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How much AC ripple in a DC power supply is too much?

Publication date: 
March 2016
Chuck Yung, EASA Senior Technical Support Specialist
Type of media: 

Once upon a time, DC motors operated from batteries or motor-generator sets. For the past 50+ years, most DC motors have operated from solid state power supplies – rectifying AC power to DC power. When motors began operating from rectified power, one of the problems experienced was the presence of AC “ripple” in the leads that were supposed to deliver DC power to the machine. Absent a specific standard, a recurring question is: “How much is too much?” Before I try to suggest an answer to that question, let’s talk about what AC ripple is and explain why it is not desirable. 

One source describes AC ripple as “the small unwanted residual periodic variation of the direct current (DC) output of a power supply … from an alternating current (AC) source.” We express ripple factor as the ratio of the root mean square (rms) AC voltage over the DC voltage. That is usually expressed as a percentage. Since it is easier to measure the peak-peak voltage when using an oscilloscope, the ripple is often reported as the peak-peak AC to DC voltage ratio.

Topics covered include:

  • Full-wave rectification
  • Bigger (power supply) is not always better
  • Using true-rms ammeter
DC motors