• Gustav (PCB Grinder)

Pot, Switch or Knob?

Updated: Jun 22, 2019

When I get questions about DIY, the terms "pot", "switch" and "knob" often get mixed up, and I think a small blog post covering the basics will be helpfull.

The Pot

A standard potentiometer is basically a three legged resistor, which consists of a continious track of resistance, and a viper moving along that track.

That may sound a little abstract, but if we look at a schematic representation of it, it will probably make sense.

The zig-zag line represents the resistance, and the arrow represents the wiper, drawn in the center position. In a 3 pin round potentiometer, it looks like this internally.

If we assume the resistive track is linear and has a total resistance of 10.000 ohm, and we place the wiper in the center, the resistance from 1-2 will be 5.000 ohms, and the resistance from 2-3 will be 5.000 ohms. If we turn the pot counter clockwise, 1-2 will decrease, while 2-3 increases proportionally, and vice versa. The sum of 1,2 + 2,3 will always be identical to the sum of 1-3 (Trace the line from 1-3). Pots are used in most audio gear, and the faders on a mixing desk are really just a pot layed out in a line. They come in different configurations and values for different applications, and the "taper" of the pot dictates how the resistance varies on the travel. Getting into the utility of the different tapers is way beyond the intended scope of this post - suffice to say, we've got options!

A pot is always continuous, but you can buy pots with detents (I offer them for the GssL compressor project), and detents are mechanical stops. This can be as simple as a center detent for pots that needs to feel "settled" in the center position, but you can get pots with a lot more stops (I know of pots with 41 detents). They still have the continuous track and the wiper underneath the operation, though. They are not switches, just because they have stops. Now, the thing about pots is, they aren't very accurate, usually +/-20%, sometimes +/-5% (Yes, even on your expensive hardware units). For some applications, it doesn't really matter, but if we want to match two controls very carefully, we can either go through a batch of pots and select the two closets to each other, or we can use a switch instead! The Switch

The most basic form of a switch is a mechanical device that can make a connection, or avoid making a connection. This is a schematic representation of the simplest switch configuration, called an SPST (Single pole, single throw)

And this is what it looks like in the form of a toggle switch (notice:just two terminals).

To make an SPDT (single point, double throw), to switch between two active connections, we need to add another terminal....Now we can make connection 1, or make connection 2.

Using a switch instead of a pot With an SPDT switch, we can already replace a potentiometer with a switch, so lets use our earlier, imagined 10.000 ohms potentiometer as an example.

We can replace it with an SPDT switch, and a 10.000 ohms resistor, if we solder the resistor across like this!

This corresponds to two settings on the 10.000 ohms linear potentiometer - fully clockwise and fully counterclockwise. One setting has 10.000 ohms on 1-2 and 0 ohms on 2-3, while the other has 10.000 ohms on 2-3, and 0 ohms on 1-2. This would be a pretty lame replacement in most cases, since the resolution is so bad, but fortunately, we can get rotary switches with much more steps, so all we have to do is figure out which values we want available on our steps. The principle is exactly same, no matter how many steps, or different connections, we spread it out on. Use a Pot or a Switch? Potentiometers are relatively cheap, they take seconds to to install, and they have an infinite amount of settings from end to end, but pots are inaccurate, nearly impossible to match, and pretty though to recall to bring back a previous setting, Switches can be perfectly matched, and recalled, but they are relatively expensive, they take much longer to set up and install, and they have a limited amount of settings within a given range. Knobs? Knobs are what you put on the shaft of rotary switches and pots to grab on to them :)


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