variable capacitor substitution?

[Mdkanz]

would this work as a variable capacitor? because when the variable resistor hits 0ohms it would short out the capacitor and cause it to be zero uf ridght? and when the resistor is at 10k ohms it would limit the capacitor how many pf would it be at 10k ohms?

 

[audioguru]

Your circuit does not show a variable capacitor. It shows a small capacitor that is variably shorted with a variable resistor.

Why not use a variable capacitor that is 100pF max?

 

[Mdkanz]

I was just wondering if this would work because i have variable resistors but i can't find any variable capacitors.

 

[audioguru]

Shorting a little capacitor with a variable resistor does not make a variable capacitor.

 

[unclejed613]

i guess that with newer electronic tuning methods, variable caps are not common as dirt anymore...

you can make a variable cap with two pieces of single-sided circuit board material and some method of pivoting one of them.

you can find a lot of them in old radio equipment. 100pf wasn't an uncommon value in equipment designed for 2-10Mhz or so. or you can order variable caps from Digi-key or Newark (Farnell's in Europe)

 

[kchriste]

The only way to make a resistive POT work as a variable cap of sorts is to pair it with a varactor diode. That may not work in all circuits either. Could you post a schematic of the circuit you'll be using the 100pF variable cap in?

 

[Hero999]

A varactor also needs a high enough bias voltage to get the desired capacitance. I thought of suggesting a small 12V battery (the type smaller than an AAA and often found in key fob transmitters) with a high value pot. 1M but then I realised the value would change as the battery runs down which might not be a problem if it's designed to be adjusted frequently.

Is it a trimmer?

You could connect different value capacitors in parallel and a DIP switch to connect ans disconnect them. For example if you permanently connect a 15pF capacitor in parallel with 47pf, 22pF, 10pF, 4.7pF and 2.2pF, each in series with a switch you'll have a 15pF to 100pF variable capacitor with roughly 2.2pF steps, even better you could replace the 2,2pF capacitor with a gimmick capacitor which will give a smoother adjustment. I'll post a schematic if you're interested.