I'm posting this question just out of curiosity and also I know that you guys are expert in this electrical stuff building projects. I used to watch electrical circuits building videos in youtube in my free time. And I just came across a video, a guy converting ATX PSU into bench power supply. And I found that all those converting PSU into bench supply are connecting a power resistor across 5V rail. As I am a pure beginner to this topic, I couldn't find why they are using a power resistor across the rail. I hope you guys will provide me the answer, so that I can understand this resistor concept.
1
-
2\$\begingroup\$ Because there is a minimum load required on the rail for the voltage regulator circuit to perform adequately, usually a couple percent of full load. Lots of good discussion here: electronics.stackexchange.com/questions/445505/… \$\endgroup\$– virCommented Nov 30 at 7:47
Add a comment
|
3 Answers
\$\begingroup\$
\$\endgroup\$
The power supplies are not intended to be used in a scenario where you only load the 12V output and use no load on the 5V output.
Older power supplies may not even start properly or regulate the 12V stably if the 5V is unloaded.
So basically, ATX supplies are not intended to be used for any arbitrary purpose in any arbitrary way, so you need to make adapter circuits to keep the supply running properly while using it in an unintended way.
5V and 12V may come from same flyback transformer and main regulation feedback may come from 5V or from both 5V and 12V. Having no load on 5V may make it unstable, unregulated, and close to trigger an undervoltage or overvoltage protection circuit when there are large load steps on 12V.
\$\begingroup\$
\$\endgroup\$
ATX power supplies usually have most of the components you’d need to build a well performing switching bench supply. But it would take a redesign, and rewinding the secondary side of the transformer.
They are great for a low voltage bench supply, say 0 to 4.5V, maybe to 5V if you can adjust the voltage setpoints higher. That is a niche application though. You would get 50A or more in a small package. The linear regulator would need to dissipate up to full PSU power, so you’d need a pair of large CPU coolers to mount the pass elements on.
Overall, it’s just not worth it. If all you want is a “0..12V” supply, find a junk 12V output transformer and build around it. That’s something a beginner could easily tackle.
Unfortunately, due to their specialization, ATX supplies aren’t good for much else without major reengineering work.
Personally, I use them as a source of magnetic components and high-voltage switches for my own designs. But that takes time, some instrumentation, and either know-how or good books. You’re dealing with 400V DC on the primary side of the transformer, with about 100A short circuit current capacity. If I drop something conductive across this HV DC rail, I’m getting hot copper splatter to the face.
answered Nov 30 at 9:58
Kuba hasn't forgotten MonicaKuba hasn't forgotten Monica
49.1k1 gold badge44 silver badges139 bronze badges
\$\begingroup\$
\$\endgroup\$
If you're working with an older ATX PSU (pre-2010 or less efficient models), adding a load resistor might still be necessary. Because the older designs used only the +5V output for feedback to a shared primary regulator, a minimum 5V load improved the performance of all outputs.
Bench PSU Conversions: Some hobbyists add resistors during bench PSU conversions as a precaution because they might not know the specific load requirements of the PSU they're modifying.
If you're converting a newer PC PSU, you might not need a dummy resistor on 5V unless the PSU shows instability during testing!
