A 500V 30mA Variable Linear Power Supply

Reaching for higher voltages, this was the next step after the 2x120V, 250V and 275V supplies. I needed only a little current (this PSU was intended as an avalanche pulser power supply), so I put everything on one PCB. The interior of this supply looks like this:

HV_PSU_500V_30mA_PictureSmall

Click here for a higher resolution picture …

At the left we have the input filter and a small 30VA 1:1 isolation transformer. In the middle, we have the PCB with a separate supply for the panel meters and the fan. The linear regulator for the 500V consists of a voltage doubler and an LM358-based HV regulator with a BUZ84 pass transistor. The schematics look like this:

HV_PSU_500V_30mA_Schematics

Click here for a Higher Resolution Picture …

As I had no SPICE model for the BUZ84 pass transistor, I used an available model for the IRFGB20 (I know its not the same). The unregulated voltage can be about 720-750V, so nothing to tamper with. The BUZ84 is old and has a rating of 800V, 6A and 125W. Here we stress it with 750V, only 30mA and 22W, and it is fan cooled. This is a design where the voltage pot sits at the low voltage side and can be grounded to the case. A similar design than the one above was used in Horowitz and Hills Art of Electronics.

The PCB looks like this:

HV_PSU_500V_30mA_PCB

This is a one-sided (mirrored) PCB.

Results were quite OK, it has only a few mV of ripple and is short circuit proof. Due to fan cooling the BUZ84 does not even get hot even when running with shorted outputs.

5 thoughts on “A 500V 30mA Variable Linear Power Supply”

  2. Hi,
    I build your according to your schematic and works fine, but when I turn off the power I get a big over voltage at the output… for example if I set 100v and turn off the output goes to 400v for about a second. There is some way to avoid this?

    Thank you and best regards

    Stefano

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    1. Hi Stefano,

      I dont know how you did it but I have a separate “Output ON” switch . For any power supply switching the primary on and off is risky because the regulating electronics enters an undefined state and overshoot or spikes can happen. For high voltage the procedure is
      1) Start PSU with no connections and output switch OFF
      2) Attach load
      3) Enable output switch
      When turning off, same in reverse.

      If you insist the following trick works:
      Make an (electronic) relay circuit that loads the output heavily (low value resistor). The relay circuit is deactivated if line power is present.
      Such circuits are normally used to bleed off the charge in high voltage capacitors.

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  3. Thank you for quick reply, I think I found a solution, increasing the value of the filter capacitor on the auxiliary power supply solve the problem, from 100uF to 20000uF gives the time to the HV capacitor to discharge leaving the control part supplied for all the time needed.

    Best regards,

    Stefano.

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  4. I am thinking of a output switch. The problem is I cannot determine the type of switch. All switches available are rated for a contact voltage of 250V max. There is no relay or switch in the market that can handle 600v safely. What can be done ?

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