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WORKSHOP POWER SUPPLY

Modified: 15:27, 27 October 2013

SPECIFICATION:
  • +5v @ 5A
  • -5v @ 1A
  • +12v @ 5A
  • -12v @ 1A
  • +17v @ 8A unreg.
  • -17v @ 8A unreg.
  • +1.2 to 13.5v @ 5A
  • +3 to 28v @ 8A-5A
INTRODUCTION

This power supply was rebuilt from an original I made using a transformer I found that was rated at 200VA with a split winding of +/-12volts. The regulator was very basic but did have some very high power heatsinks for TO3 transistors and 2N3773 - 30A transistors!! It all needed updating so I decided to revamp the supply. I was given an old piece of test equipment which was faulty but the case was ideal for a new supply, being large enough to house the rather large transformer and two massive 100,00uF capacitors used for smoothing.

The result was a meaty power supply that could deliver decent currents when I needed them. The addition of a couple of meters resulted in a supply worth at least £200. It may not have been the most efficient with switchmode regulators about but it was simple and did the job!!

CONSTRUCTION

The picture above shows the transformer and 100,00uF capacitors. Right is a view of the bridge rectifier for all the supplies. It would have been better to have individual ones maybe, but just look at the size of it. I just had to put it in - its rated about 60A I think! Anyway space was tight in this case so no room for lots of smaller ones.

Its bolted to the base of the case to help dissipate any heat generated.

Spot the join - it matched at bottom!

The front panel had loads of various holes and cutouts for the original controls and displays. The easiest way to deal with that was ot cut a piece of sheet aluminium to the size of the front panel, clamp them together with small G-clamps, then treat as a new piece drilling through both panels when fitting new fixings. The detail was photo-copied in 2 parts onto acetates as mirror images, trimmed and stuck into place with spray mount.
Pretty professional finish!

Maybe a bit corny but the sheer grunt of this supply was surprising. In testing with 2 headlights, it could deliver over 9A constant @ 12v on the high current variable supply, and over 14A until the thermal fuse kicked in. The transformer loved it humming away nicely but those heatsinks did start to heat up!

POSITIVE FIXED REGULATORS +5volt and +12volt @ 5Amps
rail. This circuit was used for +5 and +12 volt sections. The LM338K is a 5A variable regulator in TO3 case, ideal for the heatsinks. The preset P11 sets the output voltage and was adjusted accordingly on each PCB. This circuit is based on the datasheet example and features full protection from short circuits, reverse polarity and thermal shutdown - in fact almost indestructable. TF1 is a resettable thermal fuse in case of any devastating faults in the regulator. R12 was changed to 680ohms for the 12v.

Left is the driver PCB for the +5v, +12v and 5A variable supplies - all drive the regulators LM338K on the rear heatsinks.

NEGATIVE FIXED REGULATORS, -5volt and -12volt @ 1Amp
The negative fixed rails are based upon the 7905 and 7912 regulators with similar protection diodes as above, but reversed of course. These were given their own heatsink inside the case.

A view of the negative regulator PCB and the high current variable supply driver PCB - both on end.

POSITIVE VARIABLE REGULATOR, 1.2 to 13.5volts @ 5Amps
This is very similar to the fixed regulation except that P51 has a potentiometer on the front of the case connected in parallel to it. The preset is adjusted to set the maximum voltage level. This provides a slightly non-linear control but it is not noticeable.

The transistor is used as a simple switch that will illuminate the LED if the regulator voltage rises above about 1.1v. The reason is that firstly the supply is variable and if the LED was connected directly via a resistor to the output, it would vary in intensity as the voltage was adjusted. Also the regulator drops to less than a volt when the output is shorted thus causing LED to go out and indicate a short circuit.

POSITIVE VARIABLE REGULATOR, 3 to 28volts @ 8Amps
To achieve high current this regulator is based upon a standard op-amp regulator, NE531 was used but a 741 would be fine. The voltage is set by adjusting VR61 being part of the feedback for the op-amp. T54 illuminates the LED as before. The output from the op-amp is first amplified by T51 and then T52 and T53 in parallel. These are 2N3773 30A transistors and provide ample current drive. An important note that there is no short circuit protection on this supply apart from the thrmal fuse TF6 - take care. This section will deliver 8 amps or more at voltages up to about 12v dropping down to about 6Amps at 24volts.

IMPORTANT
This section uses the -17.5v connection from the bridge rectifier as its "0v" terminal in order to attain the required voltage for the supply.

DO NOT CONNECT ITS 0V TERMINAL TO ANY OTHER 0V. THEY ARE NOT THE SAME POTENTIAL.
DAMAGE WILL OCCUR!!

CHARGER CIRCUIT 50mA
This was a last minute addition for charging batteries but I don't think I actually connected it up. This is a simple charging circuit (detailed on circuits page) that uses LM317 to provide a constant current generator - ideal for batteries.

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