I Went To A Hamfest And Bought Pot(s)

Potentiometers for crying out loud…what the heck did you think I was talking about?!? And don’t get any ideas that you caught me here. I am a well-established straight-edged teetotaler. I came back from NEAR-Fest at the Deerfield Fairgrounds in New Hampshire two weekends ago with a box of variable resistors that only cost me five bucks…for more than 80 of them!

So I am spending the evening listening to the city sounds on my Bearcat scanner, and testing and sorting these little knobs in the hopes that some day they will make cool little gadgets like the Fuzz Face.

The variable resistor, i.e. potentiometer, consists of a spinnable knob attached to a wiper that moves in a circle across a resistive element. There are electrical terminals on each end of the resistive element, called “ends”, with a clockwise end and and counterclockwise end. You can see a broken potentiometer in the two pictures below. Twirling the knob around will spin the wiper, and thus change the resistance.

I am testing the potentiometer by measuring resistance with a digital multimeter, first taking the resistance measurement with the probes on each end terminal. This measurement will give the maximum resistance of the pot. Next I am checking to make sure the wiper works by testing the resistance between one end and the wiper terminal with the knob spun entirely in one direction (either reading 0 ohms or the maximum resistance of the pot), and then again when spun entirely in the opposite direction (and reading the opposite measurement — either 0 ohms or maximum resistance of the pot). Next I am taking a resistance measurement by probing one end terminal and the wiper terminal when the wiper is at the center point of the rotation of the knob. If the measured resistance falls halfway between 0 ohms and the maximum resistance, then the potentiometer most likely has a linear taper. A linear taper simply means that the resistance of the potentiometer increases linearly with the incremental spinning of the wiper. You can take resistance measurements at the quarter point and three-quarters point of the rotation to be certain it is indeed a linear increase (or decrease) in resistance.

On the other hand, if the resistance measurement at the center point is much larger or smaller than what would be expected for a linear taper, the potentiometer has a logarithmic taper. This is also called an audio taper, due to the fact that the human ear does not perceive changes in volume linearly. Additional resistance measurements will show that the resistance increases (or decreases) gradually for the first half or so of the complete knob rotation, and then increases (or decreases) precipitously.

Well, there you have it. A ragingly exciting night of DMMing and pots…

Best,

KM1NDY