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How To: Run Compaq ESP127 power supply model: PS-5501

I got ESP127 power supply from an old Compaq server some time ago, with intention to use it as my RepRap printer power supply. But I couldn't find any useful information on the internet on how to get it to power on. 
Today, I've decided to grab my multimeter and play with it to see if I will be able to get it to work. By default when you plug the power cord to it, the cooling fan will start spinning but the green LED at the back will stay off and there will be no voltage (or almost no voltage) on any of the power rails +3.3v, +5v, +12v. This power supply +12v rail can handle 25 amperes which makes it perfect for a reprap. 
After an hour or so... I saw a green light ;)

 I've soldered terminal socket to the 12v pads to make it more portable and easy to setup.


  1. hi,
    do you now also how to get out -12V,+5V and 3.3V?

  2. I'm not sure about -12V but 5v and 3.3v should be available at the bottom pads, just measure the voltage between ground and the pads

  3. Thanks, i got the same one for a project, i see you soldered/shorted in 3 places (2above, 1 below), in a normal ATX you only need to short 1. are those 3 shorts really needed? or just one? just making sure, thanks, don't won't to blow the thing up :)

    1. It was a while ago.. but I'm pretty sure these 3 shorts are needed, I had the backplate/edge connector for this psu and I have figured out the correct (hope so ;) )pinout from it.. I have 3 of these running without issues since almost 2 years :)

    2. Thanks from my prusa i3 whit new power supply ! :)

  4. Hi I have the same power suply and , as you , my green LED is not on but the fan is , how did you fix it ?


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