Why fuses blow last in cheap power supplies

[Th3_uN1Qu3]

I bet some of you have asked yourself before... Obviously, because a fuse with too high a rating was used. But why's that?

Well, the answer is inrush current. A SMPS charges one or several capacitors directly from rectified mains. As you know, a discharged capacitor appears as a dead short. So they are in effect shorting the mains for a brief period of time.

Now, there are those things called inrush limiters, basically large NTC thermistors. Whose job is to limit the current that the caps can draw when the power supply is plugged in, because they have a high resistance when cold, which then becomes negligible when they heat up. But, on cheap supplies they are either skimped on or not fitted at all, so that means that for a few fractions of a second, you have several dozen amps flowing thru the poor fuse! For the unit to not blow fuses on a cold start, a higher rated fuse than otherwise necessary must be fitted. And the higher rated fuse allows the unit to go out with a bang before it finally opens.

That's another mistery cleared for ya.

[Toasty]

Re: Why fuses blow last in cheap power supplies

That and the fact the silicon will toast faster than the fuse anyday.

[Longbow]

Re: Why fuses blow last in cheap power supplies

For the unit to not blow fuses on a cold start, a higher rated fuse than otherwise necessary must be fitted.

Not necessarily. Slow blow fuses are used specifically for this purpose and will not blow until the overcurrent is sustained for a specific time period. High inrush current is a fact of life and there are standard ways of compensating for it. No NTC device is not proof that the designer didn't know what he was doing. Hope that clears up a mystery for you.

[riotpack]

Re: Why fuses blow last in cheap power supplies

Alot of switching controller ICs also have a soft start feature available (usually by an RC network on the shutdown pin) which acts like a NTC thermistor, but does nothing for the inrush due to the reservoir capacitor charging up, of course a thermistor is useless if you quickly cycle the power when hot! And the "silicon" blows first because it is the weakest link and fuses are slow!

[c_hegge]

Re: Why fuses blow last in cheap power supplies

I actually saw this happening for first hand today. I videoed an Ultrix "450W" exploding at 320W on the load tester. Looking at the video frame by frame shows that the fuse blew 3 frames (1/10 of a second) after the transistor.