Selecting the correct fuse for a 24VDC circuit

Thread Starter

Ja Jang Myeon

Joined Nov 2, 2018
Greetings again, all!

I have an PCB that is being powered by a medical grade 24VDC power supply by Meanwell, specifically GSM120A-24-R7B. In short, it is a 120W, 24VDC, single output channel 5A power supply. Inside the enclolsure where the PCB is, the power circuit is tied directly to the PCB board's input. I have been told to place a fuse on the positive voltage side, inside the enclosure, in-line between the entry point of the power and the PCB. Seems simple enough, except I have several questions...

1) Why would I need a fuse inside the PCB enclosure when the poser supply is a highly efficient, darned expensive medical grade power supply?
2) Assuming the answer to question 1 is do it anyway, is there a general rule of thumb for selecting the correct fuse?

The total watts required by the PCB is 25.

Thank you in advance!


Joined Jun 22, 2012
The only difference with medical psus is the Isolation Voltage, usually 4KV or higher. it's the same switchmode psu as a laptop brick, rather than breaking into the psu, i would put the fuse external on the moulded lead.


Joined Sep 17, 2013
The power supply should, if well-designed, have built-in protection against excessive loads, but it doesn't hurt to have a fuse as back-up protection for it. A 2A fuse perhaps, given that the expected load is ~1A?
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Joined Jun 19, 2012
Sounds like you have been sent on a fools errand, did they tell you why it's supposed to be there?

What is the load, and how much does it draw?

In most cases, fuses are intended to be fire protection elements.
If the wire gauge is sufficiently small, such that 5A current could cause it to catch fire, then a fuse rated for the wire is appropriate.