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A few things Posted by Ari [Email]  (#2847) [Profile/Gallery]
(more from Ari) on Fri, 21 May 2004 07:06:39
In Reply to: Re: Repairing Seat Heaters, Jeff S., Thu, 20 May 2004 08:31:40 Members do not see ads below this line. - Help Keep This Site Online - Signup |
I'm not a big fan of crimping. If the crimp loosens even a little, you'll get minor arcing. That develops a carbon layer, which increases localized resistance, which increases temperature at that point, and pretty quickly the joint fails. So a good crimp should be fine, but there is NO margin for error - or you'll be back in there.
Power is NOT quadrupled. Remember P=IE and E=IR? P=E^2/R. Voltage doesn't change. The applied voltage is 12 volts, no matter what. If, just for an example, the bottom and back heaters are both 10 ohms, the delivered wattage is 12^2/20, or 7.2 watts. If one heater is wired out, the resistance is now only 10, but the applied voltage is still 12 volts - 144/10 = 14.4, or twice the wattage delivered. That's still way too much.
Which brings up the 'rheostat'. You don't mention the year of your car, but by 'rehostat' I assume you have the four-position seat controller in the dashboard. That is not a rheostat. That's a controller. In the seat there is a heater element (as you know) AND a thermistor. They share a ground, but there are two wires (high side of heater, thermistor) going to the controller. The controller is an op-amp and a relay. It uses the thermistor to measure the seat temp. The op-amp is a simple comparitor (with a little hysteresis) that compares the seat temp with the setting on the dial. If the seat is cold, it closes the relay and applies 12 volts (or whatever the bus voltage is, depending on engine speed) to the heater. When the seat gets above the set point, the relay opens up. So it supplies 12 volts or 0, through relay contacts. The hotter you set it (3 on the controller), the higher the duty cycle of the relay. With twice the current, you'll be putting a bigger load on the wiring and relay contacts.
Lastly, I consider the most important part of the seat heaters the back. My personal opinion. Warm buns are nice, but the warmth on the back and kidneys is what I find most comforting. I'd give up the seat bottom before I'd lose the back. My recommendation - repair the break with a soldering iron. Yes, it's a pain. But seat backs very rarely fail. The seat bottom is much more common. So this will be a once-in-a-lifetime experience. Go with it, and come up with shortcuts on something you'll do more often.
posted by 192.249....
, Thu, 20 May 2004 08:10:25, Fri, 21 May 2004 07:06:39 <-- Viewing This Message
, Thu, 20 May 2004 05:42:30
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