Capacitor blend pot - calculate capacitance?

[Frank1974]

I have a 100k (measures 112k) linear pot with .001uf and 10uf caps that are blended in parallel.  I've used it for finding an input cap value I like.

How do I now work out the capacitance value?

I think I need to measure the resistance value to work out the % of rotation to get the ratio between the 2 values.  But I'm just confusing myself.  There must be a simple(ish) equation to work this out.

Hope someone can help.
Thanks.

[Chill]

Meter the capacitance from the blend pot wiper to the opposite sides of the caps.  There's probably a mathy way to do it as well, but why bother when you can do it the easy way?   

[Frank1974]

If only I had a meter that could test capacitance.  My meter only does ac/dc voltage, resistance, hfe, and diode/continuity.  I don't know a way of working out the capacitance with this, although I'm sure there is a way.

Thanks.

[George Giblet]

You can't work out an equaivalent capacitance with only the values of the two caps and the resistor.  The blending circuit is no longer a capacitor it is a more complicated circuit - it has three parts.     You can come up with a number for the equivalent capacitance but the value depends on the rest of the circuit.   If you put a single capacitor of that value in place of your blend cap circuit it won't behave *exactly* the same only sort of - sort of might not be good enough.

A capacitance meter will probably show a value but it won't necessarily be representive of the capacitor value you want.

Now for your circuit, there's such a massive difference in capacitor values I suspect you will actually need the two caps and the resistor to get the sound you want.  Alternative you might be able to just rip out one of the caps.  I'm not sure what circuit you are using to blend the cap.  Can you post the blend circuit and the circuit you are using?  Hard to comment without both pieces of info.

[Solidhex]

If you're going to be using a single cap in the end you might as well just socket it and swap till you find what you like...

--Brad

[Frank1974]

Thanks for the info.  Heres the schematic.  Its a dual gang pot with the caps connected between the outside lugs on the pots, the wiper(s) used as a capacitor output.  I was using it in a fuzz face circuit instead of the input cap.


I was fed up of having to keep swapping caps everytime I make a new project, and was looking for a way to quickly dial in the capacitance whilst playing, and then be able to work out the value.  I'm now going to make a little daughter board with a dip switch to select between cap values instead.  I've used this for a sweep switch in a wah layout I made before.

4 caps in parallel added to 4 caps in series.  I'll just change the cap values to be able to go upto roughly 2uF.  Maybe have two 8-way dips instead.

It will be a handy little tool to keep for finding cap values in new projects.

[birt]

just use a box with a rotary switch and 2 alligator clips?

[Frank1974]

The dip switch allows for a lot more possible values than a rotary and its cheaper too.

For example the dip above can get the values (uF),
0.0025, 0.0033, 0.0047, 0.0050, 0.0072, 0.0080, 0.0097, 0.01
0.0125, 0.0133, 0.0147, 0.0150, 0.0172, 0.0180, 0.0197, 0.020, 0.022
0.0245, 0.0253, 0.0267, 0.0270, 0.0292, 0.0300, 0.0317, 0.032, etc etc etc upto possible 0.0937.