Electronics > Repair
Help Identifying Fuse Type
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AstutePauciloquent:
Hello everyone, I pulled this blown fuse out of the high side off a computer power supply, im not able to find a replacement online by searching for the markings on the fuse; that read 250V 5G 12A.
I know the 12A means 12 amp, and that it is a fuse designed to be used with AC current (to prevent arcing when blown), with a maximum voltage of 250V. However im not so sure about the 5G. The fuse can be seen in this image. If anyone can help me learn what type of fuse it is, it will be very much appreciated.

Adding some more info: The Power Supply is a 650 watt. According to ohms law when it comes to calculating a fuse rating, we do 650 watts Divided by 240 (For New Zealand AC power). This gives me a rounded rating of 3 amps. I am definitely missing something here.

u666sa:
250V 12A fuse

You don't have to buy exactly same fuse, just purchase a regular glass fuse, and solder some legs to it. Secure fuse in 3rd hand, add flux, use heavy tip, solder leg from resistor, cut to size, repeat the other side.
srb1954:

--- Quote from: AstutePauciloquent on November 30, 2024, 05:36:25 am ---Hello everyone, I pulled this blown fuse out of the high side off a computer power supply, im not able to find a replacement online by searching for the markings on the fuse; that read 250V 5G 12A.
I know the 12A means 12 amp, and that it is a fuse designed to be used with AC current (to prevent arcing when blown), with a maximum voltage of 250V. However im not so sure about the 5G. The fuse can be seen in this image. If anyone can help me learn what type of fuse it is, it will be very much appreciated.

Adding some more info: The Power Supply is a 650 watt. According to ohms law when it comes to calculating a fuse rating, we do 650 watts Divided by 240 (For New Zealand AC power). This gives me a rounded rating of 3 amps. I am definitely missing something here.

--- End quote ---
For a large power supply like this there is potentially quite a high inrush current of many times the normal operating current. This appears to be a standard fuse that has no ability to withstand the inrush currents and the fuse rating has subsequently been increased to prevent nuisance blowing of the fuse from this inrush current. However, this increased rating can result in the risk of the fuse not blowing so quickly, or even at all, in some fault conditions.

A better solution is to use a time delay fuse, which is usually designated by a 'T' after the fuse current rating. These type of fuses are designed to not blow in response to momentary inrush currents but still reliably blow in response to a lesser, but prolonged, overload. The fuse current rating needs to be a little higher than the calculation from PSU wattage would suggest as you need to take into account the PSU efficiency and the power factor, both of which will increase the expected AC input current. I would recommend trying a 6.3AT fuse for this application.

P.S. Before inserting a new fuse I would check that there are no faults in the PSU. From the splatter inside the fuse it would suggest that there is a severe fault in the power supply, possibly a shorted MOSFET or diode.

AstutePauciloquent:

--- Quote from: AstutePauciloquent on November 30, 2024, 07:38:38 am ---
P.S. Before inserting a new fuse I would check that there are no faults in the PSU. From the splatter inside the fuse it would suggest that there is a severe fault in the power supply, possibly a shorted MOSFET or diode.

--- End quote ---

Thanks for the information, great stuff. I have checked the low side for shorts (no shorts where found). I'm yet to check the high side, and I suspect the same thing. That fuse blew for a reason, and it is most likely a high side short, that I will locate tomorrow.
fzabkar:
Is this a dual voltage PSU with a 240/120 selector switch and two bulk capacitors? If so, then perhaps the manufacturer has rated the fuse for 120VAC regions.
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