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Desktop Power Supply re-used as Bench-top Power supply

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Compaq PS-5181-4C

I want to re-purpose a Compaq Desktop PC power unit (Model PS-5181-4C) as a bench power supply, principally for its 6 Amp 12 Volt output. The 5 volt output is 18 Amps. Total power is 185 Watts.

I cannot now recall what model of PC the power unit was fitted to.

The PCB is printed ‘1994 Compaq’ in the same style printing that the resistors are marked ‘R10’, etc, which leads me to believe that 1994 is a design date, rather than a date of manufacture.

The HP Support site says that '5181 4C' is a 'Retired Product' and is no longer supported by HP.

Many Google Search results later ...

I conclude that the PS-5181-4C is not an ATX design, but either an AT design, or a Compaq independent design.


The ATX design has a double row 20 or 24 channel motherboard connector (mine has single row connectors, one 11 channel and one 8 channel).

The ATX design has Yellow wires to carry 12 volts (mine has Orange wires).

The ATX has a single Green wire, which when shorted to a Black 0 volts wire, causes the fan to run when mains power is applied (mine has no green wire at all).

The ATX switch to initiate start-up is a non-latching, (momentary contact) design (mine has a latching, ‘push to make / push again to break’ power switch, on a twisted pair umbilical cord).


I found one web page that described the pin-out of my 11 + 8 channel connectors.

His advice to get them running was:

“(This) switching power supply is not stable (and will not work) unless one of its outputs is loaded to at least 5% of capacity (roughly).

This means you need to be continuously drawing at least 1 amp from the 5V supply or 300 mA from the 12V supply.

One way to do this is to hook a power resistor from 5 V to ground or from 12 V to ground, wasting some power as heat but ensuring that the power supply works”.


My arithmetic says that’s 5 ohms for the 5 volt line, or 40 ohms for the 12 volt line.


Even after implementing those measures, my example doesn't power up. All I get is all I've ever got, a twitch from the fan immediately after the switch is clicked 'on'. I have metered the switch, it isn't intermittant, when it's on it's ON.


Any advice please?


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I was wondering if you found anything out about this?  I have this model and want to do the same thing as you.  I'll probably just use an 8 amp Boost-Buck converter with it once I can keep it running. 



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No, I didn't find a resolution. Lowering the value of the resistor generated a stronger kick to the fan, but I convinced myself this was more a capacitive discharge kick than anything else.


I cut my losses by buying a  adjustable 200 Watt Bench top power supply. Fusion PS201ADJT.

There are two adjustments, voltage 5 - 15v, and the current limit, 0 - 15A (Yes, '15' in both cases). This has a built in digital meter to show the current and voltage.


An equavalent meter would have been an additional purchase had I got the Computer PU working, and that was limited to 6A at 12v, I didn't have a use for the higher rated 5v output, so really I'm better off with the Fusion unit.



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