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To use an ATX supply outside of a PC while preventing output voltage to run away at low/no load, can I just use a couple of diodes in series to provide some kind of voltage-dependent load?

Like:

schematic

simulate this circuit – Schematic created using CircuitLab

The idea is simply to have a base load which limits the output voltage to some level so as to not damage the PSU. The no-load output voltage for my appliance is not critical.

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    \$\begingroup\$ IMHO, ditch the diodes and use a power resistor as your dummy load. Maybe. If I understand what you're asking. \$\endgroup\$
    – dext0rb
    Commented Oct 31, 2013 at 16:27

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As I understand it you shouldn't need one. ATX supplies are supposed to be well regulated.

See

There is a footprint for a 9 Watt 10ohm load resistor on the 5 volt rail. Some older ATX supplies won't start without some sort of load.

In our experience most ATX supplies don't require a significant load on the 5 volt rail to start. An artificial load just wastes electricity and creates unnecessary heat. In the production version we include the resistor but don't solder it because most will never need it.

Some Intel ATX specifications say

3.2.1. DC Voltage Regulation

The DC output voltages shall remain within the regulation ranges shown in Table 2 when measured at the load end of the output connectors under all line, load, and environmental conditions. The voltage regulation limits shall be maintained under continuous operation for any steady state temperature and operating conditions specified in Section 5.

Table 2. DC Output Voltage Regulation

Output      Range     Min.     Nom.     Max.    Unit 
+12V1DC     ±5%      +11.40    +12.00   +12.60  Volts 
+12V2DC (1) ±5%      +11.40    +12.00   +12.60  Volts 
+5VDC       ±5%      +4.75     +5.00    +5.25   Volts 
+3.3VDC (2) ±5%      +3.14     +3.30    +3.47   Volts 
-12VDC      ±10%     -10.80    -12.00   -13.20  Volts 
+5VSB       ±5%      +4.75     +5.00    +5.25  Volts 
(1) At +12 VDC peak loading, regulation at the +12 VDC output can go to ± 10%. 
(2) Voltage tolerance is required at main connector and S-ATA connector (if used)

As noted many many websites do say that a small load is required. If you want to follow that advice, use a small power resistor. A desktop PC probably uses 2-3 W in standby so something approaching that should probably be your target.

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  • \$\begingroup\$ Interesting. From what I read I assume(d) that a load is actually required for most ATX PSUs to prevent damage. \$\endgroup\$
    – JimmyB
    Commented Oct 31, 2013 at 16:52
  • \$\begingroup\$ @Hanno: See update. If it wont meet specs under all load conditions, it isn't ATX. \$\endgroup\$ Commented Oct 31, 2013 at 16:55
  • \$\begingroup\$ Right. Now I'm more confused than before :) Many sources (like this) clearly state that "no-load" is a "no-go" for those PSUs. \$\endgroup\$
    – JimmyB
    Commented Oct 31, 2013 at 17:09
  • \$\begingroup\$ @Hanno: Hmm ... I see what you mean. A 10Ω 9W power resistor would be a reasonable choice if you want to play it safe. \$\endgroup\$ Commented Oct 31, 2013 at 17:20
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I had to put a load to make it works, I used a resistor 10ohms 25W. From what I know the Diodes had ideally zero resistence (almost zero) so it will be like a short circuit.

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I can only confirm that it is usual for ATX1.0 PSU that they don't start up with too little load. Im just preparing an old PSU that I bought for 5 bugs to power my external HDD. With an 100 Ohm resistor on 5V (250mW) it overshoots over 6V at start up and consequently shuts down immediately. Unfortunately I have no power resistors at home. I'm a little bit uneasy with trying if the 2nd hand psu voltage does not overshoot with my 16 SAS HDD 😂. I will try it with some broken HDDs first. This is also a warning: Some of today's IT accessories have considerable less power consumption than 20 years ago. If you would hang only an SSD on the PSU that I just tested and that SSD does not have pretty strong overvoltage protection diodes it will be killed at start up overshoot. @Paulo Balbino: Read about diode forward voltage to understand what the intial autor has meant.

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    \$\begingroup\$ There are/were many different manufacturers of the ATX power supply. Not all are the same design electronically although they do have a similar mechanical dimensions to adhere to. Electrically they have a set of output voltages they supply to match the system and a current minimum. The maximum current available varies quite substantially over the years. on all units. \$\endgroup\$
    – Gil
    Commented Jun 14, 2021 at 2:56
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Example of requiring a load resistor:

I have a 500 watts ATX. If I take more than 3 amps from the 12 volt rail the voltage collapses down to 8 volt. But if I attach a 33 ohm 20 watt resistor to the 5 volt rail, the output at 12 volts stays at 11.4 volt even at 10 amps of current.

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