Power supply - Littlefuse questions - Page 1
Forum section: electronics / repair

author: andersthuresson
author_rank: Contributor
timestamp: March 12, 2021, 12:43:58 pm
content: Hello smart people!I am currently trying to repair a power supply unit. This power supply is belong to a Brüel&Kjaer PSTN telefon interface from the 90's. It is actually a part of a whole test system. A while ago magic smoked escaped from it and I found that the Schaffner filter sitting on the mains was cooked. I changed that and it powered up nicely, for a little while. After that it stopped working. After another inspection I found that the +5V rail is missing. I have the +24V rail so it's not completely dead.I found some Littlefuse components (LF1A) that were blown. 2 of them infact. These fuses are connected between a transformer and a schottky in a TO220 package. After that the rail goes to a connector and out to other PCB boards in this unit. There is of course a reason to why these fuses are blown and I need to investigate it further. This is my first time coming across these Littlefuses and despite using Google I did not find all answers. I hope you can help to clear up some questions up for me.These little fuses are in a yellow axial package, marked LF1A. I did not see any other markings. How to know if the original ones are slow, medium or fast blowers? I assume they are 250V/1A rated based on there in the circuit they are located, but I would love to know if there is a way to see the voltage rating on these fuses? Can such Littlefuses be replaced by other types? DigiKey have a lot to choose from but I could not find any fuses that looks the same as the originals. I also checked other sources. A picture show the same yellow diodes with marking as I have in the power supply. I need to find suitable replacements.Cheers!

author: tooki
author_rank: Super Contributor
timestamp: March 14, 2021, 01:23:49 pm
content: Quote from: andersthuresson on March 12, 2021, 12:43:58 pmLook at the product website:https://www.littelfuse.com/products/fuses/axial-radial-thru-hole-fuses/pico-fuses.aspxInspect those products and you’ll find that only one matches. (I won’t tell you which, because I think it’s important to learn how to find information, and it just takes practice!)

author: andersthuresson
author_rank: Contributor
timestamp: March 15, 2021, 05:04:46 am
content: Thanks for the link. I been checking that website before but there are so many to choose from. Some are very quick blow and some are medium or slow. There are no markings on the original fuses, as far as I know, that will tell me which one to choose. Today I de-soldered these blown fuses and found that they are 5A and not 1A as I said before. There are in total 14pcs of LittleFuses on this board. These blown fuses are connected between a transformer that give 40VAC out and a schottky rectifier. I understand these fuses are there to protect the transformer. I added 2 photos with the markings I can see on the de-soldered fuses.I can see the common rating of these fuses on the website is 125V. So in my case it should be okay to use one 125V / 5A rated, but I still don't know if I need very fast blow or slow. Surely I am posting this question because I don't understand enough about fuses, rating and the theory behind these types of fuses. I could of course buy very fast ones and see if they will blow as soon as I turn on the PSTN interface or if they will hold.This PSTN simulation interface is obsolete and B&K in Denmark doesn't offer support or service. They didn't even manage to dig out a schematic drawing or service manual. This mean I am on my own here. 10 years ago this unit cost around US$40k so I don't want to mess this up, which is why I come here to consult people who are more knowledgeable about fuses than I amCheers!

author: tooki
author_rank: Super Contributor
timestamp: March 15, 2021, 07:02:08 am
content: Quote from: andersthuresson on March 15, 2021, 05:04:46 amThe color matters — only two are yellow, and they differ in their markings (one has a date code, one doesn’t) and available values. If you’re unsure, it certainly doesn’t hurt to send them an email.For what it’s worth, in most power supplies, slow blow fuses are used, because minor current spikes are no big deal, so you don’t want nuisance blows. (The fuses are there for short circuit protection and/or gross overload).

author: helius
author_rank: Super Contributor
timestamp: March 15, 2021, 07:26:57 am
content: Tooki is right; if you look carefully at the images on that page, there are only three possible series for the fuses you are working with.And from those three series, if you download the datasheets, you will find that one of them is immediately eliminated. Of the remaining two, only one matches the markings on your fuses.

author: andersthuresson
author_rank: Contributor
timestamp: March 15, 2021, 09:20:52 am
content: Thanks guys for your reply. I am learning a lot of new things regarding these fuses thanks to you. This is why I love EEVblog. I did not know that color matters. I know all color coding of resistors by heart but these things are all new to me.Yes, I see the 2 yellow ones on the website but the problem is that one is very fast acting and the other is slow blow. I agree that in the power supply application it is more logical that it is the slow blow version that is used. A fast acting would be more prone to blow due to power ON/OFF spikes, etc. I guess I just feel a lack of confidence when it comes to these fuses because if I get it wrong it could end very badly. It might be a very expensive BBQ. Brüel&Kjaer who is the manufacturer will not to anything to help.I therefore took the advice and contacted Little Fuse support, hoping that they can say for sure which model of fuses I need in this application.Cheers!

author: tooki
author_rank: Super Contributor
timestamp: March 15, 2021, 11:43:05 am
content: Quote from: andersthuresson on March 15, 2021, 09:20:52 amSo look at the yellow slow blow and fast blow more carefully. You’re looking for a 5A fuse, remember.

author: helius
author_rank: Super Contributor
timestamp: March 15, 2021, 08:34:23 pm
content: Do you two see the same series that I do? I see three series that are yellow, without plastic sleeve:471, 473, and 521. 521 has a date code and is only available in 2A.471 is time lag (in other words, medium speed), and 473 is slo-blo. The specific markings are a "T" on 471, and not on 473.It looks like your fuses in Reply #2 have a T in a triangle.

author: andersthuresson
author_rank: Contributor
timestamp: March 16, 2021, 07:51:10 am
content: I finally ordered from DigiTech. LF5A slow blow in yellow package. At the same time I ordered LF5A fast blow (green color) just in case. I will monitor the temperature of the new fuses with my FLIR, and also the rest of the power supply after changing fuses.