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Computer psu questions ?

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tron87

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I recently salvaged a 500 watt atx computer psu to use as a bench power supply but have some niggling question that need answers.

Power rails specs of the psu are:
+3.3v 26a max current
+5v 27a max current
+12v 22a max current
-12v 0.5a max current


As this is a 500 watt power supply is the following within limits if i max out these power rails at the same time will it be within limits of the psu just want to be sure its safe ?

5v x 27amp = 134 watt
12v x 22amp = 264 watt
Total wattage 398 watt


Is it possible to make a higher voltage rail than 12v from using existing rails if so how and how much current can i safely draw on that rail ?


Do most computer psu have overload protection and can they run inductive loads such as a motor/relay etc without fear that it will damage the transformer ?
 
Just about everything you ask depends on the quality of the unit.

Yes, most have overload protection.

No, as 12 volts is really as good as it gets. While the -12 and +12 would give 24 volts using the -12 as common your current would be limited to less that 0.5 amp maximum.

Yes, they will drive inductive loads but just like any case when the load is inductive snubbers and filtering should be used to protect the PSU as well as any additional circuits.

You really don't want to run any given rail at maximum. This is especially true of multiple rails and again is a function of the quality of the power supply. This is what is known as cross loading.

Ron
 
Just load tested the 12v+rail and got the following results what i don't understand is how can the the voltage on the 12v+ rail drop so much when loaded at not even 50% max spec at this rate at 20 amp the voltage is gonna be significantly alot lower than 12v+ so why is it spec'd at 12v+ on that rail ?

no load 12.3v
3 amp 11.3v
4 amp 11.08v
5 amp 10.8v
 
It's a computer supply, NOT a general purpose PSU - as with any multi-rail supply, regulation only applies to one of the rails (obviously you can't regulate more than one) - and it's the important one (5V) which is usually regulated.

You can modify them to regulate off the 12V rail, which will keep that stable instead.
 
They are usually switching flyback designs with multiple output windings on the flyback transformer. Only one of these outputs, usually the lowest voltage, highest current, output is sensed to control the feedback loop of the primary side switching transistor. So the non-regulated outputs will have voltage variance with load.

Many of these powers supplies do not work well with no load on their feedback sensed output. Load the 3.3v output with a reasonable amount of current (5 amps) and the variation on the other outputs should reduce.
 
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Many of these powers supplies do not work well with no load on their feedback sensed output. Load the 3.3v output with a reasonable amount of current (5 amps) and the variation on the other outputs should reduce.
Does the load current on the sensed output aka 3.3v rail have to be the same amount of current that the 12v+ rail has in order to provide a more stable output voltage on the 12v+ rail ?
 
No, it just needs to be a load suitable of getting regulation going. Thus I suggested a 10 ohm 10 watt resistor, common off the shelf. Any US Radio Shack. The link I provided earlier shows the allowable +/- 5 % tolerance of the voltages out. The 12 volt rail shall maintain between 11.4 and 12.6 volts. That being under rated load.

Ron
 
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