Electric blanket controller

... and (I forgot first time around)... why have they put a couple of turns in each of the leads?

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PTC thermistor? thermal fuse?

It might be a simple fuse. See if the resistance is zero/near zero when warm and cold. Can not tell from the photo, but looks tohave a 7A over the 120 volt.

Bingo, Andy! Good call.

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It's actually 2A in this case. Andy's suggestion was correct. But like you say, the resistance should be pretty low whereas it's actually infinite, so looks like it's blown.

Not sure about the name of the component, but I will say that today's electric blankets are awfully poor made. Years ago, when I had one while growing up, it lasted for at least 10 years before someone threw it away during cleaning. As an adult, I purchased a Sunbeam brand about five years ago and I've never had one last more than 6-8 weeks! The only type of heating blanket I've found in recent times that does last are electric throws. I have two, purchased about 5 years ago, and with heavy use, they still work fine. Problem is that they are a bit too small for bed use although I make them work that way.

It's either an energency overtemperature cutoff (like a fuse), or a thermal switch that repeatedly time-cycles according to the heating/cooling time constant of its companion resistors.

It has a '2A' rating, not a temperature rating, so probably NOT the emergency cutoff. Maybe a PTC resistor (solid state 'switch').

The diodes presumably send current to heating element A on positive half-cycles, and heating element B on negative half-cycles (so as to retain some function if one element fails open-circuit).

It's cactus, throw the thing out.

NT

I don't believe in doing that unless there's no other option. Having grown up in the overhang of postwar rationing and austerity I feel compelled to do all I can to rescue stuff if it's safe and practical to do so. Plus our 'throw away society' is not doing the planet any favours at all.

I assumed it was tucked between those resistors to sense any higher than usual warming in them. It's marked 76 degrees C in the case of this one. I'm guessing it (since it's in series with the live line) that if the temp rises above that level it will wind back the current, but this one has blown completely open-circuit and maybe that's what it was designed to do. Nothing visible, just on testing for resistance.

Sounds feasible. I'll check the resistance of the heating elements and see what they show. One may have gone partially short-circuit, causing the fault in the first place. It's a 70W blanket so at 240V they should come out at about 800 ohms in total if my quick 'n' dirty sums are correct.

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A spring loaded contact inside is held together with a wax like substance that melts at some desired temp.

Likely the unit was made to overhead by being on top of the blanket and insulated by bed clothes.

... Phil

As others noted, it is a thermal fuse.

The turns are there for pliers or other tool to absorb the heat while soldering the thermal fuse to the PCB. It's quite easy to blow a low temperature rating thermal fuse while soldering.

What't the schematic like, are the resistors connected in such a way that they would heat during some failure condition and blow the fuse ?

Ah, thanks very much for that, Mikko; the missing piece in the jigsaw!

I don't have a schematic, but that's been my working assumption throughout. It makes sense that way.