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

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



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Cursitor Doom wrote:

https://tinyurl.com/t7s6og8

Question is, what is the correct name for this 'mystery component'

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

In article , says...

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?




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.

On Fri, 22 Nov 2019 20:17:25 +0000, Andy Burns wrote:

PTC thermistor?
thermal fuse?

http://www.aolittel.com/sale-1097314...-subminiature-
pellet-ceramic-thermal-fuse-tf-cutoff-125c-250v-2a-axial-leaded-for-
air-.html

Bingo, Andy! Good call.



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On Fri, 22 Nov 2019 16:11:31 -0500, Ralph Mowery wrote:

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.

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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On 11/22/19 2:50 PM, Cursitor Doom wrote:
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!




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.

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

I refer to you this photo of the internals...

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

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).

On 23/11/19 5:34 pm, whit3rd wrote:
On Friday, November 22, 2019 at 11:50:53 AM UTC-8, Cursitor Doom wrote:

I refer to you this photo of the internals...

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

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.

On Friday, 22 November 2019 23:49:28 UTC, Starfella wrote:
On 11/22/19 2:50 PM, Cursitor Doom wrote:
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!




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.

10 years? I grew up with one several times that old.


NT

On Sat, 23 Nov 2019 18:08:11 +1100, marty wrote:

It's cactus, throw the thing out.

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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On Fri, 22 Nov 2019 22:34:01 -0800, whit3rd wrote:

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.

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.

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).

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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Cursitor Doom wrote:

----------------------


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.



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

https://www.amazon.com/10Pcs-Microte.../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

Cursitor Doom writes:

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

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

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

On Mon, 25 Nov 2019 07:57:16 +0200, Mikko OH2HVJ wrote:

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.

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

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 ?

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



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