How to Calculate Capacitors for a Bridge Rectifier

By Alexander Rudinski
The properties of diodes allow them to convert negative current to positive current.

A full-wave bridge rectifier is a specialized arrangement of diodes that converts alternating current, or AC, to direct current, or DC. While the current that flows out of electrical sockets is AC, electronic devices are powered by DC, requiring a conversion from the outlet to the device. The rectifier makes this conversion, producing a rough approximation of DC current by reversing the negative half-cycles of AC, producing positive current with the same frequency of the input AC current. This conversion is not perfect, and attaching a capacitor to the circuit improves the current conversion, producing less ripple and a purer version of DC.

Determine the necessary output voltage of the bridge rectifier. This value is given in volts, and it is determined by the specifications of the load you are powering with the rectifier. On a pre-made component, this value is given in the data sheets. If you don't have it, the value can be calculated independently by finding the component that requires the most voltage in the circuit. This component determines the minimum voltage requirements.

Add an additional 10 percent to the voltage requirement, plus 1.4v to allow for the inherent voltage drop of a bridge rectifier. For example, if you need a 15v output, the calculation would look like this: 15v + 1.5v + 1.4v = 18.9v. This is the peak voltage your transformer must produce. In this example, the voltage can be rounded down to 18 volts to avoid overloading the circuit with a 20v transformer.

Determine the peak current required by the load. Again, this is available on the data sheets, or it can be determined by the component that requires the most current to operate.

Multiply the peak current by 5. For example, if the peak current is 5 amps, the calculation would be 5 x 5 amps = 25 amps. This value will be considered the peak current for this equation.

Determine the peak output voltage of your bridge rectifier. This is determined by subtracting 1.4v from the total output voltage of your transformer. Using the above example, the equation is as follows: 18v - 1.4v = 16.6v.

Multiply the value from Step 5 by the frequency of the transformer's output, normally 60 hertz.

Divide the value of the peak current by the value found Step 6. In this example, the equation appears as follows: 25 amps/996 = 0.0251.

Express the value found in the equation in Step 7 in micro-fards to find the total capacitance required of the capacitor. In this example, the value is 25,100 micro-farads.

About the Author

Alexander Rudinski has been writing professionally since 2008. His work appears on the Nerve website, where he continues to work as a photographer and writer. Rudinski has a Bachelor of Science in communications, concentrating on documentary video, photography and professional writing. He graduated from the University of the Arts, Philadelphia.