[V8] Seat Heater switches

[Dave Saad]

There is no measuring of temperature or anything at the seat. That would be far too expensive and complicated. There might be a hi-temp cutoff switch for safety but I doubt even that. I did repair my heaters years ago and don’t remember anything in the seat except the heater wire.

To control the heat you really have two options. Vary the current, or vary the duty cycle. A current regulator would require a large transistor with a heat sink somewhere. The Audi system does not have this so it must vary the duty cycle. I think you can actually hear clicking when the heaters are on coming from the relay panel. This clicking is the “duty cycle”. All that means is that the relay clicks on/off at a time interval determined by the thumb switch. The more “on” time, the hotter the seat gets. I am guessing the duty cycle is measured in seconds. Just for example, assume the duty cycle period is 30 seconds. At 50% duty cycle the “on” time equals the “off” time and would be 15 seconds. So the seat would heat for 15 seconds, then cool for 15 seconds. The thumb wheel lets you vary the ratio of on/off time. If you look at the current supplied over 30 seconds, it is 50% of maximum.
This is a very simple circuit and easily implemented with a small cheap integrated circuit called a LM555 timer.
Here is some basic info:
http://www.learnabout-electronics.org/Oscillators/osc44.php

Dave

> 
> Thanks for the ideas.  I am wondering on the seat heaters, the current
> should be dictated by the heating element, right?  So that the heat
> adjustment should be accomplished by varying voltage, was my guess.  Not
> sure, obviously my electrical/electronics knowledge is fairly rudimentary.
> I've brushed with it for years at work, but mostly stick to mechanics or
> simple circuits..
> 
> On Mon, Feb 2, 2015 at 4:31 AM, CavalloGT <cavallogt at gmail.com> wrote:
>