Turning

Subject:
replacing the pot

John K Jordan
Understand that potentiometers have more specs than just the resistance value in ohms (10K, etc). Some have a linear taper (meaning half-way through the turn a 10K pot will read 5K) and some have a non-linear, e.g., logarithmic taper. Some have a carbon film, others could be ceramic or wire wound to handle more current. (A wire-wound pot will feel different when turning.) If you replace it with the wrong type it might not work the same (i.e., could change the speed too slowly or too fast on one end of the rotation) or could even burn up from too much current. A pot intended for audio control may be different from one in a grinder. Note that I have never looked at what type of pot is in such a grinder - the type used depends largely on the speed control circuit in the device.

With no markings I'd probably take it apart and analyze it - usually there are metal tabs you can pry up to remove the cover. Once open you can easily see if it is wire-wound, verify the taper, and might even be able to see what the problem is, for example, the resistive element might be cracked or burned where riveted at one end. Either way, you can probably measure the resistance of what is left and determine the resistance of the original pot.

I agree that the problem is most likely a bad pot. Keep in mind that it could be something else in the circuit. If the disconnected pot reads a smoothly increasing/decreasing resistance when measured from the center terminal to both of the outer terminals, the pot is probably OK.

Note that a replacement pot (of the correct specs) does not have to be the same physical size as long as it will fit in the space and fit through the hole.

You mentioned you like the lower speed. Until you find a replacement pot one simple thing you can do is substitute a fixed resistor (or a pair, if the circuit was connected to all three terminals), picking the resistance values to get the fixed speed you want.

If unfamiliar with such things, someone with experience could test and fix this easily (assuming the problem is in fact the potentiometer). If they are like me, they will have spare parts on hand and be glad to help. To find such a person, one place to check is with a local Maker's club.

JKJ

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