Electronics > Beginners

Curious about Potentiometers and Tuning coils use all 3 terminals


  Going through my (precious) Forrest Mims book, started getting curious, about motivation for using all 3 terminals in many common circuits...
In the case of a Tuning Coil, many circuits literally have a short-circuit negating a portion of coil, while adjusted position 'selects' the remaining coil turns.  Same idea for Potentiometers, usually keeping all 3 terminals in circuits.
   Am I missing that some measure of contact noise gets worse, when sliding motions cause near-open connection, or, worse, if middle slider (briefly) loses any contact ?
Otherwise, why not just use (most) circuits having only 2 connections, for controlling volume, or controlling tuner inductance.

Terry Bites:
A potentiometer is a 3 terminal device, a variable voltage divider rather than a single variable resistor. A pot can be connected as a variable resistor if you need one. Why make two types of product when one will do both? Its unusual to see a variable resistor out side of gain, time or frequency preset applications. The same three terminals on an inductor are there for the same reason.
Wiper noise is only a problem in old, dirty or cheap and nasty pots. Or the pots from hell... the slider pot. Tying one end to the wiper is a good idea, an open circuit is an entry point for noise and leakage currents.

Ask yourself what happens if the wiper, even briefly, fails to make contact in a 2-terminal application.  Like a 2-terminal pot in a variable power supply...What happens to the output voltage?  And can the connected device handle that situation?...Should it have to?


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