Rheostat vs Potentiometer Connections

[Steve Mavronis]

I'm in the process of checking a PCB layout I'm getting ready to etch for a compressor clone. I have the sensitivity (compression) control attached to a rheostat configuration, with a transitor connected to pin 3 and the IC through a resistor connected to pin 2 of the rheostat. But in my PCB layout I also have pin 2 connected to pin 1, which normally floats free as in the schematic. The reason is because of routing a trace I have no room to connect directly to pin 2. But in a rheostat configuration isn't essentially pin 2 and pin 1 the same connection anyway if tied together as far as pin 3 is concerned in creating variable resistance with the wiper action? I just want to make sure to know if it's still valid having rheostat pins 1 and 2 tied together to proceed and etch my PCB. Thanks....

[PRR]

> isn't essentially pin 2 and pin 1 the same connection anyway?

No.

Pin 3 to pin 2 is variable zero to 1K.

Pin 3 to pin 1 is fixed 1K.

Move the trace.

[Steve Mavronis]

I don't understand. If pin 1 is left unconnected, what is the difference between the wiper pin 2 and pin 3 in the "normal pin 1 open" configuration vs having pin 1 and 2 connected the whole time as the wiper moves towards pin 3? I mean pin 1 isn't grounded completing any circuit anyways, so it should be at the same potential as pin 2 connected to pin 1 or not.

My way seems to be in agreement with The Trimmer Resistor section on this Beavis Audio page: http://www.beavisaudio.com/techpages/Pots/

Here we have a potentiometer where lug three is the input, and lugs 1 and 2 are connected together to form the output. As you turn the shaft, the resistance decreases. You are forming a simple resistor whose value is variable.



By attaching lug 1 to lug 2, we are building in a "fail-safe." This ensures that the circuit is never completely open—there will always be some resistive path in case the wiper goes south.

Might not be able to move the trace positioning. It's too tight. I'd probably just not solder the pin 1 leg of the PCB mounted pot if you are right. The pot being used is 500K reverse log by the way as in the original MXR Dyna Comp.

[PRR]

> " in my PCB layout I also have pin 2 connected to pin 1"

I did not see that the first time.