How to Choose a Rectifier Capacitor

By Alexander Rudinski

i Hemera Technologies/ Images

A full-wave bridge rectifier uses diodes to convert the negative portion of alternating current, called AC, to positive. This creates an approximation of direct current, called DC, that can be used by electronic devices. Because AC current comes out of household outlets, rectifiers are required for most electronic devices. By pairing a capacitor with a full-wave rectifier, the current produced by the rectifier is filtered into a cleaner version of DC that is more effective and efficient. Choosing the correct capacitor requires calculating the total amount of capacitance, expressed in microfarards, that the capacitor needs.

Determine the output voltage of the secondary winding your transformer. This value, given in volts, is provided on the data sheet that accompanies the transformer. For this example, we will use 18 volts.

Subtract 1.4 volts from this value to determine the total output voltage of the bridge rectifier. In this example, that value is 18v - 1.4v = 16.6v.

Multiply this value by the frequency of the transformer's secondary winding. In most cases, this is 60 Hz. In this example, the value is 16.6 x 60 = 996.

Multiply the total current, expressed in amps, required by your load by 5. For example, if your circuit requires 5 amps, 5 x 5 amps = 25 amps.

Divide the value found in Step 4 by the frequency multiplied by the output voltage. In this example, the equation looks like this: 25 amps / 996 = .0251

Express this value in micro-farards by multiplying the value in Step 5 by 10^6. The final value for this example is 25,100 microfarads. This is the rating required of your capacitor.