Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems.

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Default Do thermal fuses fail from old age?

On Mon, 26 Mar 2012 00:53:26 -0500, The Daring Dufas
wrote:

Back about 4 decades when I tried my hand at TV repair, me and the guys
I worked with had to replace a lot of what were called
"glowbars/globars", they were actually PTC devices hooked to the
degaussing coil around the CRT of a TV set to power the degaussing coil
for a short period every time a TV set was turned on.


That type of thermistor works backwards from the flavor I'm proposing.
For the degaussing coil, the thermistor has a low resistance when
cold. When it gets warm, the resistance increases dramatically. When
installed in series with the degaussing coil, it allows an initial
blast of current through the coil, followed by effectively turning
itself off.

In the case of protecting the thermal fuse, it's the other way around.
When cold, the thermistor has a fairly high resistance, thus limiting
the peak inrush current. As the current heats up the thermistor, the
resistance decreases, allowing the device to operate normally.

The only problem would be
coming up with a thermistor that could handle the current of the heating
element when it's connected in series with it.


It's not current handling but rather energy handling capacity. If the
inrush current surge lasts too long, the thermistor will get rather
hot. If this were a design exercise, I would need the steady state
current, the peak inrush current, and the approximate time duration in
order to calculate the energy dissipated (in joules or watt-seconds)
and eventually the maximum thermistor resistance.


--
Jeff Liebermann
150 Felker St #D
http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
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Default Do thermal fuses fail from old age?

On 3/26/2012 2:13 AM, Jeff Liebermann wrote:
On Mon, 26 Mar 2012 00:53:26 -0500, The Daring Dufas
wrote:

Back about 4 decades when I tried my hand at TV repair, me and the guys
I worked with had to replace a lot of what were called
"glowbars/globars", they were actually PTC devices hooked to the
degaussing coil around the CRT of a TV set to power the degaussing coil
for a short period every time a TV set was turned on.


That type of thermistor works backwards from the flavor I'm proposing.
For the degaussing coil, the thermistor has a low resistance when
cold. When it gets warm, the resistance increases dramatically. When
installed in series with the degaussing coil, it allows an initial
blast of current through the coil, followed by effectively turning
itself off.

In the case of protecting the thermal fuse, it's the other way around.
When cold, the thermistor has a fairly high resistance, thus limiting
the peak inrush current. As the current heats up the thermistor, the
resistance decreases, allowing the device to operate normally.

The only problem would be
coming up with a thermistor that could handle the current of the heating
element when it's connected in series with it.


It's not current handling but rather energy handling capacity. If the
inrush current surge lasts too long, the thermistor will get rather
hot. If this were a design exercise, I would need the steady state
current, the peak inrush current, and the approximate time duration in
order to calculate the energy dissipated (in joules or watt-seconds)
and eventually the maximum thermistor resistance.


That's why I thought an indirect approach might work and last longer. ^_^

TDD


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Default Do thermal fuses fail from old age?

On Mon, 26 Mar 2012 00:13:49 -0700, Jeff Liebermann wrote:

On Mon, 26 Mar 2012 00:53:26 -0500, The Daring Dufas
wrote:

Back about 4 decades when I tried my hand at TV repair, me and the guys
I worked with had to replace a lot of what were called
"glowbars/globars", they were actually PTC devices hooked to the
degaussing coil around the CRT of a TV set to power the degaussing coil
for a short period every time a TV set was turned on.


That type of thermistor works backwards from the flavor I'm proposing.
For the degaussing coil, the thermistor has a low resistance when
cold. When it gets warm, the resistance increases dramatically. When
installed in series with the degaussing coil, it allows an initial
blast of current through the coil, followed by effectively turning
itself off.


That's a PTC (positive temperature coefficient) thermistor. Its resistance
goes up with temperature.

In the case of protecting the thermal fuse, it's the other way around.
When cold, the thermistor has a fairly high resistance, thus limiting
the peak inrush current. As the current heats up the thermistor, the
resistance decreases, allowing the device to operate normally.


Nope. A PTC protection device (a.k.a. "polyfuse" or "polyswitch") is also a
PTC device. It has very low resistance when cold. When excessive current
flows through it, it heats and becomes open, more or less. It's only reset
when the voltage is taken off and it's allowed to cool. It wouldn't work if
it had a high resistance when it was cold.

Both devices are PTC thermistors, albeit with different characteristics. NTC
thermistors are normally used as temperature sensors.

The only problem would be
coming up with a thermistor that could handle the current of the heating
element when it's connected in series with it.


It's not current handling but rather energy handling capacity. If the
inrush current surge lasts too long, the thermistor will get rather
hot. If this were a design exercise, I would need the steady state
current, the peak inrush current, and the approximate time duration in
order to calculate the energy dissipated (in joules or watt-seconds)
and eventually the maximum thermistor resistance.


Any sane person would just buy the one designed for the application.
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Default Do thermal fuses fail from old age?

the machines are roll laminators that put plastic on paper, think of
menus, often they are laminated.

the high inrush current leads to all sorts of troubles, with burned
out switches, fried connectors, burned out boards etc..
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Default Do thermal fuses fail from old age?

On Mon, 26 Mar 2012 06:53:12 -0700 (PDT), bob haller wrote:

the machines are roll laminators that put plastic on paper, think of
menus, often they are laminated.

the high inrush current leads to all sorts of troubles, with burned
out switches, fried connectors, burned out boards etc..


That may be true, but what does it have to do with the price of eggs in China?


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On Mon, 26 Mar 2012 06:53:12 -0700 (PDT), bob haller
wrote:

the machines are roll laminators that put plastic on paper, think of
menus, often they are laminated.


One of these?
http://www.ledcolaminator.com
I do computah work for a print shop that has several of these. I
don't recall hearing about any thermal fuse failures.

the high inrush current leads to all sorts of troubles, with burned
out switches, fried connectors, burned out boards etc..


Do you have any specs or measurements for one of these machines?
1. Operating voltage:
2. Peak inrush current:
3. Steady state current:
4. Duration of inrush surge:
If you want, I can then grind the numbers and offer a recommended
thermistor. I have that horrible feeling that the power consumption
may be high enough that a suitable inrush thermistor might not work
(or exist).

--
Jeff Liebermann
150 Felker St #D
http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
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Default Do thermal fuses fail from old age?

The Daring Dufas wrote in news:jkf3na$uvu$1
@dont-email.me:

On 3/21/2012 11:03 PM, jeff_wisnia wrote:
Over the years I've had a few thermal fuses in houshold appliances go
open for no apparent reason. When I've replaced them with ones with

the
same temperature rating they stayed working fine for years more.

A couple of days ago our three year old Bunn coffee maker quit,

because
a 141 degree celcius overtemperature thermal fuse on the water tank
opened. It might possibly have happened because the water tank
thermostat stuck closed, but after I repaired it the thermostat cycled
just as it should.

I was suprised to find that there were TWO identical thermal fuses
connected in series located right next to each other, the bodies were
actually touching. That seemed like a belt and suspenders approach,
unless there's a significant likelihood that a thermal fuse won't open
when it should?

Comments?

Thanks guys,

Jeff



I could understand parallel fuses because one may not handle the
current. Perhaps in series it's safer because the unit will shut
off if one fails to open? o_O

TDD

That is because they dont trigger on current(mainly).
They trigger when their surrondings get to hot.
Imagine a current carrying spring, soldered to the other
side with solder of a particural melting temperature.
Of course you can also heat them by massive overload in
current, but that is not the way they ought to work.
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Default Do thermal fuses fail from old age?

On Thursday, March 22, 2012 12:03:41 AM UTC-4, jeff_wisnia wrote:
Over the years I've had a few thermal fuses in houshold appliances go
open for no apparent reason. When I've replaced them with ones with the
same temperature rating they stayed working fine for years more.

A couple of days ago our three year old Bunn coffee maker quit, because
a 141 degree celcius overtemperature thermal fuse on the water tank
opened. It might possibly have happened because the water tank
thermostat stuck closed, but after I repaired it the thermostat cycled
just as it should.

I was suprised to find that there were TWO identical thermal fuses
connected in series located right next to each other, the bodies were
actually touching. That seemed like a belt and suspenders approach,
unless there's a significant likelihood that a thermal fuse won't open
when it should?

Comments?

Thanks guys,

Jeff


--
Jeffry Wisnia
(W1BSV + Brass Rat '57 EE)
The speed of light is 1.8*10e12 furlongs per fortnight.


With use thermo fuses do fail over time if they are working near to their temperature rating. They are in place to prevent a disaster if there is any over heating of the appliance or device.

The designed lifespan for most appliances is 3 to 5 years depending on the appliance. I realize that this is not the answer the average person wants to hear, but that is the fact! Therefore your appliances are doing well if you only need to replace a thermo fuse to keep them going.

Jerry G.
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Default Do thermal fuses fail from old age?

On 03/23/12 22:02, Michael A. Terrell wrote:

N_Cook wrote:

Michael A. wrote in message
m...

N_Cook wrote:

Perhaps the voltage rating was lower than the required use, ie in fused
state not rated for the service voltage across the broken section so

someone
thought, I know , we'll put 2 in series ;-)


Which would do nothing, if they couldnn't handle the voltage.

Have you ever looked at the wiring in a furnace? They use multple
thermal shutdowns, for liability in the US.


so you have no visual sense for emoticons



I did too many fire restoration jobs after a pair of switches failed
and people died. I see no humor in people dying from failed and jury
rigged repairs. Laugh all you want. It fits you.



I see humor in you dying from sodomizing an 8 year old girl, pencil dick.
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