Fusible resistor value help needed - Page 1
Forum section: electronics / projects

author: Rob2
author_rank: Newbie
timestamp: March 20, 2022, 01:22:45 pm
content: I have assumed this is a fusible resistor but am struggling to be sure of the specs. It is the first component on the live input of a 25W charger Li-ion battery charger.

author: Ian.M
author_rank: Super Contributor
timestamp: March 20, 2022, 02:53:17 pm
content: You can't be sure of the specs as there is no way to determine the manufacturer, resistor series, and thus locate a datasheet, but reading the color code gives 1 ohm 5%, and any fusible resistor with the same body size (and thus similar wattage) should do as a replacement.However your problems don't end here as a blown out fusible resistor frequently indicates blown semiconductors down stream of it. If you are lucky, one of the diodes in the mains rectifier is blown (if its in a discrete bridge replace as a pair with the one on the opposite side that carries the return current for the failed one), but its even odds its the chopper (or PFC boost transistor if it has PFC) blown, which usually take out their controller ICs and often surrounding parts when they go.

author: Rob2
author_rank: Newbie
timestamp: March 20, 2022, 10:21:27 pm
content: Thanks very much indeed for your concise reply; it certainly gives me a good starting point.

author: Berni
author_rank: Super Contributor
timestamp: March 21, 2022, 07:40:39 am
content: A lot of these fusible resistors you might find in cheep PSUs might not actually be a fusible resistor but just some random low value resistor. All of the modern trough hole resistor designs are actually an insulating core with resistive material added on the outside. So once way too much power is put trough them the thin resistive material on the outside burns off, leaving the insulating base material.Even the resistors designed to be fusible are not precise things. You won't have one reliably pass 1A and then quickly fail once you place 1.5A across it. The resistor is there for when something goes very wrong in the PSU and it becomes practically a short circuit, sending a massive amount of current that blows this resistor up before the huge current can cause more damage.So as long as you replace it with a similar resistance and size fusible resistor you should be fine.But as others have said there is usually a reason why it blew. Sometimes you can have lightning blew these, but most of the time it is because the PSU failed in a bad way. So replacing the resistor might just blow it again. I would check all the power semiconductors in the PSU first.

author: Rob2
author_rank: Newbie
timestamp: March 22, 2022, 09:56:10 am
content: Thanks very much for the extra input, I tend to get lost down rabbit holes so am avoiding lots of research on this! I will attempt a fix this morning and may try to meter what is going on at the location as all easily measured diodes and the rectifier test ok. I certainly am way below any digital fault finding or real circuit analysis so tend to rely on pretty basic knowledge, a lot of visual inspection, some simple meter work, and good luck

author: gamalot
author_rank: Super Contributor
timestamp: March 22, 2022, 12:40:47 pm
content: FYI, the size of the 1/2W 1 ohm wirewound fusible resistor I found is 9*3mm

author: Rob2
author_rank: Newbie
timestamp: March 26, 2022, 11:30:32 am
content: That is certainly a very good physical match, thank you - is fusible mentioned and do you have a link?.

author: Jeff eelcr
author_rank: Frequent Contributor
timestamp: March 26, 2022, 11:41:47 am
content: That is a metal film resistor.Jeff

author: gamalot
author_rank: Super Contributor
timestamp: March 26, 2022, 01:19:32 pm
content: Quote from: Rob2 on March 26, 2022, 11:30:32 amYes, they claim it's fusible, unfortunately, maybe you've already noticed the Chinese in the picture, they sell it on Taobao in China.https://item.taobao.com/item.htm?spm=a230r.1.14.63.3ede6b69HfoGEn&id=607641731287&ns=1&abbucket=15#detailThere are many sellers selling similar resistors, the link I gave is just one I randomly selected, I have nothing to do with the seller.

author: Jeff eelcr
author_rank: Frequent Contributor
timestamp: March 27, 2022, 12:28:32 am
content: I have also seen 1 W resistors the size of 1/2 W and I thought wire wound were marked by a wider first band?Metal film can be used as (fuseable or flameproof.)Jeff

author: Ian.M
author_rank: Super Contributor
timestamp: March 27, 2022, 12:45:02 am
content: Not all metal film resistors are flame-proof. e.g. many with glossy coatings can emit flames and sustain combustion when severely overloaded often before they go open circuit.Not all flame-proof metal film resistors are fusible - a fusible resistor, (like a fuse) is rated to clear a fault (up to a particular fault current and supply voltage) when it blows.Assuming our O.P's board doesn't have a mains fuse, odds are it *SHOULD* be a fusible resistor. However if it was designed and manufactured in a country with relaxed safety standards, its quite likely that its just a flameproof resistor with no certifications for use as a fusible resistor.

author: Jeff eelcr
author_rank: Frequent Contributor
timestamp: March 27, 2022, 10:16:15 pm
content: Ian yea sometimes you just have to go with best guess and pray but known manufactures usually have safety parts marked and will supply replacements or would, now?Now they make something, throw it in a box to you, for you to QC it for them.We actually use a 000 ohm surface mount (resistor) as a 3.2A fuse in one unit. (MFG no longer supplies parts to us.)Jeff