Quick beginner question about fuses

I am trying to fuse a power supply and dont have much experience. The power supply calls for a 750ma fuse, I am using 117ac volatage. The very closest i can come to this fusing from digikey in a panel mount configuration is probly like 350ma@125vac. So I am wondering what to do? Is there any way around this? Do i have to resort to putting a pcb in just to mount a stupid pcb fuse?

Thanks, arit

Arit,
I'm not entirely sure I understand your question.
Are you saying you can't find a 750mA fuse, or you're needing methods in order to mount it?

peter

You should not worry too much about voltage rating as long as it is at least as much as you have in your application. It is common to use 250V fuses on mains applications. Just make sure you match the current rating as close as possible.

use 1 amp fuses?

800mA fuses should be easy to find.

Best regards,

Mikkel C. Simonsen

peter-I am looking for a fuse and fuse holder that is panel mount.

thomas-So it is ok to use 250 on 117? Is it just current then that blows fuses? I was thinking watts.. if that makes any sense. If 250 works though that would be perfect, then i can just use a 3ag size, nice and familiar.

mikkel- i tried looking for 800ma at digikey, not much there either, they just dont have much for 117, or 125 in a panel mount model until you start getting over around 5 amps.

Thanks

excellrec,

Volts don't really figure into the equation, as far as when the fuse blows, since the fuse element is of a low enough resistance that negligible voltage is dropped across it. Rather, it's the current that's important. Since power dissipated in the fuse element is what causes it to blow open:

P = Vsq/R or Isq * R

HOWEVER, the voltage rating of a fuse is still very important for other reasons. The voltage rating refers to the maximum open-circuit voltage permissible, so it's perfectly OK (and quite common, in fact) to use a 250V-rated fuse in a 120V circuit. If a fuse having a lower voltage rating than the circuit voltage is used, in the event of overcurrent, it may "flash over" and fail to protect the circuit, explode, etc.

Good reading about fuse ratings and terminology:
http://www.connectingindustry.com/news_story.asp?story_id=13649

ok, thanks alot for the clarification!

> then i can just use a 3ag size, nice and familiar.

Yes. Use 3AG or 20mm, standard fuses. You can easily find them down to 1/2A.

"250V" is the maximum safe voltage when blowing. If you used a little car-fuse in a 13,000V power-line, then after the fuse "blew" the high voltage would just jump-arc across the tiny fuse and current would continue to flow. Using "30V" car-fuses on 120V is slightly dangerous: they won't arc, but they may suffer a lot of damage before they break 120V pressure, which could overheat the fuseholder. The standard fuse for either 120V or 240V power is the "250V" rated fuse.

The fuse-holder itself may be rated much higher than you need. I think good 3AG fuseholders are rated 10A or 20A. But you can still use a 1A fuse in them.