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I am building a power supply unit





  • Port A: LM117 Regulated 1.2v - 13.5v Output x2
  • Port B: LM117 Regulated 1.2v - 13.5v Output x2
  • Port C: LM117 Regulated 1.2v - 13.5v Output x2
  • Port D: 7805 Regulated 5v Output x2
  • Port E: 7812 Regulated 12v Output x2
  • Port F: Raw 18v Capacitor Output x1
  • 3A Amperometer
  • Variable Threshold Overcurrent Beeper Circuit





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How about a PC power supply? I've heard of folks hacking those, can get several voltages from them. I've thought about it and have a few to experiment with.

Yes now I use a modified PC power supply, that's it


but i need a psu with more regulated/variable  outpus

so i decided to build a new one and keep the old one as a backup pcu...

About the needing for shotcircuit protection I think there is no need for it becuase 78XX and LM117 have internal overheat and shorcircuit protection


I have a question:

Where to conect ground wire from wall power network? the case is all plastic , so i was thinking to connect it to electrical 0V output (GND) 

Very cool PC supply! I'd think it would have most of what you need, being able to deliver higher current levels. I'd wondered about adding meters (digital) to one, since seeing current draw is handy in keeping something from frying itself sometimes.

I wouldn't connect the wall power earth ground to anything on your power supply output, but only to the frame of the transformer.  Here, if someone has miswired the house, could lead to a nasty surprise. 

a friend told me PC psu are switching psu and this may not be good because they are noisy...


Yes, PC power supplies are switching types.  They can be noisy at certain frequencies, but a bigger problem is that some switching PSUs need a minimum load to work properly.  That is, some kinds can malfunction if they have too small a load, or no load at all (open circuit).  Lab bench power supplies are normally analog linear regulators.

i know the problem of the needing of a minimum load for switching psu

infact I placed a fixed resistor in the case in order togive a certain current drain

please confirm:

I have to connect the earth wire to  transformer chassis AND 0V output...



Definately connect the transformer's metal parts to the mains Earth.  As for 0V, it's optional, and many commercial lab power supplies bring an earth connection out on the front panel in the form of a green 4mm socket/terminal.  That way, the use can connect 0V to Earth if required, and disconnect it if a floating supply is needed.  You can see the green terminal (lower right) in my photo of my Farnell bench power supply:


BeamBots would want to live in such a house!