Types of fuses: Slow Blow - Time Delay - Anti-Surge - Page 1
Forum section: electronics / projects

author: AllTheGearNoIdea
author_rank: Unknown
timestamp: May 22, 2018, 02:44:05 pm
content: I’m having some problems with fusing a power supply. The power supply has a toroid transformer and some relatively large smoothing caps. Fuse often work fine and then for some installation just blow. Im putting this down to combination of low source impedance from one site to another and also part of mains sine wave when power is applied. I’m currently using a 20mm time delayed fuse and idealy want something with a longer delay for the high inrush. I have done a bit of reading into various types i.e TD slow blow and anti-surge and I’m struggling to understand if there is any difference between these terms or is this just marketing wank etc for exactly the same device.So what type of fuse will have the slowest blow characteristic ?I dont have any more information at the moment im awaiting a datasheet for the transformer. Its 110V primary probably 200VA with two 24V secondary windings. I think we are using a 3.5A TD fuse at the moment.I want to install the largest fuse possible without the TX going on fire with a short in the secondary etc.Many ThanksRegards Chris

author: rstofer
author_rank: Super Contributor
timestamp: May 22, 2018, 03:54:35 pm
content: I can't help with the marketing stuff but most reputable fuse manufacturers produce a graph showing clearing characteristics. You are supposed to compare the graphs of various types to see how they differ in your area of concern - essentially the first few milliseconds.http://www.littelfusesales.com/public/docs/POWRGARD-fuse-characteristic-curves-charts.pdfNote that I said reputable manufacturers, WonHungLow fuses probably don't qualify.Google for 'time delay fuse clearing graph', there is a lot of information out there.Get a copy of the Buss or Littlefuse catalogs and you can look into the various types of fuses. Fuses are available that are aimed at transformer inrush.

author: AllTheGearNoIdea
author_rank: Unknown
timestamp: May 22, 2018, 05:12:23 pm
content: Thanks for the reply I was hoping to just establish if there’s any difference between these before I start pouring over the data sheet. I suspect they are all one and the same with characteristics to match particularly standards. So my question would be is a slow blow the same as a anti-surge or time delayed. I think they are probably the same.

author: AllTheGearNoIdea
author_rank: Unknown
timestamp: May 23, 2018, 12:19:26 pm
content: After doing some more investigation there is another type of fuse TT which is super slow blow. It would apear that this is aimed at such very high inrush applicationx. So if slow blow, timed delayed or anti-surge wont cut try super slow.Regards Chris

author: Siwastaja
author_rank: Super Contributor
timestamp: May 23, 2018, 06:10:10 pm
content: As usual in electronic engineering, these kinds of terms are not very specifically defined; even if they were, they would be mixed and misused. As usual, you need to go out for the actual curves, and it's irritating that you need to read dozens of datasheets just to find whether a product could be suitableat all, but that's what you need to do.The type terms can be useful in parametric search only if you want to exclude a certain extreme. For example, you can often decide that you definitely don't want a "fast" fuse; or sometimes you want either "fast" or "semiconductor" fuse (the latter being hilariously misleading).

author: SeanB
author_rank: Super Contributor
timestamp: May 23, 2018, 07:40:57 pm
content: Add inrush limiting, using a NTC thermistor in the incoming feed. A 1R unit, as used in PC power supplies, will handle the inrush current quite well, yet will still allow the fuse to blow fast on fault situations without failing itself. Very common on a toroid for the inrush to be high, and the NTC inrush limiter is a very common solution, solving both the inrush problem and the high peak currents in the rectifiers degrading them, as they are stressed beyond the single cycle non repetitive current on switch on. Without it you find they will eventually fail, either short circuit or by breaking apart.

author: rstofer
author_rank: Super Contributor
timestamp: May 23, 2018, 09:53:57 pm
content: In the old days we used to joke that transistors were put in the circuit to protect the fuses.

author: AllTheGearNoIdea
author_rank: Unknown
timestamp: May 23, 2018, 10:17:30 pm
content: Thanks for the tips. This is an existing product and I don’t think we will do anything beyond looking at the sluper slow blow option. But we will probably re-spin the board in the next couple of years due to obsolescence and will almost definitely add something to reduce the inrush. I’m also investigating low inrush transformers, which I think it’s likely to be another way of putting a positive spin on copper losses.The equipment only gets switched off under power failure and so the components rarely see inrush. We usually find if the fuses doesn’t blow on the first day it’s good for the life of the product. I think this probably wasn’t tested fully by me when I did the original development due to poor supply impedance within my test facility.Actually if I re-spin the board I will probably remove the transformer and go for a switch mode brick instead