# Substituting capacitors

I have a nicely-spec'd wireless router that doesn't work any more. (Sorry, I don't have a schematic, but it's an Asus RT-N16.) I popped it open and found that it has one capacitor with a bulging top. It's labeled 16V 680uf, but I don't have a capacitor with those exact specs. But as long as I find another capacitor with roughly the same capacitance, I should be able to substitute that, right?

Q=CV
Q=16v*680uf
Q=10880C


I have a ton of random junk that has various capacitors with other specs. For example, one is labeled 35v 470uf.

So, suppose I want to substitute this 35v capacitor.

C=Q/V
C=10880C/35v
C=311uf


Since the proposed substitute capacitor's 470uf > 311uf, is it likely that this substitution will work, or is it the case that I'm oversimplifying this and need to find something with closer specs printed on the side?

## 2 Answers

The voltage is the maximum the capacitor can withstand before the dielectric breaks down, and does not factor into the capacitance (measured in farads, $F$). Use a capacitor with the same capacitance, and the same or higher voltage rating.

Also, $Q$ is charge, and although it is the product of voltage and capacitance, you must use the applied voltage, not the rated.

• I figured it couldn't be so simple...and I guess I was mixing up my terms. Thanks! I'll accept your answer as soon as the site lets me. – rob Oct 26 '13 at 3:25
• @rob, also most of the time you can use a slightly bigger cap if you can't get the exact capacitance. Just make sure the ESR is similar to the original cap. – Pentium100 Oct 26 '13 at 6:15
• It depends on what the cap is used for though. Supply caps can be increased a certain amount without too much harm, but bypass, filter, and oscillation caps almost always have to have the same capacitance. – Ignacio Vazquez-Abrams Oct 26 '13 at 6:22
• Good to know; thanks for the follow-ups! – rob Oct 28 '13 at 3:32

There's more to it than just capacitance and Vmax rating. If your replacement cap has higher ESR (resistance), it will soon overheat and blow up. The same will happen if you pick a cap with a significantly higher capacitance, since increasing capacitance usually increases ripple currents.

I suggest you go for as close of a match as possible. If you will order a new cap, make sure it's from the same manufacturer and product line. If not, watch out for caps which are significantly smaller than the one you're about to replace: those usually have higher ESR and ironically can't dissipate as much heat due to their size.