Help Identifying Fuse Type - Page 1
Forum section: electronics / repair

author: AstutePauciloquent
author_rank: Contributor
timestamp: November 30, 2024, 05:36:25 am
content: 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.

author: u666sa
author_rank: Frequent Contributor
timestamp: November 30, 2024, 06:17:23 am
content: 250V 12A fuseYou 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.

author: srb1954
author_rank: Super Contributor
timestamp: November 30, 2024, 06:57:01 am
content: Quote from: AstutePauciloquent on November 30, 2024, 05:36:25 amFor 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.

author: AstutePauciloquent
author_rank: Contributor
timestamp: November 30, 2024, 07:38:38 am
content: Quote from: AstutePauciloquent on November 30, 2024, 07:38:38 amThanks 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.

author: fzabkar
author_rank: Super Contributor
timestamp: November 30, 2024, 07:43:36 am
content: 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.

author: Harry_22
author_rank: Frequent Contributor
timestamp: November 30, 2024, 08:01:38 am
content: Yes, 12A is definitely a lot. I don't even have these in spare parts. Such an input fuses do not blow without a reason.You can use the legs for a new one. To do this, crush the glass and remove the caps.

author: cte
author_rank: Regular Contributor
timestamp: November 30, 2024, 09:27:52 am
content: That's a standard 5x20mm glass fuse (rated 12 A) withaxial lead fuse capsattached to it for direct soldering.You should be able to remove the caps to re-use them. But you should check thoroughlywhythe fuse has blown.

author: coromonadalix
author_rank: Super Contributor
timestamp: November 30, 2024, 12:16:34 pm
content: depends if it was a slow or fast blow

author: Harry_22
author_rank: Frequent Contributor
timestamp: November 30, 2024, 02:31:31 pm
content: Quote from: coromonadalix on November 30, 2024, 12:16:34 pmIt is universal(sorry, I got confused)

author: AstutePauciloquent
author_rank: Contributor
timestamp: November 30, 2024, 08:28:56 pm
content: I found what would have blown the fuse, this high voltage MOSFET. I have it removed and searching for a replacement. Chances are there are more blown components that i will have to find. But hopefully not.

author: Harry_22
author_rank: Frequent Contributor
timestamp: December 01, 2024, 07:04:15 am
content: Review elements at the Gate. If the track goes directly to the PFC controller via resistor, then it needs to be replaced.If you want to check, then unsolder this transistor, apply DC from 50V instead of mains and watch the gate with an oscilloscope.Check that it is PFC, otherwise anything can happen.Be careful. There will be 400V at bulky capacitor!

author: tooki
author_rank: Super Contributor
timestamp: December 01, 2024, 10:39:09 am
content: Quote from: Harry_22 on November 30, 2024, 02:31:31 pmThere is no such thing as a universal fuse.