I am some what new to electronics. I have been designing a variable bench power supply from an ATX.

I want to add two output power switches: One to the low current low voltage variable output rail (min 1.8 V 20 mA and max 12 V and 3A) and another to a higher voltage and current output (min 5 V 800 mA max 30 V 10 A.) These two outputs will be accomplished by two different DC-DC converters. Both “rails” will be isolated from each other via an on-off-on toggle power switch, so both rails won’t be used simultaneously. A 12V PSU will be added to drive the meters and possibly the switches as well.

I have been looking into relays and also using MOSFETs as switches.

So the question:

  • What kind of switch would better suit my needs?
  • Any other idea besides relay or MOSFETs as switch?
  • If I use MOSFETs, how do I choose the right one for a variable voltage and current?

I hope I was clear enough and provided sufficient information.


1 Answer 1


It's good to add output control switches to homemade power supplies. It makes things a lot more convenient.

The simplest approach is to just simply route your output through a switch. However, that means your switch needs to be rated above the voltage/current you intend to run.

Option 2 would be to use a relay, controlled by a switch on the front panel. The relay, so long as you use one with good ratings compared to your usage, will be adaptable and durable to whatever you put through it. It's easy to find a relay whose contacts are rated for 20-30 amps and over a hundred volts. A bonus is that you can also easily wire in a LED indicator for the front panel too, showing that the switch is in the ON position. The front panel switch can be small and convenient, too (a toggle, a latching button, or even a lighted latching button) since it is only conducting the amperage required for the relay coil, which is usually not much.

If you have a spare 12 V for control, then you can get a relay with a 12 V coil. The switch can control 12 V to the coil, and the panel indicator can be in parallel with the coil. The result will be reliable and look nice.

I would not recommend a MOSFET for this. FETs need to be biased well to run well, and there may be cases where you set for a voltage where the bias doesn't work. Also, FETs have a body diode facing back to your power supply, which could cause problems in a dynamic lab setting.

  • \$\begingroup\$ I understood. Thanks a lot for the answer. I do have a few 12V relays. But not as module, just the relays. Now I have to figure out how to proper wire it. Let’s do some research. \$\endgroup\$
    – Leopardi
    Jul 13, 2022 at 9:35

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