ATX PSU Output 12 V DC Voltage

By deevarnado ·
I have been checking Desktop PSUs, even my new 1 shows 12V DC out. I read 24V AC on the same yellow wire. Is this just the way cheap PSU operate? I thought the 12V DC output should not have any AC. If AC is on the outputs is that a defective rectifier?

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AC Ripple is perfectly normal

by robo_dev In reply to ATX PSU Output 12 V DC Vo ...

How are you measuring it? DMM or Analog meter? Under load or unloaded?
If it's a DMM, is it a 'True RMS' meter or not?

Some meters are way off with respect to quickly-moving AC signals.

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Personally I've always found Multimeter's

by OH Smeg In reply to ATX PSU Output 12 V DC Vo ...

Useless for Computer Power Supplies.

If you want to test one grab yourself a Power Supply Tester like the one here

These place a Load on the PS and give correct readings.


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Switching Power Supplies

by TheChas In reply to ATX PSU Output 12 V DC Vo ...

I find it very hard to believe there is 24 VAC of ripple voltage on ANY computer power supply. More likely there is a ground loop present in your test setup and you are reading what would have been AC hum in the days of hi-fi.

The best easy way to quantify the noise and ripple on a switching power supply is to measure the output using an oscilloscope.

If you did have a bad rectifier in the power supply, it would not work at all. Or, at least work very poorly.

Further, on a computer power supply (and most other switch mode supplies) there should be no 60 hertz ripple on the output. The raw main AC power is rectified and filtered. Then chopped at a comparatively high frequency. The high frequency AC is then fed to the transformer or voltage regulators. This allows the use of much smaller and less expensive transformers and capacitors. The high frequency is also why the AC range on most handheld DMM's is useless to measure PC power supply ripple and noise. The frequency is just too high for the filter circuits in the DMM to pass onto the measurement circuit.


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Thanks for response!!

by deevarnado In reply to ATX PSU Output 12 V DC Vo ...

I'm using a DMM and when I checked PSU Voltages(Yellow 12V to Black Gnd) with it, i noticed 12V DC was correct. When I switched to AC 200V lowest AC setting I would read 24V AC. Checked good computer while it was running and on 12V DC my meter still reads AC double what the DC Voltage is. No doubt a scope would be nice. I really didn't think my DMM would read AC on the output. Must be my meter.

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