Reading Trimmer Pots
by Bob Roberts
I mention this several other places, but I think it's time to give it a brief page of it's own. "How do I know what value my pot is? ", is the Q most often asked. Short of pulling it from the circuit & measuring it with a meter, the numbers on it can be read with ease....|
Pots can be read by taking the first two numbers and adding the third number as the amount of zeros after it, e.g., 103 would be 10 with 3 zeros after it or 10,000 ohms which when shortened to the Greek abreviation K for kilo which translates to 1,000, would be 10Kohms.
Hence, 201 would be 200 ohms & 202 would be 2000 or 2Kohms. Now, with the advent of much smaller parts with less room for stamping these numbers, they've shortened this same system, so that now you may find numbers like "23" representing 2Kohms... 2 & three zeros or 2000 ohms. A pot value of 200,000 ohms, 200Kohms, would be expressed as "25", 2 with 00000 for 200,000 ohms.
In short, your pot may have the value written out, 200K ohms, "204", "25" or perhaps nothing at all in which case you would have to remove it from the circuit & measure it with a meter to the 2 outside legs. The reason for this removal is to get an accurate reading of the pot since in-circuit it may parallel other components or have the center tap shorted to one of the 2 outside legs. If the pot in Q is mounted on a remote board that is disconnected from the rest of the circuit, but you can see it has one leg shorted to an outside leg, it is possible to get a rough reading in-circuit by rotating the pot from it's max to min, or vice versa, while reading with the meter. Depending on how it is mounted your reading will vary from zero to the rough value at the other extreme.