How to Measure AC Ripple on a Computer Power Supply

Written by ed wagner
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How to Measure AC Ripple on a Computer Power Supply
An oscilloscope displaying a complex waveform. (Stockbyte/Stockbyte/Getty Images)

Output voltages from a computer power supply must be tightly regulated. Ripple, noise and voltage spikes can compromise system performance, causing problems with I/O circuits, displays and even the central processor. If a unit exhibits anomalous failures that cannot be easily traced, the power supply is a good starting point for troubleshooting.

Skill level:

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Things you need

  • Oscilloscope
  • Digital multimeter

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  1. 1

    Attach an oscilloscope to each voltage source in turn. Use circuit schematics to find test points, or find the large electrolytic filter capacitors on the outputs. Be aware that some computers have separate grounds for the input and output sides. Couple the o-scope for DC and measure the DC output voltage. If the vertical menu is set for 1 volt per division and the trace detects 5 divisions, it's a 5-volt supply. Couple the probe for AC and change the vertical menu to 10 millivolts per division. Adjust the horizontal sweep for a stable display by changing the threshold voltage and time base as necessary. Make note of the peak-to-peak voltage of any ripple. Watch for noise and spikes on the o-scope display.

  2. 2

    Calculate the ripple percentage by dividing the peak-to-peak value by the power supply DC voltage, and then multiply by 100. So if you measured a 50 millivolt peak-to-peak voltage, for instance, the ripple is 1 per cent. Some power supply specifications will use percentage, while others simply state the peak-to-peak voltage.

  3. 3

    Measure ripple voltage with a digital multimeter. Attach the leads across each power supply output. Set the meter to read DC voltage. Most advanced meters use auto-ranging, but if it's not available, choose an appropriate range. Make note of the voltage, then switch the meter to read AC volts. Perform the calculation for the percentage of ripple.

    How to Measure AC Ripple on a Computer Power Supply
    A digital multimeter can measure power supply ripple. (Polka Dot Images/Polka Dot/Getty Images)

Tips and warnings

  • When performing any of these measurements, route the test leads carefully, keeping them away from transformers or other test equipment that can induce voltages in the leads. Use the short ground leads supplied with o-scope probes for the same reason.
  • Exercise care around power supplies as they can produce painful electrical shocks even when the power is off. Large capacitors can discharge unexpectedly through an unwary hand or tool.

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