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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 Oct 31 '13 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 Oct 31 '13 at 16:52
  • \$\begingroup\$ @Hanno: See update. If it wont meet specs under all load conditions, it isn't ATX. \$\endgroup\$ – RedGrittyBrick Oct 31 '13 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 Oct 31 '13 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\$ – RedGrittyBrick Oct 31 '13 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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