Effective voltage rating of paralleled capacitors

[powersys]

Say I have 2 pcs of 50V, 10uF capacitor. We know that when both capacitors are connected in parallel, the effective capacitance of the paralleled capacitors will be 20uF. What's about the effective voltage rating of the paralleled capacitors? Still 50V?

What will be the effective voltage rating if both the capacitors are connected in series?

Thanks.

 

[Kral]

powersys,
The effective voltage rating for multiple capacitors in parallel is equal to the lowest voltage rating. In your example, the ratings are both = to 50V, so the rating of the combo is 50V. If you had a 50V and a 20V capacitor, the rating of the combo would be 20V. The capacitance values do not enter into the determination of voltage rating
Regards,
Kral

 

[newelltech]

When you put multiple capacitors in series, the voltage across them divides according the the impedance - just like for resistors.
If you put your 2 capacitors (50V, 10uF) in series, ideally the network will have a capacitance of 5 uF and a voltage rating of 100 V. Ideally.

BUT, this is bad practice because the voltage distribution will change depending on the capacitance. For example, if the capacitor tolerance is +/- 25%, then one capacitor could be 7.5 uF and the other could be 12.5 uF. Now your network has a capacitance of 4.68 uF and if you apply 100 V across the network, the 12.5 uF capacitor will get 37.5 volts and the 7.5 uF capacitor will get 62.5V. So in this case you really have to use capacitors with a 62.5 V rating for an effective 100 V network rating.

 

[lambtron]

newelltech makes a valid point about uneven voltage distribution across series capacitors.

Another important issue that has not been mentioned is that the "50V" voltage rating probably must be derated if the circuit will operate across the full operating temperature range, and so each cap may in fact have an effective voltage rating well below the nominal 50V specification.