Electric blanket controller

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Gentlemen,

I refer to you this photo of the internals of a typical modern cheap and  
nasty blanket controller. All the components it uses are shown on this  
board: just 5 diodes, 3 resistors and some 'mystery component' whose  
function is presumably to sense overheating from the two resistors it's  
between and cut-back the mains voltage to the blanket itself if needed.

https://tinyurl.com/t7s6og8


Question is, what is the correct name for this 'mystery component' and  
what should it read resistance-wise when cold?

Thanks!



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Re: Electric blanket controller

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



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Re: Electric blanket controller

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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 ?

--
mikko

Re: Electric blanket controller
On Mon, 25 Nov 2019 07:57:16 +0200, Mikko OH2HVJ wrote:

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

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



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Re: Electric blanket controller
Cursitor Doom wrote:

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PTC thermistor?
thermal fuse?

<http://www.aolittel.com/sale-10973142-aupo-p3-f-pico-subminiature-pellet-ceramic-thermal-fuse-tf-cutoff-125c-250v-2a-axial-leaded-for-air-.html

Re: Electric blanket controller
On Fri, 22 Nov 2019 20:17:25 +0000, Andy Burns wrote:

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pellet-ceramic-thermal-fuse-tf-cutoff-125c-250v-2a-axial-leaded-for-
air-.html>

Bingo, Andy! Good call.



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Re: Electric blanket controller
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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.



Re: Electric blanket controller
On Fri, 22 Nov 2019 16:11:31 -0500, Ralph Mowery wrote:

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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.



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Re: Electric blanket controller
On 11/22/19 2:50 PM, Cursitor Doom wrote:
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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.

Re: Electric blanket controller
On Friday, 22 November 2019 23:49:28 UTC, Starfella  wrote:
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10 years? I grew up with one several times that old.


NT

Re: Electric blanket controller
On Friday, November 22, 2019 at 11:50:53 AM UTC-8, Cursitor Doom wrote:

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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).

Re: Electric blanket controller
On 23/11/19 5:34 pm, whit3rd wrote:
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It's cactus, throw the thing out.

Re: Electric blanket controller
On Sat, 23 Nov 2019 18:08:11 +1100, marty wrote:

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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.



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Re: Electric blanket controller
On Fri, 22 Nov 2019 22:34:01 -0800, whit3rd wrote:

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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.

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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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Re: Electric blanket controller
Cursitor Doom wrote:

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 ** FFS -  it's a common thermal fuse.

https://www.amazon.com/10Pcs-Microtemp-Thermal-Cutoff-SM110A1/dp/B072SV9QFB

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  



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