuit to misbehave quite badly. By connecting the unused terminal to the wiper, then if the wiper goes open circuit your circuit still 'sees' the end-to-end resistance of the pot instead of an open circuit. That's generally less likely to instigate a magic smoke release, so it's generally a good idea.

author: m98
author_rank: Frequent Contributor
timestamp: May 06, 2016, 10:39:14 pm
content: I'd think that it's just a simple "eh, why not?" design decision. Maybe when it's old and scratchy, there might be a bit lower amplitude current noise while moving, because the max resistance you can get is the end-to-end resistance. And as ajb stated, it's also good practice for reliability. But when that's critical, you should probably think about adding a parallel resistor. Min/Max shouldn't be affected.

author: RandallMcRee
author_rank: Frequent Contributor
timestamp: May 06, 2016, 11:58:45 pm
content: Thanks!Very helpful. If someone had asked me, about this, and I felt "authoritarian" I could have mumbled something about intermittent wipers but have never experienced this myself and was never sure that this actually ever happens...Randy

author: nctnico
author_rank: Super Contributor
timestamp: May 07, 2016, 07:25:01 pm
content: When making a volume control I use a voltage controlled gain amplifier driven by the voltage from a potentiometer. Add a filter capacitor and it will take a lot of deterioration of the potmeter before causing audible problems.

author: bson
author_rank: Unknown
timestamp: May 07, 2016, 07:43:54 pm
content: Quote from: T3sl4co1l on May 06, 2016, 10:10:13 pm^ this, what he said - tie the open end to the wiper for an adjustable resistorIt's also worth mentioning that if the open end is grounded you get an adjustable voltage divider. This is generally what's desired for attenuation purposes.

author: moffy
author_rank: Super Contributor
timestamp: May 08, 2016, 11:59:06 pm
content: Ran across a Linear Technology application note, AN42 appendix C, which talks in depth about trimming procedures:http://cds.linear.com/docs/en/application-note/an42.pdf

author: AF6LJ
author_rank: Unknown
timestamp: May 09, 2016, 12:13:28 am
content: Good thread for young players, even a few who are not so young.It is mostly common sense when you think about it...

author: StillTrying
author_rank: Super Contributor
timestamp: May 09, 2016, 01:33:46 pm
content: I always try to connect the 'strapped end' to the lowest impedance part of the circuit. I imagine (at least) it picks up less noise that way.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 09, 2016, 04:30:00 pm
content: That's another good point, when you can arrange it. Examples: variable resistor to GND or VCC; variable divider (potentiometer).You might connect a resistor in series with the trimmer, where you need a variable resistor over only a certain range; putting that resistor on the "high resistance" (sensitive) side of the circuit allows the pot to be on the lower resistance (less sensitive) side.Same goes double for variable caps and inductors in tuned circuits, where the capacitance (and noise coupling) of an adjustment screwdriver can have profound effect!Tim