Protecting an AC to DC Power Supply - with a fuse

Thread Starter

Lumenosity

Joined Mar 1, 2017
518

24v - 20A Switching Power Supply
Is there a reason you could not put a fuse on the V+ line coming from this power supply to protect it from over current?
Specifically, an inline ATO 20Amp fuse

Condition 1).
You accidentally connect this power supply to a 36v battery. Would a fuse on the positive line(s) protect the Power Supply?

Condition 2).
You accidentally short the positive and negative leads.

Condition 3).
Any possible current overload beyond 20Amps

Thanks
 

Thread Starter

Lumenosity

Joined Mar 1, 2017
518
Except for the purejunk supplies, they all have an incoming power fuse on the PCB. Some are overload protected and some are not. Anoutput fuse might protect an outside load and wiring, and thus bee a good idea. Only an external series diode will protect it from damage by connectingto an external higher voltage source.
Thank you.
Connecting to an external higher voltage source needs a diode for protecting the PSU.
This is exactly what I needed to know.

Is there a formula to determine the size diode needed or will any diode do?
And I assume the diode will need to oriented with the cathode facing the external voltage source (on the Positive line) ?
 
Last edited:

Tonyr1084

Joined Sep 24, 2015
5,990
The diode you choose must be capable of handling the full 20 amps the supply is capable of putting out. Otherwise you'll blow the diode every time you draw more amperage than the diode can handle. In your case I'd recommend overkill. That is to say - use a diode capable of handling 1 1/2 times to 2 times the 20 amp rating. (30 to 40 amps). This way you'll never run your diode hot. Even if you're not using the full 20 amps.
 

anniel747

Joined Oct 18, 2020
1,122
Only an external series diode will protect it from damage by connectingto an external higher voltage source.
Or a crowbar.
Thank you.
Connecting to an external higher voltage source needs a diode for protecting the PSU.
This is exactly what I needed to know.

Is there a formula to determine the size diode needed or will any diode do?
And I assume the diode will need to oriented with the cathode facing the external voltage source (on the Positive line) ?
A crowbar is a far better solution. https://en.wikipedia.org/wiki/Crowbar_(circuit)
 

anniel747

Joined Oct 18, 2020
1,122
The diode you choose must be capable of handling the full 20 amps the supply is capable of putting out. Otherwise you'll blow the diode every time you draw more amperage than the diode can handle. In your case I'd recommend overkill. That is to say - use a diode capable of handling 1 1/2 times to 2 times the 20 amp rating. (30 to 40 amps). This way you'll never run your diode hot. Even if you're not using the full 20 amps.
Don't forget the voltage drop of the diode, a crowbar doesn't have that problem.
 

Tonyr1084

Joined Sep 24, 2015
5,990
Well, if 24V is critical, yes, a crowbar circuit. But 23.4V is pretty close to 24V, so I don't think a diode will cause any issues.
 

Tonyr1084

Joined Sep 24, 2015
5,990
1621442037879.pngOK, a trim pot. Not what I'd normally consider an adjustable PS. This doesn't answer what I was thinking of - is this one of those adjustable supplies with panel meters and such that are designed for bench top adjustability. So - I would assume - and we all know how to spell ass-you-&-me - it probably can be adjusted at least 0.6V above 24VDC. But that's just an assumption on my part.

For my money (what little there is of that) I'd opt for a 30 amp diode as opposed to a crowbar circuit. But that's just one opinion; and opinions are like arse holes - - - everybody has at least one, and some of them probably stink.
 

anniel747

Joined Oct 18, 2020
1,122
If we had the part number/datasheet there would be no assumption necessary. For what we know there is probably a crow bar already in there as most do except for some no name chinese copies I saw rebranded with fake reputable pedigrees for immense profits.
 

Thread Starter

Lumenosity

Joined Mar 1, 2017
518
Yes, we're talking cheap Chinese PSUs, where many YouTube reviewers have opened them and found many used parts inside,
crushed capacitors, Mosfets not making contact with the heat sink etc....roll of the dice.
They are readily available on Amazon and ebay. I do have a few better PSU's on hand such as Mean well. They really are better.

The diode solution is just so much easier to implement. I'm not concerned about less than a volt drop.
Just something to help guard against connecting to a higher voltage rating than the PSU.
I haave a bench full of battery banks some in parallel, some in series.

But one question: Wouldn't I need to get not only a 30A diode, but also one with the correct voltage rating?
How will the 30A diode know to allow the 24v to pass.....but block anything above it? Just buy a 24v 30A diode ?

I looked and couldn't find such a beast.....DigiKey
 

anniel747

Joined Oct 18, 2020
1,122
Yes, we're talking cheap Chinese PSUs, where many YouTube reviewers have opened them and found many used parts inside,
crushed capacitors, Mosfets not making contact with the heat sink etc....roll of the dice.
They are readily available on Amazon and ebay. I do have a few better PSU's on hand such as Mean well. They really are better.

The diode solution is just so much easier to implement. I'm not concerned about less than a volt drop.
Just something to help guard against connecting to a higher voltage rating than the PSU.
I haave a bench full of battery banks some in parallel, some in series.

But one question: Wouldn't I need to get not only a 30A diode, but also one with the correct voltage rating?
How will the 30A diode know to allow the 24v to pass.....but block anything above it? Just buy a 24v 30A diode ?

I looked and couldn't find such a beast.....DigiKey
https://www.digikey.com/en/products/filter/diodes-rectifiers-single/
 

anniel747

Joined Oct 18, 2020
1,122
Yes,
As posted above, I looked on DigiKey and there was no such thing as a 24V 30A diode
Mostly all the diodes filtered for you are ok (except some Digi-Key quirks that let thru a few 25A or lower) for your usage. Why would yo want a 24V diode if you mentioned 36V batteries?
 

MrChips

Joined Oct 2, 2009
23,536
Well, if 24V is critical, yes, a crowbar circuit. But 23.4V is pretty close to 24V, so I don't think a diode will cause any issues.
At 20A the diode forward voltage is not going to be 0.7V.

I was thinking more about the dissipation caused by the drop. The power supply can be adjusted to compensate for the drop.
At 20A the diode forward voltage could be 3V.
That's 60W power dissipation. Yikes!
 
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