How Do You Arrange the 3 Legs of a Trimming Resistor? Where Does the 3rd Go?

[Potomac]

Another beginner question regarding trimmer resistors.  I'm wondering what to do with the 3rd lead of a trimmer.

I've been playing around with trimmer in series with an analog sensor (Omron EE-SX1140).   

I have two of the sensors on a perfboard.  I snipped off one of the fixed resistors I had in front of one of the sensors previously, and soldered in two of the leads of the trimmer.  I had tested those two leads and found that they gave me the resistance values I wanted (wiping up and down around 218 Ohms)

Now I'm wondering what to do with the seemingly non-essential 3rd leads that I have bent off to the side and unconnected?  What is the purpose of the 3 leads typically?

I attached a photo showing what I mean

[David_AVD]

The unused end pin can be connected to the wiper pin if you want the resistance not to rise above the full pot value if the wiper contact (inside) were to ever go open circuit.

[rstofer]

I don't know that I would cut it off but when the device is being used as a variable resistor (as a rheostat, not a voltage divider), the third leg is often redundant.

https://en.wikipedia.org/wiki/Potentiometer

[Potomac]

Thanks for the quick reply.

David, which pin is the wiper pin? Is it the middle pin?

Rstofer, I am definitely using it as a rhetostat, not a voltage divider.  I'll read you can just solder it to the middle pin rather than having it bent to the side.  Does that sound like a best practice?

[rstofer]

I have often seen the center leg connected to one end of the resistor.  I agree with David, this would be the way to go should the wiper run off the end.  It costs nothing to do it so why not?

You can find the wiper with an ohmmeter.  Between the two end points, the resistance will stay the same regardless of the wiper position.  So, after identifying the ends, the only lead left is the wiper.  It would be shown on the body of the resistor but maybe it isn't...

[Lee Leduc]

Wiper is the middle pin.

[Potomac]

Thanks Lee

So Could I just solder the left pin to the middle pin to clean things up?

[rs20]

So Could I just solder the left pin to the middle pin to clean things up?

Yes, solder the middle pin to either the left or the right pin. Which side you choose determines whether clockwise means an increase, or a decrease, in resistance.

[Dave]

I have often seen the center leg connected to one end of the resistor.  I agree with David, this would be the way to go should the wiper run off the end.

That is not going to happen. The real problem is the resistive coating tends to wear out over time and the wiper momentarily loses contact from time to time as it slides along the coating. If you have just two leads of a potentiometer wired, it means they are going to go open circuit for a moment and you could lose something like feedback in a control loop, which would mean that the circuit could do something unexpected and potentially destroy something. If you connect the wiper to one end of the resistor, the resistance will just jump to the end-to-end resistance instead of open circuit.

This used to be a much bigger problem back in the day when graphite was the resistive compound of choice for potentiometers. The resistive surface was much rougher and little grains of carbon would slowly start to break loose and cause problems. See image below to get the idea.