Smoothing Capacitor Calculator

Use this calculator to find the smoothing capacitor you need after a rectifier, when you make a DC power supply from a transformer.

A rectifier turns AC into pulsing DC. The capacitor charges up at each peak, and feeds the load until the next one. The voltage drops a little in between, and that drop is the ripple:

C = \frac{I _{L O A D}}{f _{r i ppl e} \cdot V _{r i ppl e}}

With a full-wave rectifier, like a diode bridge, the ripple frequency is twice the mains frequency. With a single diode, it’s the same as the mains frequency, so you need twice the capacitance.

Transformer Output Voltage (AC, RMS)

Rectifier

Mains Frequency

Load Current

Allowed Ripple (peak to peak)

Peak voltage: -
Average DC voltage: -
Lowest voltage (bottom of the ripple): -
Capacitance needed: -
Nearest standard capacitor (next higher): -
Capacitor voltage rating to use: -

How to Use the Result

Pick an electrolytic capacitor with at least the capacitance the calculator shows, and a voltage rating above the peak voltage. The calculator suggests one with some margin.

Small transformers give a higher voltage than their rating when the load is light, so measure yours if you can. The calculator counts about 0.7 V for each diode the current goes through.

If your circuit needs a steady voltage, add a voltage regulator after the capacitor. Then make sure the lowest voltage, at the bottom of the ripple, stays above what the regulator needs. Want to learn more? Check out the linear power supply tutorial.