Capacitors in Parallel

Capacitors in parallel

Wire capacitors in parallel and the equivalent capacitance is just their sum: Ctotal = C₁ + C₂ + … + Cn. Each one sees the same voltage, which means the lowest voltage rating in the group sets the ceiling for the whole bank. The charges add up too, Qtotal = Q₁ + Q₂ + …, and the same goes for stored energy. This is how you get large capacitance out of cheaper, smaller parts, and it lets you blend different technologies to cover a wider frequency range. Example: a 100 µF aluminum electrolytic alongside a 220 µF one gives you 320 µF at the same voltage rating. Throw a 100 nF ceramic into the parallel mix and the bulk capacitance barely moves, but your high-frequency decoupling gets a lot better, since ceramics carry far less ESR and ESL than electrolytics do.

Applications: power supply filtering and power factor correction

You'll see this most often in power supply filtering. Parallel banks of electrolytic caps (bulk, low frequency) sit next to ceramics (decoupling, high frequency) to smooth ripple coming off the rectifiers and kill switching noise right beside each IC. In industry, power factor correction (PFC) banks placed in parallel with inductive motors cut the reactive power (kVAR) and keep the utility from charging penalties. Supercapacitors get paralleled to bank up a lot of energy at low voltage for backup duty and regenerative braking. Audio crossovers, snubbers and EMI filters lean on the same trick, stacking parallel capacitors of mixed values.

FAQ

Why mix electrolytic and ceramic in parallel? An electrolytic holds plenty of charge but reacts slowly thanks to its high ESR and ESL. A ceramic is tiny yet keeps up with MHz transients. Put them together and you've got the whole frequency range covered.

What about different voltage ratings? The lowest one wins. Pair a 10 V with a 50 V and you still can't safely go past 10 V.

Why do PFC banks use parallel and not series? Motors call for more reactive compensation, so the capacitance has to add rather than divide. Going parallel also leaves the full line voltage across each capacitor.

Is there a limit to how many I can parallel? Not electrically. The catch is inrush current at power-on, which climbs with total C, so large banks want a soft-start circuit.