# B Capacitance question

1. Oct 5, 2016

### RED119

So I read a page where someone said they had put 18 400v 3900uF capacitors in parallel and that this came out to 5600 joules... Now I'm sure I am doing something wrong but when I did it out I only got 28.08 joules...
I used this website: http://www.rapidtables.com/calc/electric/Volt_to_Joule_Calculator.htm
plugged 400 in for volts and 0.0702 in for coulombs, if someone can let me know what I am doing wrong here please let me know.

Second little question, i know if you put capacitors in parallel you just add the Farads, but if they all have different volts, say one has 20v 3000F, the other has 400v 90uF, what would that end up being in parallel?

2. Oct 5, 2016

### lewando

5600 J looks like the correct answer. Use: energy = CV2/2.

If you put the capacitors in parallel, by definition they will all assume a single voltage across all connected capacitors. The parallel capacitance is independent of the voltage.
 Sorry--if you mean what is the resulting voltage, you have to do a charge analysis.

Last edited: Oct 5, 2016
3. Oct 5, 2016

### RED119

Thanks so much for the reply, is the C in that equation coulombs or capacitance?

4. Oct 5, 2016

### lewando

Welcome. C is capacitance. 'q' is usually represents the charge.

5. Oct 5, 2016

### davenn

hi ya'welcome to PF :)

just remember tho, you cannot mix voltage values ... that is, the applied voltage to the capacitors must not exceed the lowest rated capacitor voltage value

eg. as in your example of a 20V and a 400V cap, the circuit voltage shouldn't exceed 20V else that lower voltage rated capacitor will fail

but having a 50V, 100V etc rated cap in a 20V circuit isn't a problem

Dave

6. Oct 9, 2016

### Pete_L

The voltage rating of a capacitor is the voltage that the capacitor can withstand before being destroyed. At this moment I forget whether that is peak or RMS voltage, but it is general practice to overrate capacitors in a circuit. So if one capacitor in a network of paralleled capacitors is rated 20 volts, you can't apply voltage greater than 20 volts without risking damage to the one rated at 20V.