Need advice : thermal fuse, is it iOK to down or uprate them slightly ? - Page 1
Forum section: electronics / projects

author: Vince
author_rank: Super Contributor
timestamp: November 24, 2020, 07:50:45 pm
content: Hi all,Not too sure if I should post this here or in the "repair" section....I am repairing something but don't need help on the repair itself, just help on component replacement.A colleague of mine gave me his old waffle cooker to fix. No electronics in it but just a thermal fuse that blew.Axial type, brand "SEFUSE", model "SF129E", 10Amps, cut-off at 133 °C.Problem : local shops and even Farnel don't have a 133°C fuse. It's either a bit less at 130°C or a fair bit more at 142 or 144°C.I DID find the exact same fuse, same brand model and rating etc, exact same device but.... only on Ebay & co, dirt cheap and from obscur vendors, and straight from china. Chances of counterfeit devices is too high, not sure I wanna take risks for a safety device, not to mention the usual 3 weeks delay (when all goes well...).So I would rather buy from a reputable vendor and/or local shop (no shipping costs and wait ), but that means I have to use either 130 or 144 instead of 133°C....The fuse is located inside the hinge of the " clamshell ".I have no clue about thermal fuses and waffle cooker designIf I take a 130°C fuse, it would blow more easily, and at 144°C maybe it will blow but when damage is already done, so... what would you do ?Is the rating not that critical ? If I use 130°C I should not be worried about the fuse blowing during normal use, for example ?Advise much appreciated, thanks in advance !!!

author: TheMG
author_rank: Frequent Contributor
timestamp: November 24, 2020, 08:08:22 pm
content: I wouldn't go any lower for sure. Often these things end up blowing not from an actual over-temp condition (but do verify the main thermostat operation to be sure) but from repeated thermal cycling too close to the max temp, or developing a bad connection leading to heating of the connection and fuse.Going to 142C (9C higher) I don't think should be a problem. 142C isn't really hot enough to set anything on fire as far as I know.Chances are 133C thermal fuses is what the manufacturer could easily get their hands on at the time. The exact value isn't too critical, as long as it's not too low so as to blow during normal operation of the device, but low enough that is blows before any components of the device reach temperatures that could result in fire.PS: this should go without saying, but is easy to overlook, DO NOT solder the replacement thermal fuse, crimps only!

author: Siwastaja
author_rank: Super Contributor
timestamp: November 25, 2020, 10:43:11 am
content: Lower blow temperature is the safer thing to do.Higher blow temperature solves a problem of nuisance blowingifsuch problem exists, but reduces any safety margin the original design has and thus, increases the risk of fire.The best thing to do would try to analyse why it blew in the first place, but it may be impossible to say.My guess is as the previous poster said, repeated baking cycles beyond what the intended usage has been. A well-designed waffle cooker would have a resettable protection that would trig in the first place, then the non-replaceable thermal fuse as a backup in case the primary protection fails, but a cheap unit may just get bricked if you like waffles more than the designer expected you to do.Personally, I would pick theslightlyhigher temperature rating fuse and take the increased risk of fire because I'm not leaving the thing unattended anyway, but this is something I cannot recommend on the interwebz forum, hence, pick the lower temperature rating fuse.

author: Ian.M
author_rank: Super Contributor
timestamp: November 25, 2020, 11:21:08 am
content: Order both, fit the 130 deg C one, and snake a thermocouple probe in there so you can monitor the max. temperature the thermal fuse is exposed to if you cook a large batch of waffles in quick succession.CAUTION: Treat the thermocouple probe as mains live!Many thermal fuses have metal bodies connected to one terminal and its difficult to adequately insulate the tip of the thermocouple while still getting good thermal contact. Therefore its preferable to use the thermocouple with a DMM that supports one or at least, a plastic cased, battery powered temperature meter, so its safe floating.If you see the temperature creeping up too close to 130 deg C, you may be able to live with the limitation of having to allow some cooling off time when batch cooking rather than fitting the 142 deg C one which, although many of us would probably do it for our own use, we cant recommend in general or for use by a third party.Remove the thermocouple before letting anyone else use it!

author: Vince
author_rank: Super Contributor
timestamp: November 25, 2020, 06:32:44 pm
content: Thanks all for you feedback, much appreciated.Just back form my local electronics shop, got very lucky : despite his online inventory show only 130 and 142°C fuses, inventory was not very accurate and turns out the the drawer supposed to hold the 130°C fuses, carried 133°C ones, just what I needeed ! What's more... his fuses are also the exact same brand and model as the original ! SEFUSE SF129E ! Bought 3 of them...So no need for thermal cameras and thermocouple, don't have any of that. Cooker is worth less than the price of the fuse, really... this is not a "project" repair...As for crimping... I don't have the required hardware, and not economical to buy any tools for this repair. Would be cool for future repairs though, but I have zero money for equipping my lab for now, other priorities, like fixing the car and house.Guy at the store gave me tips though, to kinda make a crimp, sort of.. .said it worked every time for him, so might give it a shot.However the manufacturer datasheet for the fuse, does say that it CAN be soldered, given some precautions. Like soldering at the very tip of the lead to keep as much lead lenght as possible, as well as reducing the iron temperature and using low melting tip solder like silver solder, which I happen to have. Also most important, use pliers or similar to clamp the lead between the body and the tip of the lead, so as to soak the heat before it gets to the body of the fuse. Maybe dipping the fuse in water while soldering might help as well, who know...That's why I bought 3 fuses, so I can experiment...However, the bad news is that maybe the fuse is not the root cause of the problem. Guy at the shop said he had the exact same thing happen on his waffle cooker and the problem turned out to be the bi-metallic thingy that's adjustable and let's one adjust the temperature of the cooker. It goes bad/drifts over the years, and the cooker cooks hotter and hotter.... which blows the fuse. So replacing the fuse may not solve anything... and cooker might still end up in the trash anywayIt's old and shitty and a brand new on costs pennies, but still, I don't like losing a battle !I agree with bypassing the fuse : if you cook waffle you should not leave it unattended anyway, you are expected to stay nearby at all times until you are done and you finally unplug it. Still, if I can replace that fuse that would be preferable of course !Thanks again for your feedback,Vince

author: Haenk
author_rank: Super Contributor
timestamp: November 26, 2020, 09:52:05 am
content: Low temp solder might be not suited - depending on the type of solder it might melt as low as 135 degrees, i.e. it will unsolder itself and might connect live to chassis (worst case) before the fuse will act.

author: akimpowerscr
author_rank: Regular Contributor
timestamp: November 26, 2020, 12:08:05 pm
content: I volunteer in repair cafes and we often face this kind of problem.Our choice would be the 142 ° C thermal fuse.There is no real fire danger, these thermal fuses are very imprecise and their operation depends a lot on the thermal coupling with the heat source.Pay particular attention to the thermal coupling, it is very important.If the thermal fuse blows repeatedly, further research into the causes of this fault should be done.To connect them, never solder them.We use hollow brass rivets with the wires passing in the hole and crushing them with ordinary pliers.https://www.ebay.fr/itm/M3-M4-Brass-Metric-Eyelet-Rivets-Thread-Through-Nuts-Hollow-Hole-Grommets-Craft/382865351050?hash=item59248da18a:g:P5AAAOSwNE5YW4s3