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I'm attempting to power my Raspberry Pi with my own power supply. As you can see from the picture, that is the micro usb I have modified to connect to my power supply. However, when I connect it to my Raspberry Pi, nothing happens. I have tested the pins on the micro usb when it is connected to the psu and there is 5.1V at the power pin and 0V at ground pin. I haven't damaged the rpi as if I connect back to it's original adapter, it boots up fine. The psu is supplying about 5.1V and can go up to 12A (I know this is a large amount, but the rpi only draws what it needs so it shouldn't matter).

Any suggestions as what could be wrong?

  • @tomasby No, there are no lit LEDs at all. Its an old computer psu, so theres 12V, 5V and 3V3. I can get a pic of the whole thing, but not right now as I'm not at home. – sparpo Mar 28 at 10:16
  • You can try to connect the wires to the GPIO (pins 4 & 6) instead of the micro-USB port. Then you can use the voltmeter directly on the connection. – Tomas By Mar 28 at 11:33
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    have you tried powering something else with your frankencable? – Jaromanda X Mar 28 at 12:56
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    do not assume that the cable wires are color coded ..... also, do not assume that you cut up a good cable – jsotola Mar 29 at 0:23
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You said in a comment:

... No, there are no lit LEDs at all. Its an old computer psu, so theres 12V, 5V and 3V3...

That may be the case when the PSU is properly powered up but if there is only a tiny load on just one output (the 5V one) the Switch-Mode Power Supply is probably unstable and may be shutting down because the draw on it's outputs is so little. Have a look at the output ratings on the PSU and you will probably find that it is expecting to provide tens of amperes of current at 5V and probably several at 12V. Under conditions of underload the output voltages may start to rise and if they get too high it may be shutting down to prevent damage to the circuitry to which it is connected.

Also, depending on the era of the unit it may need a switching voltage to be provided to a control line (or a terminal to be grounded), see the connection labelled Power on here: https://en.wikipedia.org/wiki/Power_supply_unit_(computer)#Wiring_diagrams.

If you have a spare automotive direction indicator bulb you could wire up one or two of those between a 12V output and ground.

One thing to bear in mind is that: although those outputs are at a nice low, safe, voltage, they can provide a fair amount of current and if you short them out with say a ring or a metal watch strap that current is going to make those things get very hot very quickly - the standards seem to say that each output should be limited to providing no more than 240VA (effectively Watts) and at 5V that corresponds to nearly 50W - OTOH the heat from that should cauterise the stump should a red-hot wedding ring burn your finger off...

  • "cauterise the stump" +1. However, OP said. "I have tested the pins on the micro usb when it is connected to the psu and there is 5.1V at the power pin and 0V at ground pin. " – Seamus Mar 29 at 16:47
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The possibilities seem limited.

  1. The cable wires are too thin to carry the power required by the Pi.
  2. The wrong pins have been used.
  • Power wires being too thin? If that was the case, shouldn't there at least some power going to the board? Shouldn't there still be a LED lit up on the board? Maybe i have connected the wrong pins... But, I connected the 5V wire to the red wire on the micro usb and the gnd to the black one, so i don't what else could go wrong. – sparpo Mar 28 at 10:20
  • @sparpo I can't think of any other possibilities. – joan Mar 28 at 10:28
  • It probably wouldn't do anything, but would soldering the other wires to +5V or GND help? – sparpo Mar 28 at 11:03
  • Nope - on the RP power input the Data lines are - not unreasonably - open circuit IIRC - and if you plug the frankencable (lovely term that @Jaromanda X!) into anything else it may not do it any good! – SlySven Mar 30 at 2:35

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