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I have a generic PC powersupply I'm using to power a BFL ASIC Bitcoin Miner. It takes an input similar to a PC Video card with two connectors that look like this

Connector 1   Connector 2
 OOO           OOO
 OOO           OOO

The generic PC PSU requires a motherboard to send power to the AUX outputs. Is there any way I can "trick" the PSU into powering the rest of the other devices safely?

Suppose there is no DIP switch to press, can I just connect one pin of the Motherboard output to ground? (or similar)

I'd prefer to do this with no additional soldering/resistors etc.

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  • \$\begingroup\$ Did you short the green wire to a black wire on the ATX24 connector? \$\endgroup\$ – Lior Bilia Oct 6 '13 at 3:41
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    \$\begingroup\$ Some PSUs require a large load on the 12v or 5v rails as a safety precaution. You might not be able to do without. \$\endgroup\$ – Passerby Oct 6 '13 at 7:24
  • \$\begingroup\$ @Passerby: what's the particular nature of the hazard in an unloaded 12V or 5V rail, and can it be mitigated? \$\endgroup\$ – Li-aung Yip Oct 7 '13 at 2:29
  • \$\begingroup\$ @Li-aungYip the internal circuit depends on the load for regulation checks. No load, and it won't turn on, and if it did, something in the circuit could damage from attempting to turn on. Mainly mosfets or transistors. Same reason open circuits in some led drivers will cause the internal fet to blow. The safety is for the PSU, not exactly a physical danger. The only way to mitigate it is by adding a load, or by delving deep down into the PSU circuit, reverse engineer and change it. \$\endgroup\$ – Passerby Oct 7 '13 at 3:25
  • \$\begingroup\$ @Passerby: Would putting a few watts of resistive load on each rail be sufficient? Given that PC power supplies are cheap, it doesn't matter if the PSU blows up so long as it doesn't catch fire, or fry attached equipment. \$\endgroup\$ – Li-aung Yip Oct 7 '13 at 3:58
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Sure thing! Those connectors you're describing seem to be 6 pin PCI Express power connectors, where top width of the pins will be 12 V (usually yellow cable) and the others (bottom row) connected to ground. These are usually shared across rails inside the PSU (usually different physical switching semiconductors, to spread the load).

In order to fire up an ATX power supply, you'll need to pull the power-on signal low by connecting it to ground. This is pin 16 (a green wire) on the main 20/24 pin connector. Just short that to any ground (black) wire on the 20/24 pin main connector.

Whilst a little unsafe, it can be done.

Also, as the PSU uses a switch-mode configuration to regulate line voltage down to ATX voltages you need to consider the loading on the power supply. An unloaded switch mode power supply without any protection can oscillate in ways that were not intended for its usually operation. Some PSU manufacturers cut costs by omitting components that help stabilise the power supply with no/low loading and count in it being used to power a PC only.

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    \$\begingroup\$ The guide Vasily linked in his answer suggests a 10 ohm, 10 watt power resistor, connected between +5V and GND, in order to provide a little bit of load even when nothing is connected. (It will dissipate 2.5W so it needs to be heatsinked - the guide recommends supergluing it to the metal case of the PSU.) \$\endgroup\$ – Li-aung Yip Oct 7 '13 at 2:27
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For my graduation engineering project I needed a bench power supply which can deliver ~80W. I used this manual to convert a usual ATX power supply into a bench one. The resulting power supply is outstanding!

If I understood your question correctly, you're not interested in a totally general bench power supply, but just want to make the usual power supply work without actually connecting it to the motherboard, right?

If so, then you should make the following connections:

  1. Green wire (POWER_ON) to black wire (GND). The PSU will be controlled by the rear switch.
  2. Brown wire (SENSE) to orange wire (3.3V). If this doesn't work, try a red wire (5V) instead.

Safety:

As long as your PSU powers some internal circuitry and its outputs are hardwired to some PCB, you may not worry about safety issues or minimal load conditions. In this case you may omit the power resistor which is suggested in the article.

However, if you plan to move the connections and may incidentally touch the outputs (even if the PSU is off), you MUST add the bleeder power resistor as suggested in the article.

P.S. The trick with the fan instead of a bleeder resistor is ingenious!

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  • \$\begingroup\$ Anon downvoter: Why the downvote? If it's due to a safety consideration, say so. \$\endgroup\$ – Li-aung Yip Oct 7 '13 at 2:36
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    \$\begingroup\$ I didn't down-vote, but one issue I see with this answer is that it's a link-only answer. THe answer text itself doesn't explain what's in the article, so if something happens to the article, we don't have a source of information on what to do. Paraphrasing article in text of the answer together with a link to the article would have been better. \$\endgroup\$ – AndrejaKo Oct 7 '13 at 5:51
  • \$\begingroup\$ @AndrejaKo, thanks for informative comment. I edited my answer to better address the question. \$\endgroup\$ – Vasiliy Oct 7 '13 at 12:31
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There is a very nice video tutorial on the topic available:

JumperOneTV tutorial on how to convert an ATX power supply to lab bench power supply.

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