Non resettable fuse question. SF152E - Page 1
Forum section: electronics / beginners

author: Another Dave
author_rank: Contributor
timestamp: February 08, 2017, 03:31:31 am
content: I have 2 non resettable thermal fuses (SF152E)'sPopped the meter onto them and no current passes through. Thought have they blown?My mind got thinking, being thermal do they require a set temperature before they startto conduct? Heated one up and yes, it started to conduct. I didnt get a chance to checkthe temperature it started to conduct. Number on the side shows 157 degrees C.This part...http://uk.rs-online.com/web/p/non-resettable-thermal-fuses/1769306/If this how they work? Or has it blown and heating it caused it to conduct again?Thanks.

author: mikerj
author_rank: Super Contributor
timestamp: February 08, 2017, 09:18:51 am
content: Quote from: Another Dave on February 08, 2017, 03:31:31 amIt's blown. They contain a low melting point alloy inside, and once melted it's all over.

author: Another Dave
author_rank: Contributor
timestamp: February 09, 2017, 02:04:24 am
content: Thanks, so when i heated it the alloy just melted to create the connection and failed again as it cooled.Rather strange, they are from a breadmaker thats had very little use and was OK last time we used it.Both to fail at the same time. Very strange. I may have a tinker and replace them or just bin it.Maybe 157 degrees C was a little too low for the application?Thanks for that.

author: mikerj
author_rank: Super Contributor
timestamp: February 09, 2017, 09:56:01 am
content: That does sound odd. When you say you checked them with a meter and "no current passes through them", were you measuring the resistance?

author: Another Dave
author_rank: Contributor
timestamp: February 09, 2017, 09:15:28 pm
content: yes measuring the resistance and no reading at all until heated. Then the resistance starts reading i forgetat what point it starts, but as it gets hotter the resistance decreases to almost zero at which pointi stopped heating it and the resistance started climbing again.They are stamped at 157 degrees. And it was repeatable. I heated it again and the same thing happened.Heat it and it starts to register a reading then falls away as it gets hotter, and climbs as it cools.

author: Gregg
author_rank: Super Contributor
timestamp: February 10, 2017, 12:00:27 am
content: These little thermal fuses are used in many household small appliances. They act as current as well as temperature fuses. The 157 degrees C seems a bit low for a bread maker; but it all depends on the location of the fuse and its proximity to the heating element(s) within the appliance. Sometimes they enclose them in a fiberglass sleeve to keep radiant heat to a minimum (and to hide them from those of us that take things apart to fix them).If for some reason the current suddenly went over 10 amps, it is no surprise that both of them went open circuit. Similar could happen if the thermostatic control was stuck on full heating mode. It certainly is worth further investigation before tossing it.I would start with an ohm meter to verify the heating elements have a reasonable resistance and are not shorted to the frame. Then I’d try bypassing the SF152Es via a fused or otherwise current protected source that is easily switched off and measuring the amperage draw. You probably should put some water in the bread pan to help keep the heat from being localized (could be the reason the fuses blew--if it was turned on with nothing in it). The current should go down as the heating element gets hot; in any case, it never should draw the full 10 amps and should settle down to 5 or 6 amps (the label should tell you the wattage) within about 30 seconds. If it draws way too much current, it probably isn’t worth fixing. Next step would be to check the thermostat to ensure it turns off the heat reasonably close to the set temperature. If the thermostat is bad, it may or may not be worth fixing. A search on the internet may find a substitute fuse that is a little higher temperature rated, but the same 10 amps.

author: Another Dave
author_rank: Contributor
timestamp: February 10, 2017, 01:02:59 am
content: Thanks again. Not checked the element resistance. It did read something so not failed. Will check againand see what it shows.Yes its sleeved in a fibreglass type material, luckily its open at either end so i can slide the sleeve along the wire to view the fuse. Its also clamped to a head spreader also.Never been switched on empty, and it has not had much use at all really. Packed away whilst the kitchen was done and brought back out and not working.No power to anything at all not just the heating. Digital display no power. Only got as far as checking the fuse and power coming in, through the switch and then to the circuit board with 6 connectors on it. 2 fuses a temp sender. element etc.With no power to the board im wondering if its going to be something like my lathe where it was either a couple of caps oran 78L05. I replace 3 items in one go and it worked. getting 0.7v out instead of 5v. Thats been working fine ever since.It was mentioned not much kills a 78L05 but it did appear to be the issue. This boards a bit more than 5v though, 94v possibly,thats printed on one edge.But assume a similar method. What voltage should i get and what do i actually get, and work backwards.Thanks.

author: Gregg
author_rank: Super Contributor
timestamp: February 10, 2017, 02:52:08 am
content: Power to the control board may come from a circuit after the two open fuses. Often making a sketch of the connections and then try to follow them with an ohm meter writing down the results on the sketch will point you in the right direction. When troubleshooting anything, always start with what is known and move on to what is easiest to test. Don't overlook the possibility of safety switches on the lid or in case it tips over. Some of these can be really sneaky like a magnet in the lid and a hidden reed switch in the case.