A High Power Fixed Voltage Power Supply for the HB Bench

Bench power supplies  I’ve been wanting to build bench supplies for a long time and this week I finally got around to it.

This supply is essentailly a header for a PC power supply.  The intent here is to have a lot of current at the ready, of a few of the most common fixed voltages

The PC power supply has all except the +9V – which I provide via an internal 3-terminal regulator.  Each line is individually switched and the LED help to provide a visual indication as to what is turned on and what is not.  That also ensures that power is cut immediately and there is no "bleed donw" delay.

The negative voltages are down-set from the rest of the positive voltage outputs to avoid late night accidental confusion.  The power switch is a toggle that activates the entire power supply.  

Using the PC power supply is a great choice when ultra critical demands are not present.  Most modern units feature short-circuit protection, overload, under and over voltage crowbars and automatic overload reset.  They are quite robust compared to the traditional linear supply we built back in the day. 

The enclosure is a 5.5x3.5x2.5” radio shack aluminum box – I really like these boxes – cheap and at the ready.  Holes were drilled for all the items, then I sanded the surface to give it a uniform matt finish.  The labels are black on clear.  And a top coat of a “fog” colored clear coat protects the labels.  The slight haze tint in the paint is really great at hiding imperfections.

  

 

In this interior view, the 9V regulator is mounted on the back.  The wiring is 14ga and the terminal strips serve as a stress relief.  Power is fed via a DIN plug mounted at the bottom.  All inputs are bypassed at this jack.  The rest of the design is unremarkable.

  

 

Load test results:

      Post             Voltage                                       Current capability

  • -12v       -11.85v idle                                    125ma max
  • -5v         -4.99v idle                                      250ma max
  • +3.3        3.41v idle, 3.3v @ 2A                    Infinite (30A?)
  • +5           5.27v , 5.08 @ 3.6A                       Infinite (25A?)
  • +9           9.01v idle, 8.90 @ 1.1A                 About 1A (limited by 12v supply line)
  • +12         12.08 idle, 11.85@4A                    Infinite (10A?)

Rear view showing the added heat sink.  Probably not needed given the 9V reg will not be dissipating more than a few watts.  

  

The most time consuming part of the project was making up the cable that runs from the power supply to the DIN.  

  

 

  

Finished unit mounted and ready for action.  

Since these pics were taken, I have moved to a "new" used 430W unit - the 12V supply on the initial supply was defective (which is why I had it in my junkbox).  So now all taps are functional and of full power.  

These PC fixed power supplys really needs to have a minimum load of approximately 5-10% of rated power - to ensure the regulation works properly.  I have some power load resistors to mount in the box someday when I get eager.  But as it is now, the regulation is FB for my purposes.

I think when I get the resistors mounted, I will put in some ceramic bypass units and some large electrolytics to help improve the regulation further in these lightly loaded conditions.  

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