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I have a 450 watt power supply that I am trying to use as a DC power supply to learn about electronics. I watched some videos that show grouping the red, yellow, black wires so I did that but when I test the output, the group of red wires shows 5 volts but only about .2 amps, the group of yellow wires shows 12.18 volts but only about .5 amps. The power supply shows DC output of 22 amps for the yellow and 25 amps for the red. What am I doing wrong? Thanks

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    \$\begingroup\$ how are you measuring the current? \$\endgroup\$
    – jsotola
    Jun 2 '21 at 2:24
  • \$\begingroup\$ I put the red wire from my meter on the group of yellow from the power supply and the black from the meter to the group of black from the power supply with DC amps selected on my tester. it shows about .5 amp. I also connected the power supply to a car amp and the meter shows the same even when connected to the car amp. the car amp will turn on and work but with very low sound coming from the speaker \$\endgroup\$
    – ds1234
    Jun 2 '21 at 2:37
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    \$\begingroup\$ Measuring the current by shorting out the power supply through your meter will tend to damage the meter and/or power supply. If your meter still works, consider yourself lucky and don't do that again. \$\endgroup\$ Jun 2 '21 at 2:56
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    \$\begingroup\$ May I suggest not using a 450W power supply to learn about electronics. It's a question of safety in case something goes wrong. You can do a lot with just 1 amp of current at a low voltage and you'll be dealing with a lot less power. \$\endgroup\$
    – ErikR
    Jun 2 '21 at 3:04
  • \$\begingroup\$ Thanks. I will remember that. \$\endgroup\$
    – ds1234
    Jun 2 '21 at 3:05
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The way you measured current did short out the power supply via the meter, so the meter fuse should have blown.

Voltage is measured by putting meter in parallel with output, current is measured by putting meter in series with output (and load, so without load it is in series with a short circuit).

If the meter has no fuse it is dangerous, I don't even dare to think what could have happened if you had shorted out a 12V car battery (melting wires?) but fortunately the PC power supply has overcurren protection, so it detected the short and shut down.

Also note that PC power supplies are not that great for general purpose power supplies. If you only load the 12V output, and have no load on other outputs, they may have trouble keeping the output voltage at 12V or they might shut down easily under variable load, especially if the load suddenly is disconnected. So don't expect it to work great with your car amp.

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  • \$\begingroup\$ This may be a dumb question but here it is. When an amp meter is added in series with a load to complete a circuit, isn't the meter shorting out the power source? What is the difference in completing a circuit and shorting it out. I understand a little about parallel and series circuits but can you explain more when you said " Voltage is measured by putting meter in parallel with output, current is measured by putting meter in series with output (and load, so without load it is in series with a short circuit). Thanks \$\endgroup\$
    – ds1234
    Jun 2 '21 at 5:48
  • \$\begingroup\$ No, when measuring current, the meter leads can be thought as a piece of wire that completes the circuit from supply to load so you can measure current that flows. The meter only short circuits the supply when there is no load, and there is no load when you stick current measurement leads directly into power supply. \$\endgroup\$
    – Justme
    Jun 2 '21 at 9:49

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