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I'm working on a power supply for my camera (powershot SX150IS) in order to leave it on for long periods of time. So I took an old power ATX power supply and connected the 3.3V output to the camera, but it never powered on, not even in playback mode. Every time I tried to power the camera on, the power supply shut down.

The rated current for the 3.3V output is 15A and, according to the specs of the ACK-800, which is the power adapter for my camera (see http://www.cnet.com/products/canon-ack-800-ac-adapter-kit/specs/), the current the adapter drives is 1.5A. So, the power supply wasn't even able to drive 10% of the rated current for the 3.3V output.

Do you guys know why this is happening?

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    \$\begingroup\$ Add a load to the other voltage outputs. \$\endgroup\$
    – Majenko
    Dec 6, 2014 at 23:02
  • \$\begingroup\$ Have you shorted the PS_ON to ground? \$\endgroup\$ Dec 6, 2014 at 23:04
  • \$\begingroup\$ Of course ;) The power supply was on. \$\endgroup\$
    – Charlie
    Dec 6, 2014 at 23:05
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    \$\begingroup\$ Depends on the PSU. Try a large resistor, and reduce it till it works, but don't go too low or the resistor will draw too much current and overheat. \$\endgroup\$
    – Majenko
    Dec 6, 2014 at 23:18
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    \$\begingroup\$ To just one of each voltage. \$\endgroup\$
    – Majenko
    Dec 6, 2014 at 23:23

4 Answers 4

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According to the ATX specifications (Google for "ATX12V specifications") there are certain minimum load requirements:

  • 12 V: 1 A
  • 5 V: 300 mA
  • 3.3 V: 500 mA

This means that to run the power supply in spec, you will need:

  • 12 V: 12 Ω or less (12 watts)
  • 5 V: 16.6 Ω or less (1.5 watts)
  • 3.3 V: 6.6 Ω or less (1.65 watts)

However, with the power supplies I have here, it seems that the current available on the 3.3 V line is somehow proportional to the current drawn on the other rails.

  • 5 V @ 10 mA = 300 mA max @ 3.3 V
  • 5 V @ 277 mA = 2.1 A max @ 3.3 V
  • 5 V @ 500 mA = 6.1 A max @ 3.3 V
  • 12 V @ 193 mA = 3.2 A max @ 3.3 V

With each of these tests, the 3.3 V line started dropping in voltage as the above current levels were approached, and once the output dropped to around 2.8 V that's when the supply shut down. This makes it seem like it's not so much current limiting protection as it is undervoltage protection, with the total available power at 3.3 V proportional to how much power is being drawn on either the 5 V or 12 V rails.

So it seems that the minimum load you put on 5 V or 12 V will depend on how much power you need from the 3.3 V line.

With my tests, the 5 V and 12 V lines did not have any problems delivering plenty of current, and the dummy load only needed to be placed on one of these rails to get the 3.3 V line to deliver current (i.e. either the 5 V or the 12 V line could be left unused). However there seems to be some variation between supplies so this may not apply for all of them.

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  • \$\begingroup\$ If we are using only the 12V (all 12V wires connected to a single one) do we need to only add a load to the 12V rail (to a single 12V wire) or also to the other rails (e.g. 3.3V)? Does a single load suffice? \$\endgroup\$
    – JinSnow
    Jun 3, 2023 at 14:50
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    \$\begingroup\$ @JinSnow: Technically yes you'll need the minimum load on all rails to meet the spec, however you will likely find that for the 12V rail many supplies will deliver full current without the minimum load on the other rails. You'll just have to try it with the supplies you have and see if it works. Worst case the power supply will switch itself off and you can try again. \$\endgroup\$
    – Malvineous
    Jun 4, 2023 at 11:31
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Most ATX power supply require a minimum load to stay on, add a 10 ohm resistor between 5V and ground and it should stay on.

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This is simply because the ATX power supply has the overcurrent / over voltage circuit protection. At the start if it drain more power than was is set it will automatically shutdown. Fortunately you can bypass the circuit protection if you know how to hack the psu..i can even run my 12 volt makita grass trimmer with it to cut 25 meters lawn and it really good

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  • \$\begingroup\$ So you are suggesting that the OP's digital camera overloads the 15A supply? How could this happen? \$\endgroup\$ Jan 13, 2021 at 20:27
  • \$\begingroup\$ 15A is what stated on the power supply --- but when you run it especially during the initial start the power supply doesnt go up to that much in fact when it exceed certain Ampere i.e. 1.5A it could immediately shut down...to test this get your power supply and amp meter then run a motor that at least run 2A you will during the first start it require more than that and the power supply cannot even go up to that. Thus it will shut down \$\endgroup\$ Jan 15, 2021 at 4:17
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Sounds like short circuit or thermal overload protection circuits kick in. Does the power supply work when connected to a different load?

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