I'd like to make lab power supply out of my old ATX Power supply. I am beginner in electrical engineering and this project seems pretty straightforward. I've checked many tutorials about it and creators of these instructed, that there should be resistor connected to 5V or 12V wire depending on max output current that they can provide. psu sticker

Here comes my questions: Should I put dummy load on 5V or 12V? Is 24ohm 5W resistor fine for connection?

Thanks in advance :)

  • 2
    \$\begingroup\$ Just load all the lines a little bit. You're a beginning EE so try this...calculate the power dissipated by your 24ohm, 5W resistor when connected across the 5V line and when connected across the 12V line. Does your result tell you it's okay? \$\endgroup\$
    – DKNguyen
    Commented Jul 3, 2019 at 18:50
  • \$\begingroup\$ You will likely need a jumper from "power on" lead to ground to get the supply to turn on. A 1W load may be below the minimum for a 450W supply to be in regulation. \$\endgroup\$
    – stark
    Commented Jul 3, 2019 at 19:03
  • \$\begingroup\$ There are a plethora of articles and videos explaining how to turn an atx supply into a lab supply, there are also adapters for this purpose. \$\endgroup\$
    – Voltage Spike
    Commented Apr 11 at 18:09

1 Answer 1


A 24Ohm/5W is okay for the 5V rail, but not sufficient for the 12V rail. 12V / 24Ohms = 0,5A * 12V = 6W Your resistor will probably get very hot, possibly fail, and possibly result in smoke or flames.

  • \$\begingroup\$ For better step Load ripple response a 5% preload is suggested from my experience. Also newer PSU’s <10y old, >=450W, run fine with no load. Older ones 250W,300W PSU’s needed a preload just to be stable on primary power output only. all others are forward coupled by transformer and share primary switched current of SMPS. Ie. not individually regulated. Some HIGH power PSU’s have independent 12V outputs shown in datasheet might suggest preload, but it won’t hurt with(without) it. \$\endgroup\$ Commented Jul 3, 2019 at 19:37

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