replacing fusistors?  - Page 1
Forum section: electronics / repair

author: coppercone2
author_rank: Super Contributor
timestamp: January 07, 2023, 02:09:36 am
content: So alot of equipment I have has fusistors inside of it, which are described as a slow blow fuse that has a resistor inside of it.Is this just a different mechanism for slow blow, where the resistor acts like a heater to blow the spring?If I replace with a equal rating slow blow fuse, does the protection work the same?Does it serve any purpose for inrush limiting, where the modern slow blow fuse will not act as a inrush limiter? Or by design is the resistance kept the same?I don't know what value of 0.8 amp fusistor I had. Its just a 3AG fuse with a resistor in it (with bands, but their destroyed), glass body.I bough a 0.8 amp replacement slow blow fuse for the circuit from digikey, but I am wondering if I should be thinking about soldering a series resistor in the circuit now?

author: tooki
author_rank: Super Contributor
timestamp: January 07, 2023, 05:20:27 pm
content: It’s just another mechanism for giving it the slow-blow behavior. One end of the resistor is attached using low-temperature solder. The resistor heats up, that solder melts, and the spring pulls the resistor away, opening the circuit.Every fuse has resistance — the dissipation is how the fuse wire heats up to melt and open. Don’t add a series resistor.Just look for T (slow-blow) or TT (extra slow-blow) on the original fuse and replace with an equivalent. If it doesn’t say, assume it’s a T. Of course, you can always share what is printed on it. Or Google the service manual and see what they specified. (In fact, doing that is never a bad idea. You never know if someone replaced the fuse with the wrong kind in the past. I recently helped out a professor where a multimeter input fuse blew and he needed a replacement. Turned out, instead of the 2A standard blow it was supposed to have, it had at some point been replaced with a slow-blow 800mA fuse. Good thing I checked the service manual instead of handing him a like-for-like replacement.)Here’s a discussion about it, with a hilariously wrong TV repairman replying with incredulity at the mere existence of such fuses:https://www.electro-tech-online.com/threads/how-to-replace-this-fuse.157617/

author: wraper
author_rank: Unknown
timestamp: January 07, 2023, 05:40:55 pm
content: If you mean fusible resistors, no usually you cannot replace them with a fuse. They are resistors which are made to fail open predictably and safely. There are no springs, construction is similar to usual resistors. Most often can be found in low power SMPS replacing both inrush limiting resistor and fuse.But I suspect you are incorrectly calling a type of fuses as fusistors.

author: wraper
author_rank: Unknown
timestamp: January 07, 2023, 05:54:45 pm
content: Do you mean these? They have about the same resistance as usual fuses so are not effective for inrush current limiting.

author: coppercone2
author_rank: Super Contributor
timestamp: January 07, 2023, 10:41:23 pm
content: yeah thats the one. The fuse I have says MDL1 on it, when I search for it, you get a different fuse without a resistor, and the manual says 800mA Slow Blow fuse.So I bought a 800mA slow blow fuse.If the resistance of the resistor spring fuse and the new slow blow fuse are the same it does not really matter. I was wondering if maybe the new fuses had generally lower resistance, and if that was the case, to add some series resistance.I have 3 of the same unit, and I took the fuse out of the other one, this one looks like instead of a resistor they have a carbon block. This one has 8/10 Littlefuse written on it, and maybe "313". It measures 1 ohm approximately.And the other unit has a 1A fuse (even though its supposed to be 800mA) that looks like it has a thin hair of wire inside, that one measures only 0.25 ohm. I think this might be a fast blow that someone put in there.My plan was to replace all of them with the new 800mA slow blow fuse from digikey anyway.https://www.ebay.com/itm/372653032611(but thats not the same one as the first amp, which had the fuse that has a labeled resistor instead of a carbon block) IDK why some fuses use the painted resistor and other use the carbon block (cost?)####So basically no good reason to try to find the old fuse that might have some kind of carbon block or resistor in there and just use new slow blow fuses from manufacturer because the change in series impedance will be negligible.? My concern was that maybe the small series resistance adds some necessary dampening during turn on and over current protection of some sort. But I figure the impedance of the transformer is >> any fuse you can put in there anyway, so its not doing much, only if the transformer shorted to ground on the first turn it would be significant, but that still seems pointless to worry about since the fuse will blow really quickly in that case regarldess of what kind it is @ that current rating, and it might be better to have just metal there, since the carbon is kinda a bad material compared to wire anyway. I bet the industry moved away from them because of reliability reasons of some kind, the helix on ceramic bobbin seems better then having in line carbon to metal bonds.

author: tooki
author_rank: Super Contributor
timestamp: January 07, 2023, 11:20:33 pm
content: Littelfuse 313 are still made, and they’re indeed slow-blow (or Slo-Blo™ to be precise). So yeah, any slow-blow should be just fine.

author: coppercone2
author_rank: Super Contributor
timestamp: January 08, 2023, 02:43:21 am
content: oh yeah but they don't have the resistor inside, its just supposedly the same performance

author: coppercone2
author_rank: Super Contributor
timestamp: January 08, 2023, 02:45:33 am
content: but for the 313 fusethe one thats on the website now, for 800mA, says its 0.54 ohms. The one I measured (same series with the carbon cylinder) leads a little over 1 ohm (the one that says 8/10 littlefuse 313).While its a small difference ohms wise, its 100% decrease in series resistance. I assume this means a instantaneous fault current goes from 120A to 240A (assuming 120V line voltage). Putting a 1/2 ohm resistor seems silly to me, but it would be to spec with what the unit shipped with (I assume)...

author: wraper
author_rank: Unknown
timestamp: January 08, 2023, 08:43:22 am
content: It's nominal, not maximum resistance spec. Carbon resistors often drift to high side over time too.

author: tooki
author_rank: Super Contributor
timestamp: January 08, 2023, 01:32:16 pm
content: Quote from: coppercone2 on January 08, 2023, 02:43:21 amAnd this is a problem… why?The resistor is just one implementation. It’s not a design goal in and of itself, nor is the series resistance.Quote from: coppercone2 on January 08, 2023, 02:45:33 amWhat makes you think any of that matters? It’s a line fuse. Whether 120 or 240A flow for an instant doesn’t matter, that current will blow the fuse instantly either way.The fuse’s purpose isn’t to limit current. It’s to protect against faults. A slow-blow fuse simply avoids nuisance blows due to initial inrush that would blow a fast-blow fuse.