Capacitance and Voltage rating

[jfdo]

Hi All,

I have a technical question about selecting voltage ratings for a capacitor.

e.g. 1 Let's say I need to use a 2uF cap in a 5V circuit.
As I have understood it is a common practice to use twice the supply voltage as for the cap voltage rating for safety (to avoid blowing up the cap). 
That means for my 5V circuit I should use 2uF cap with 10V rated voltage.

With the cap equation, Q = C  x V , where Q = charge (in Coulomb), C = capacitance (Farads), V = voltage (Volts)

1) 2uF cap with 5V ratings will have maximum charge of,

 Q = 2*10^(-6) * 5 = 10 * 10^(-6)  Coulombs

2) 2uF cap with 10V ratings will have maximum charge of,

Q = 2*10^(-6) * 10 = 20 * 10^(-6)  Coulombs

Now if I use 2uF, 10V cap in the 5 V circuit it will have maximum capacitance of

 C = Q/V = 20* 10^(-6) / 5 = 4 * 10^(-6) = 4 uF. Now capacitance value has been change from 2uF to 4uF. 

In this case, if I want to double the voltage rating (for safety) of a cap, should I halve the capacitance in order to have the same capacitance (2uF)? 

Thanks for any comments.

[attila]

Hi @jfdo

A 2uF capacitor rated to 10V has 2uF capacitances also at 5V.

It also important at what frequency you are using the capacitor. Electrolytic capacitors have high ESR, so at high frequencies the series resistance of these become significant. 
This 1uF capacitor at 200kHz behaves like a 0.265uF capacitor, or 0.8uF with a 1.4Ohm series resistor: