I have some old solenoids that operate at the unusual voltage of 72VDC. I'm powering them with a 48V and a 24V power supply in series (and controlling them using an Arduino with MOSFETs and the necessary protection circuitry). I have also placed protection diodes across the power supplies to handle one supply starting up before the other. This system has been working fine.

Now, I want to add some additional solenoids that operate at 24V. Assuming the 24V power supply is on the low end of the circuit (i.e., its ground is the one tied to the microcontroller's ground), and it is rated for sufficient current, would there be any issue with using the 24V supply to drive the additional solenoids? I can't think of anything specifically wrong, but it seems like the type of situation where theres some ground loop or inductive load risk or something hiding in there.

Summed up as a diagram:Image of existing circuit and proposed circuit

The reason I’m hoping to do it this way is because there are several dozen of the 72V solenoids, and I’d rather not replace all of them, and I can’t easily source any new solenoids with those specs. My backup option would be to just power the 24V ones off of their own supply, but if I can avoid adding a third supply it would be nice.



1 Answer 1


You have already specified that the supplies are rated for sufficient current, so normal operation will be OK.

There is a possible problem when you have all the 24 V solenoids energised. If you were to turn them all off at the same time, their energy would be dumped, via the protection diodes that you will probably have provided, onto the 24 V power supply.

If the 24 V supply is not capable of sinking current, and if the 72 V relays are off so it's not sourcing current to them, its output voltage will rise, possibly threatening over-voltage on the 72 V side.

This may not turn out to be a serious problem. The lift in the 24 V output will be limited by any bulk capacitors there are on its output. Relays are pretty robust devices, and your 72 V relay drivers should not have been designed 'down to the bone' so you should be able to tolerate some lift on their supply.

Do the experiment with just the 24 V supply and all the 24 V relays, to see whether there is any significant lift on switch off.

If it turns out that this could be a problem, then more positive steps to reduce or eliminate this lift would include

  • increasing the bulk capacitance on the 24 V supply
  • putting a clamp across it, maybe 26 V
  • returning the 24 V relay diodes to their own snubber, clamp or rail, rather than the 24 V supply, so their stored energy is directed elsewhere.
  • \$\begingroup\$ This was helpful and I'll give it a try, thanks. For anyone else's reference, the circuit protection I have for each solenoid has a flyback diode across its terminals, the MOSFETs controlling them are rated for up to 200V Vds, and the MOSFETs also have a zener diode to ensure Vgs is clamped to a safe level for the microcontroller. \$\endgroup\$
    – iii
    Commented Jul 4, 2023 at 21:06

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