I'm building a replacement power supply for a Twinvision dot matrix LED sign (as would be found on the side of a shuttle bus). The sign requires four unique power supplies: 3x 3v3@4a and 1x 5v@1a. The 5v power rail feeds all of the TTL logic that drives the sign's constant-current LED drivers and the 3v3 supplies each go to a third of the 14x108 LED display.

I've gotten a working design using four RT8298 switching regulators fed from a 12 to 24 volt DC input. I'd like to send all of my PSU's outputs through appropriate fuses that will bust if the sign draws more than the desired 4a/4a/4a/2a for too long.

I'm not sure exactly how long these fuses should stay intact during excess current draw... The sign draws excess current on powerup but I'm not sure how much it draws or for how long. I don't know if there's a way to measure this using only my Rigol DS1054 and my Fluke 87V?

Even if I knew that figure, I don't know how it applies to "fast-blow" and "slow-blow" fuses which are usually rated for a certain amperage at a voltage far above 3v3.

Anyways, how do I go about selecting proper fuses for this system?

 

Typically fuses can pass 1.6 times their rating for a considerable time, and at 2.1 times their rating will operate within 30 seconds.

Don’t forget that you will need to supply appropriate fusing to protect the regulators – this maybe the most appropriate location for all fuses, protecting the 12-24V input from an overcurrent situation.

 

Protecting the voltage inputs of the regulators hadn't occurred to me, and that seems like a great idea. But if the psu is fed with 12v, wouldn't it blow much more slowly than under 24v?
Since they break in 30 seconds under 2.1 times their rating, would we be shooting for a ~2amp fuse on a 3v3@4a regulator input? Wouldn't that mean that the psu will still fry its fuse after a particularly long time?

 

The input current to the regulators at an input of 12V is likely to be twice that at 24V, therefore if you base your protection on the current draw at 12V you should be OK.

But there is no harm in fusing each of the 3.3V/5V outputs, just make sure that the chosen fuse rating does not result in nuisance fuse operation.

Bear in mind that in a vehicle the main purpose of fusing is to protect against a fire in the event of excessive current flow – whereas in mains powered circuits there will also be an element of protection against electric shock provided by the fusing.