70V 3A Lab Power Supply Upgrade

A few years ago I bought a second-hand Systron Donner HR 70-3AI power supply. This power supply works with a pre-regulation by “dimming” the transformer to limit the power dissipation. This works ok, but the psu has a rather big output capacitor (3300µF), which makes it unsuitable for testing circuits.

After a few months the psu broke down, and instead of fixing it I decided to upgrade it by adding a new lab power supply part. For this I used a board I got from an action on circuitsonline. The heat sinks are rather small and can not handle the 210W (70V x 3A), therefore I decided to reuse the pre-regulation and have it regulate about 10V above the output voltage. For this I designed an interface circuit.

Old power supply drive board / pre-regulation driver:

New lab power supply circuit:

Interface circuit rev 1:

Interface circuit rev 2:

Interface circuit rev 3:

Circuit diagram:


The boards mounted:

Other alterations that have been done on the power supply:

  • The power transistors have been changed to transistors of the MJ11016 type
  • An extra 2x9V AC PSU has been added
  • The output capacitor has been changed from 3300µF to 150µF
  • The potentiometers are changed to different values.

The new output cap:

Total overview:

On the outside nothing has been changed to the enclosure.



1 ping

    • Fernando J. Pompeo on July 29, 2018 at 03:05
    • Reply


    My name is Fernando and I need a 70V x 3A power supply to test underground train onboard equipment.

    Sorry my english.


    Fernando – Call sign PU2PLL

    1. Hello,

      Sorry, I do not have full schematics of this one. I think with some searching a similar power supply can be found on auction sites.


  1. […] with the previous PSU build kit (Lab Power Supply 0-70V 0-3A) I was not completely happy with the specs (/wanted to make it more of a challenge) and decided to […]

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