Seat Heater Improving / Thermistor Question

[iso9001]

I have these seat heaters for my car that are really nothing more then a big wire feeding to the unit and a smaller wire thats wound inside a foam pad that goes under the fabric in the seat.

Well... The way they keep these things from buring up is they have a small disc of about .5" in dia and .25" tall... Inside (i'm 99% sure of this) is a wound metal strip... When it expands it breaks the connection and cuts the power to the seat...

Thats great and all, but the downside is that once the seat gets up to temp, the coil expands and breaks contact, and will not get contact again untill the coil cools down enough to reconnect...

While the seat is warm and my bottom keeps it from cooling of too soon, I am not getting the amount of heat I should be... In fact, the seat goes from Nice and Hot to Luke Warm every 8-10min it seams, with most of the time being in Luke Warm mode.

So instead of nice steady heat, I get a square wave of pulses.


Now.... How to maintain the saftey of a thermomitor while keeping my butt as absolutely warm as possible ?


One idea I had was to replace the expanding/contracting coil with some set of passive electronics (a cap, transistor, and thermistor maybe ?) that is all small enough to fit inside the original cylinder...

Any ideas ?

 

[bmcculla]

You can buy thermaly triggered relays that trigger at various temperatures. If I remember correctly they don't require any power supply and can be bought to trigger at a variety of temperatures. The ones I saw were in a TO220 package so you could mount the tab near the heating element to reduce the cycling time.

You also might try to find a positive temprature coefficent thermistor with a knee (sharp increace in resistance) near the temperature you want.

Brent

 

[iso9001]

The relay sounds hard to get,

Yea, the thermistor part is the one I'm interested in....

I need somthing that cuts the power based on the thermistor's value, but also sits inline with the 1 wire I have coming in to the seat.

Is that possible ?

 

[iso9001]

Oh, isee what your saying,

get a thermistor with a low (REALLY LOW if possible) and just put that in line... so it cuts the voltage itself.

I doubt they make one that is 1-2ohm and 100k at 100degrees

...

 

[McGuinn]

Doubt if the thermistor is the way to go. It would have to pass a very high current, so would heat up.
It sounds like you have a bi-metalic strip of some sort.
What you could try doing is decreasing the difference between the off temperature and the on temperature (I can't remember the name of this) and maybe by removing some of the insulation from around it, it will cool faster, enabling faster switching...?

 

[ante]

Does it look anything like this?

Ante :roll:

 

[ante]

Sorry, here is the picture.