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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.

Why is there a thermistor in the power door lock circuit.



 
 
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  #1  
Old September 21st 11, 02:52 AM posted to sci.electronics.repair
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Posts: 3,763
Default Why is there a thermistor in the power door lock circuit.

What would the role of a thermistor be in the power door lock circuit
of a car? It's in series with the door lock "motor", which might be
a solenoid or maybe a rotating motor.

Is it possible that it is meant to lower or turn off power to the
mechanism if the circuit is left closed for some reason and the
solenoid would otherwise overheat?

The symbol it uses for a thermistor is a resistor symbol in an
elongated circle, like a high school running track around the football
field. Is that the usual symbol for a thermistor.




After 40 years of resistance, just bought a foreign car, a Toyota, and
the shop manual is not nearly as clear as the ones I've had for Ford,
GM, and Chrysler. Part of that is because the car is 5 years newer
than the last one and so cars are more complicated, and some is
because parts of it are in bad English, and other parts in
incomprehensible English, but parts of it just don't include as much
information.
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  #2  
Old September 21st 11, 03:03 AM posted to sci.electronics.repair
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Posts: 3,763
Default Why is there a thermistor in the power door lock circuit.

On Tue, 20 Sep 2011 20:52:53 -0400, micky
wrote:

What would the role of a thermistor be in the power door lock circuit
of a car? It's in series with the door lock "motor", which might be
a solenoid or maybe a rotating motor.

Is it possible that it is meant to lower or turn off power to the
mechanism if the circuit is left closed for some reason and the
solenoid would otherwise overheat?

The symbol it uses for a thermistor is a resistor symbol in an
elongated circle, like a high school running track around the football
field. Is that the usual symbol for a thermistor.


Well this says the symbol is retcangle with the stick figure of a
hockey stick across it.
http://www.best-microcontroller-proj...c-symbols.html
What's with Toyota anyhow? Are they wrong about the symbol, or maybe
they're wrong when they call it a thermistor!

  #3  
Old September 21st 11, 03:07 AM posted to sci.electronics.repair
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Posts: 3,763
Default Why is there a thermistor in the power door lock circuit.

On Tue, 20 Sep 2011 21:03:09 -0400, micky
wrote:


The symbol it uses for a thermistor is a resistor symbol in an
elongated circle, like a high school running track around the football
field. Is that the usual symbol for a thermistor.


FTR, the symbol in the Toyota manual looks a lot like the symbol for a
light-dependent resistor at the URL below. Except the one in the
Toyota manual has more rounded corners.


Well this says the symbol is retcangle with the stick figure of a
hockey stick across it.
http://www.best-microcontroller-proj...c-symbols.html
What's with Toyota anyhow? Are they wrong about the symbol, or maybe
they're wrong when they call it a thermistor!


The
  #4  
Old September 21st 11, 04:15 AM posted to sci.electronics.repair
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Posts: 1,569
Default Why is there a thermistor in the power door lock circuit.

On Tue, 20 Sep 2011 20:52:53 -0400, micky
put finger to keyboard and composed:

What would the role of a thermistor be in the power door lock circuit
of a car? It's in series with the door lock "motor", which might be
a solenoid or maybe a rotating motor.

Is it possible that it is meant to lower or turn off power to the
mechanism if the circuit is left closed for some reason and the
solenoid would otherwise overheat?

The symbol it uses for a thermistor is a resistor symbol in an
elongated circle, like a high school running track around the football
field. Is that the usual symbol for a thermistor.


It's probably a PTC resistor or a polyswitch. The latter works like a
resettable fuse.

- Franc Zabkar
--
Please remove one 'i' from my address when replying by email.
  #5  
Old September 21st 11, 05:37 AM posted to sci.electronics.repair
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Posts: 379
Default Why is there a thermistor in the power door lock circuit.

What would the role of a thermistor be in the power door lock circuit
of a car? It's in series with the door lock "motor", which might be
a solenoid or maybe a rotating motor.


It might be a positive temperature coefficient thermistor, such as a
"polyfuse". These have a very sharp PTC "knee", and can be used as
self-resetting overload switches/fuses.

At a guess, this one might be selected/timed so that it would remain
cool (closed) if the door lock is operated for a second or so, but
would heat up enough over several seconds to "open" and disconnect the
solenoid/motor thus protecting the motor from overheating or stripping
a gear if somebody holds the door-lock button down continuously.

It could also protect the power circuitry from damage in the case of a
short circuit at the motor/solenoid itself.

Is it possible that it is meant to lower or turn off power to the
mechanism if the circuit is left closed for some reason and the
solenoid would otherwise overheat?


That would be my guess.

--
Dave Platt AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!
  #6  
Old September 21st 11, 10:56 PM posted to sci.electronics.repair
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Posts: 42
Default Why is there a thermistor in the power door lock circuit.

Yep, the shop manuals for Toyotas are almost idiot's guides. They
give specific tests, but no clue as to how the system operates. I
think Toyota wants you to pay for a course to find that information
out. I've seen pieces of Toyota training publications and they are
more generic, but do define how various systems function.

The Chevrolet guides I've used (2000) has a lot more design type info
in their manual. A similar year Toyota is just different,

PTC thermisters are regularly used for battery pack protection and
solenoid protection. They are way more reliable for this application.
  #7  
Old September 22nd 11, 02:17 AM posted to sci.electronics.repair
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Posts: 1,001
Default Why is there a thermistor in the power door lock circuit.

micky wrote:
On Tue, 20 Sep 2011 20:52:53 -0400, micky
wrote:


What would the role of a thermistor be in the power door lock circuit
of a car? It's in series with the door lock "motor", which might be
a solenoid or maybe a rotating motor.

Is it possible that it is meant to lower or turn off power to the
mechanism if the circuit is left closed for some reason and the
solenoid would otherwise overheat?

The symbol it uses for a thermistor is a resistor symbol in an
elongated circle, like a high school running track around the football
field. Is that the usual symbol for a thermistor.



Well this says the symbol is retcangle with the stick figure of a
hockey stick across it.
http://www.best-microcontroller-proj...c-symbols.html
What's with Toyota anyhow? Are they wrong about the symbol, or maybe
they're wrong when they call it a thermistor!

Are you sure it isn't a circuit breaker? Those are BI-metal
and will open if they get too hot. They are also used in a
time applications in cars.



Jamie


  #8  
Old September 22nd 11, 05:02 PM posted to sci.electronics.repair
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Posts: 3,763
Default Why is there a thermistor in the power door lock circuit.

On Wed, 21 Sep 2011 20:17:01 -0400, Jamie
t wrote:

micky wrote:
On Tue, 20 Sep 2011 20:52:53 -0400, micky
wrote:


What would the role of a thermistor be in the power door lock circuit
of a car? It's in series with the door lock "motor", which might be
a solenoid or maybe a rotating motor.

Is it possible that it is meant to lower or turn off power to the
mechanism if the circuit is left closed for some reason and the
solenoid would otherwise overheat?

The symbol it uses for a thermistor is a resistor symbol in an
elongated circle, like a high school running track around the football
field. Is that the usual symbol for a thermistor.



Well this says the symbol is retcangle with the stick figure of a
hockey stick across it.
http://www.best-microcontroller-proj...c-symbols.html
What's with Toyota anyhow? Are they wrong about the symbol, or maybe
they're wrong when they call it a thermistor!

Are you sure it isn't a circuit breaker?


Well, at the beginning of the the Toyota Electrical manaul, are two
pages with 36 electrical symbols and what they mean, and for this
circuit, the diagram uses a symbol which it says at the start of the
manual represents a thermistor, but it's not the symbol everyone else
uses.

Plus they use the same symbol** for the intake air temp sensor, the
engine coolant sensor, the Air Conditioning room temp sensor, and the
AC ambient temp sensor. . Might those really be thermistors too?

**a zigzag resistor symbol inside a circle that has been elongated in
one direction, like a model train track cricle with extra straight
track in opposite sides. .

I haven't taken apart the door yet, so I don't know what is actually
there.

My plan was to add to the passenger door-unlock output of the
door/burglar alarm Eloectronic Control Unit a wire to a relay, which
relay would unlock the trunk. What's connected to that output now is
the door lock/unlock motor (solenoid?) and this "thermistor", in
series, according to the diagram, and I wanted to understand what is
there now and what it does, before I start fiddling with the circuit.
The other end of the door lock circuit goes to ground and so will the
other end of my relay circuit.

I'm reticent because with the previous car and the previous alarm,
that I installed from scratch, everything was fine, except to open
the trunk, I had to hold down the third button for several seconds.
Because my turnk lid didn't move when unlatched, I thought it would
work better for me if I connected the trunk relay to the both-door
unlock output of the alarm I installed, in parallel with the
door-unlock relay. . However that appeared to screw everything up,
so that many of the alarm functions no longer worked. I must have put
too much drain on that output, even though I just doubled the drain
(two standard automotive relays instead of one) I shoulld I guess
have put the second relay in series with the first one, so that when
it powered the door-unlocks, it also powered the trunk relay.

I'm trying not to make the same sort of mistake here.

Thanks.

Those are BI-metal
and will open if they get too hot. They are also used in a
time applications in cars.



Jamie


  #9  
Old September 22nd 11, 05:08 PM posted to sci.electronics.repair
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Posts: 3,763
Default Why is there a thermistor in the power door lock circuit.

Thank you Franc and Dave.

Please see my reply to Jamie. There's another question towards the
end.

On Wed, 21 Sep 2011 13:56:10 -0700 (PDT), "Ron D."
wrote:

Yep, the shop manuals for Toyotas are almost idiot's guides. They
give specific tests, but no clue as to how the system operates. I


The manuals even start out by saying roughly, "There is not enough
information in these manuals for you to fix the car"

think Toyota wants you to pay for a course to find that information
out.


Maybe so, but I only own one car and I'm not paying!

I've seen pieces of Toyota training publications and they are
more generic, but do define how various systems function.


Hmmm. That would help.

The Chevrolet guides I've used (2000) has a lot more design type info
in their manual. A similar year Toyota is just different,

PTC thermisters are regularly used for battery pack protection and
solenoid protection. They are way more reliable for this application.


Okay, so all three of you say they are thermistors.
  #10  
Old September 22nd 11, 06:26 PM posted to sci.electronics.repair
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Posts: 12,430
Default Why is there a thermistor in the power door lock circuit.


micky wrote:

Well, at the beginning of the the Toyota Electrical manual, are two
pages with 36 electrical symbols and what they mean, and for this
circuit, the diagram uses a symbol which it says at the start of the
manual represents a thermistor, but it's not the symbol everyone else
uses.

Plus they use the same symbol** for the intake air temp sensor, the
engine coolant sensor, the Air Conditioning room temp sensor, and the
AC ambient temp sensor. . Might those really be thermistors too?

**a zigzag resistor symbol inside a circle that has been elongated in
one direction, like a model train track cricle with extra straight
track in opposite sides. .

I haven't taken apart the door yet, so I don't know what is actually
there.

My plan was to add to the passenger door-unlock output of the
door/burglar alarm Eloectronic Control Unit a wire to a relay, which
relay would unlock the trunk. What's connected to that output now is
the door lock/unlock motor (solenoid?) and this "thermistor", in
series, according to the diagram, and I wanted to understand what is
there now and what it does, before I start fiddling with the circuit.
The other end of the door lock circuit goes to ground and so will the
other end of my relay circuit.

I'm reticent because with the previous car and the previous alarm,
that I installed from scratch, everything was fine, except to open
the trunk, I had to hold down the third button for several seconds.
Because my turnk lid didn't move when unlatched, I thought it would
work better for me if I connected the trunk relay to the both-door
unlock output of the alarm I installed, in parallel with the
door-unlock relay. . However that appeared to screw everything up,
so that many of the alarm functions no longer worked. I must have put
too much drain on that output, even though I just doubled the drain
(two standard automotive relays instead of one) I shoulld I guess
have put the second relay in series with the first one, so that when
it powered the door-unlocks, it also powered the trunk relay.

I'm trying not to make the same sort of mistake here.



A properly chosen thermistor will heat up and limit current if the
motor is stalled. It will also provide more a bit more current on cold
days, when you need more torque because of stiff grease in the
mechanism.

Car companies are not in the electronics business, so they use what
they feel that mechanics can understand.


--
You can't have a sense of humor, if you have no sense.
 




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