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What are Ripple Measurement Techniques for the Power Supply?

Electronic equipment that plugs into a wall socket uses a power supply to change the alternating current to direct current. No power supply is perfect, however, and the resulting DC voltage always has a small amount of AC riding on it, called ripple voltage. Check it with an oscilloscope, an RMS voltmeter or a multimeter.

Power Supply
- An AC voltage waveform from a standard household wall socket varies from positive to negative peaks at a rate of 60 cycles per second. A power supply contains rectifier circuits to change the voltage to all positive waves. Filters then smooth the waves to a DC voltage with the quality of the filter determining the smoothness of the final voltage. The ripple voltage varies the final DC voltage by a small positive and negative amount. For example, plus 100 volts may vary between plus 99.5 and plus 100.5, leaving a ripple voltage of 0.5 volts.
Oscilloscope
- An oscilloscope presents a picture of an electronic waveform on a screen. A grid on the screen is calibrated to represent a specific amount of voltage for each division. A power supply may produce other unwanted signals such as thermal noise or interference from other signals. By connecting an oscilloscope to the output of a power supply, you can see an image of the signal and distinguish between the ripple voltage and any other unwanted signal. You can measure the exact amount of ripple with the calibrated grid.
RMS Voltmeter
- An RMS voltmeter measures audio and low frequency radio signals. Connected to the output of a power supply, it can also read the amount of ripple but won't separate it from other noise. You can make a "go, no-go" check to see if you need to troubleshoot a power supply further. For example, if a power supply has an output specification of plus 100 volts with more than 0.5 volts of ripple, and the RMS meter shows 0.4 volts, you know the ripple is within specifications. At 0.6 volts, you have to measure exactly with an oscilloscope.
Mulitmeter
- Do a "go, no-go" ripple measurement with a multimeter.
  
  A multimeter with an AC voltage scale can also provide a "go, no-go" ripple measurement on a power supply, but with less accuracy than an RMS voltmeter. Set the meter to the lowest AC scale and connect it to the output. Because the voltmeter is designed to read higher signals, it is less accurate at the lower scales and will show a varying reading. If you have a 0.5 volt ripple specification, a signal varying between 0.1 and 0.5 would be good, but one varying between 0.3 and 0.7 would require further troubleshooting.

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