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I have some basic knowledge about electronics. I am trying to repair an ATX 250W power supply.

In the picture below, I measured 0V with my DMM between points C and D (AC inputs of diode bridge rectifier.) I was expecting 220V AC.

Measuring with the DMM set to "diode," the bridge rectifier entries give me this:

  • FC = 0.5
  • FD = 0.5
  • EC = 0.5
  • ED = 0.5

These measurements lead me believe that the bridge is OK. Why is there then 0V between the AC inputs?

Measuring the capacitors with DMM set to "capacitor" gives me the right values (off by about 5% or so.)

When measuring the capacitors with the DMM set to resistance 200M, I get 1. I was expecting the value to rise as they charge but since they are 2.2nF (orange) and 4.7nF (yellow), I suppose they may be charging fast.

The voltage between points A and B is 220V AC (Since A and C are the same point, is this mean C and D are shorted ).

Does anyone have an idea what could be wrong these observations?

The image below shows the wiring of the components in the power supply.

enter image description here

the back of the board enter image description here

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    \$\begingroup\$ Could you draw a normal schematic for this? What's the red coil thing at the left - a transformer? Why are the two yellow caps in series with your AC to the bridge? \$\endgroup\$
    – mike65535
    Jun 21, 2021 at 12:56
  • \$\begingroup\$ the point between yellow capacitor is linked to PE(earth) through the shield, but since i have only two AC wire in my room, i supposed that it's like it doesn't exist. the coil is a ring coil with 4 pins (each coil have half the ring) \$\endgroup\$
    – miracle genuis
    Jun 21, 2021 at 13:07
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    \$\begingroup\$ i have a doubt that yellow capacitor are indeed varistor !! \$\endgroup\$
    – miracle genuis
    Jun 21, 2021 at 13:09
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    \$\begingroup\$ @miraclegenuis My bet would be on Y2 capacitors, but yellow is an odd color choice. \$\endgroup\$
    – winny
    Jun 21, 2021 at 13:12
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    \$\begingroup\$ There you go. It’s an Y2 capacitor. \$\endgroup\$
    – winny
    Jun 21, 2021 at 15:26

3 Answers 3

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With those two capacitors in series (the yellow ones, 4.7nF,) you will have about 1.4Mohm in series with the input to the rectifier. The rectifier will effectively short circuit that tiny little bit of current.

If that's supposed to be the circuit from an existing power supply, then I'd day you traced it out wrong.

If that's a circuit you came up with yourself, then you need to really think about this before going any further.

Please be careful while working on this. It is very easy to accidentally connect yourself to the full line voltage while make measurements.

  • Use clips to connect the meter to the circuit.
  • Disconnect the circuit from power when you move the multimeter clips to measure different parts of the circuit.
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  • \$\begingroup\$ i add a picture. so you can see what i schematize. \$\endgroup\$
    – miracle genuis
    Jun 21, 2021 at 14:11
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If the fuse is OK and the AC voltage is not present at the points C and D, I suggest you to check all the devices which are on the path from the AC inlet to the points C and D. Check also the switch. Sometimes there are faulty soldering points or faulty filtering chockes, which interrupt one of the paths.

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Is your meter set to "AC Volts" when you measure the AC voltage between points C and D? Your meter will (should) read 0vDC if you connect to an AC signal.

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