ELECTRONICS

Selecting capacitors with toggle switches Rick Viola 2-18-2011

These are a few examples of how to switch capacitors in a circuit to select different values. This can be handy in audio circuits that require selection of high and low pass points rather than using potentiometers. In some situations, a few positions may be all you need.

Figures A and B illustrate how you might select two capacitors with a SPST (single pole, single throw) toggle switch by adding a second cap in parallel. Since capacitors add in parallel the value is increased to the sum of the two values you choose.

Figure C illustrates switching in a capacitor in series. The total capacitance of two in series is calculated by the product divided by the sum. If both values are the same, the total is one half the value.

Figure D is an example with a SPDT (single pole, double throw) switch. This would select any two values of choice easily.

Figures E and F are examples with toggles that have a center off position. The DPDT (double pole, double throw) switch is handy to have a place to physically mount the capacitors.

Notice however, no matter what values you choose, you cannot wire a 3 position switch to yield 3 values that increase sequentially. This may not be a problem, but it would be more user friendly if for example in a filter circuit, the toggle could be labeled low, medium then high.

I had once seen a switch in a catalog called a SP3T (single pole, triple throw). These are sometimes referred to as DP3T or just as "on-on-on". Figure G shows the contacts in the three positions.

Normally a jumper is added by the end user for the purpose of having one input and 3 outputs.
Well, I thought that was very interesting. Especially since they did not cost more than a DPDT, (or just a little more)

Figure H shows how you would normally use a SP3T switch.

So, as in figure J, you could wire 3 caps to the 3 output paths and Voilà! You can have any cap value you want for any switch position.

Of coarse, I could not stop there. I knew there had to be something more. After a little thought. I came up with the idea as in figure K. This uses 3 caps, all the same value. It sequences from low, medium to high value. All in parallel, to just the bottom cap, to all in series.
I like it. you just have to buy one value and you get a range from 1/3 the value, to one times the value to 3 times the value.