How to Compensate for Frequency Ripple in DC Adapters

The lowest frequency ripple from a linear wall plug-in power adapter which uses a full bridge rectifier is at twice the line frequency­120 Hz or 100 Hz depending utility supply. The capacitance for the filter capacitor can be approximated by

C1 (uF) = [ ( ILoad ) /(120* Vp-p ripple) ] Farads

(multiply by one million go get microfarads). A quick insight into this formula is that at 1 mV ripple a 1 amp current will need to flow into the capacitor for 1/120th of a second, and out of the capacitor for 1/120th of a second. The amount of charge that goes in and out of the capacitor is delta V times the Capacitance, so C = ILoad/(120*Vp-p ripple). This solution doesn’t provide the performance of a tuned filter, requiring higher capacitance than, say, a pi filter, but it is the most economical way to solve the problem..

The capacitor can be built into either in the AC/DC adapter, or in your product.

Generally, the voltage rating of the capacitor should be above that of the ripple of the raw signal, or 1.414 times the desired DC voltage. You always want to kill noise where it originates, so it is preferable to have the filter capacitor in the power adapter rather than let all those harmonics radiate down the cord and handle the filtering in your product.

Keep in mind, there will be frequency harmonics above 120 Hz, but the capacitor should be able to eliminate, them too. It is less practical to try to use a capacitor to filter out a half-wave rectified wall transformer.