Series Capacitance

I was thinking about putting caps in series to get a smaller, less available capacitance.... but it seems like it might not be a good idea. If you put two capacitors in series, the time constant is doubled (or increased by some ratio?), even though the total capacitance is (C1+C2/(C1xC2)) ?


Thanks Guys

> putting caps in series to get a smaller, less available capacitance.... but it seems like it might not be a good idea. If you put two capacitors in series, the time constant is doubled

No, halved.

You can buy 3pFd caps. How small do you need? But yes, if you want 0.1uFd and have a bunch of 0.2uFd, use two in series to get 0.1uFd.

Thank youfor clearing that up PRR... I did do some more looking and found the 10pf polystyrene I was after at farnell, not one of my usual suppliers. But, I am glad I asked because now I know just a little bit more.


Thanks again.

And the general formula for the equivalent series C for two caps is the inverse of the formula you posted. For more than two in series get the total of the reciprocals of each cap and then take the inverse of that.

Another trick, if you are using ceramic disks, is just to clip off or grind off some of the cap until you get the desired value. This is not recommended for anything but experimenting, since the conductive material is exposed and the metal may be close to shorting. As well you may induce other fractures than the one you want and have some nasty aging effects. You can examine the clipped or ground edge with a loupe or microscope and clean it, then seal with some material or other, if the operation is a success and you want it to last.

I've used this in a pinch when a few tenths of a picofarad out of 3 or 4 were important.

At the heart of a cooled charge preamp I did circa 1982 I had a wire in teflon tubing running alongside the JFET input gate lead for a short distance. This was the feedback C that set the charge-to-voltage conversion gain and was 130fF. It was an interesting challenge to calibrate this.

Bcarso, thanks for the info, and the ceramic tip. So total series capactiance = 1/((1/C1)+(1/C2), etc..).

Thanks.