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Pot confusion – the electrical type

Posted by is9582 on March 7, 2015 with 2 Commentsas , , , , , , , , , , , ,

I tweeted earlier that I’d write an article today, relating to this subject. This is for anyone that might need to replace a potentiometer in a circuit, or is building an electric guitar. I thought about writing this, as I finally finished wiring my bass this morning, and at first I couldn’t recall which of the connectors on the pot went to which wire. After I thought about it for a moment, and had my coffee, it came back. But, what if someone else doesn’t have any prior electronics training or experience?

Most pots have three lugs, to which wires or other components can be attached, and then controlled by rotating the shaft of the pot.

The same pot, showing the three lugs, with no probes. The lead to the left, in the photo, I bent from it's original position, like the other two.

The same pot, showing the three lugs, with no probes. The lead to the left, in the photo, I bent from it’s original position, like the other two.

Many times a pot will be used as a volume or tone control, especially when talking about electric guitars and basses. I’ll provide a very basic explanation as to what is going on inside the pot, which allows this to “control” the volume. When the shaft of the pot is rotated, a wiper inside is moved in a section of a circle. The material the wiper rubs against inside is usually a path of carbon, which behaves much like a carbon resistor, and when made a specific amount of carbon is placed in the track. This allows them to create a specific resistance, for certain uses.

The wire from the guitar/bass pickup attaches to the center lug, and one of the end lugs is grounded, while the other end lug connects to the output jack. You can use a multimeter to determine which end lug you should connect to the output, depending on how you use the pot. I’ll walk you through this test.

First, turn the shaft of your pot in the direction you usually turn it, in order to raise the volume, and turn it until it stops. For most audio equipment, and instruments, this would be turning the shaft clockwise. This is what I did in my example.

Now, with your multimeter turned to it’s resistance mode, place one of the probes’ metal tips into (or against) the lug on the right end, and the other probes’ tip into the center lug.

Shaft of pot rotated clockwise until it stopped, then holding multimeter leads on center and right lug.

Shaft of pot rotated clockwise until it stopped, then holding multimeter leads on center and right lug.

When you look at your meter’s screen, you will see how much resistance is between the two tested lugs. The results from my first test are shown below.

With the multimeter set to Ohms (resistance), it reads .4 ohms, which is extremely low.

With the multimeter set to Ohms (resistance), it reads .4 ohms, which is extremely low.

Next, remove the probe from the right end, leaving the center probe in place, and place the moved probes’ tip into the left lug.

Shaft of pot rotated clockwise until it stopped, then holding multimeter leads on center and left lug.

Shaft of pot rotated clockwise until it stopped, then holding multimeter leads on center and left lug.

The results from my second test are shown below.

With the multimeter still set to Ohms (resistance), it reads 416.7K ohms, and this is a 500K ohm pot.

With the multimeter still set to Ohms (resistance), it reads 416.7K ohms, and this is a 500K ohm pot.

If you think about how the wiper would move, inside the pot’s case, you can visualize where it is when you reach the end of the shaft’s travel. Whichever outside lug the wiper is closest to, when the shaft stops turning, will have a very low resistance value. This is because there is very little resistance material between the wiper and that outside lug. Hopefully this makes sense. This is exactly why the results in my above tests, turned out like they did.

While I’m talking about some of the things relating to the bass’s electronics, and basic wiring, I’ll also share a few tools I use that are helpful. Don’t worry, I won’t go into great depth at this point, but may in the future if anyone is interested.

One of the really useful tools I own is a wire stripper, that is somewhat automatic, in the sense that it can handle a range of wire thickness, without the user needing to do anything extra to have it do it’s job. On some strippers, they have small half-circles in the blade section, that only fit a very small range of wires. You have to choose the half-circle that is the right size for whatever wire you need to strip, and hopefully there is one that will work. This may show just how nice the stripper I have can make this work. I might have three or four different wire sizes in a project, and not worrying about anything other than just squeezing the handles, for it to do its job, is great.

Automatic wire stripper, used to remove insulation a specific distance from it's end, prior to soldering.

Automatic wire stripper, used to remove insulation a specific distance from it’s end, prior to soldering.

This stripper also has a “stop” against which, you hold the end of the wire, so the amount of insulation removed is the same each time. This stop can be moved to a range of distances, that the manufacturer thinks are most useful, when working on electronics. I may have needed a slightly longer uninsulated section once or twice, but it was easy enough to make this work, too. Below you can see the blue wire has the insulation from both ends already removed.

Blue wire in background has the insulation from both ends already removed with the stripper.

Blue wire in background has the insulation from both ends already removed with the stripper.

Another useful tool is a third hand. Actually, this is what I call it, but I honestly can’t recall what it was called, when I bought it. Basically its a hefty stand that has two small clamps, with which you can hold a wire or components, when soldering or performing an operation that is difficult with just two hands. The photo below shows one of the clamps grasping a metal lead on the output jack for my bass. The red arrow points to the “hot” lead, or in other terms, the lead that will have the signal carried on it.

Output jack, held by the stereo lead, so I can solder a wire to the

Output jack, held by the stereo lead, so I can solder a wire to the “hot” lead.

 

Perhaps this will whet some appetites, and hopefully also answer a question or two that may have led some to have others work on their gear, when understanding the basic layout might allow you to do the work yourself.

Thank you for checking out the article and as always, let me know if you have any questions or comments. I’ll be glad to answer or help, if I can.

 

Lee Laird