re Re: Seat heater switches

["Smith, Kirby A" <kirby.a.smith@lmco.com>]

w.r.t. the stuff appended below:

The seat heaters in my 90q work via a 6 position switch that is, internally,
a circular carbon track that moves relative to a wiper.  I believe this acts
as a  potentiometer to determine a "power" or voltage level to the seats,
although I haven't measured the exact effect.  When the carbon track to
wiper connection becomes marginal, the controlling relay chatters.  Usually
this  is ameliorated by moving the switch to an intermediate setting.  The
contact problem seems to be related to the more used positions of the wiper.

kirby


In message <36166E3D.24567074@nh.ultranet.com> Huw Powell writes:

> > The seat heater switches in the ur-quattro are the thumbwheel type -
> > On/off when moving from 0 to 1, and then gradually more heat up to
> > a setting of 6.
> >
> > Anyone know the nominal resistance for each setting?
>
> The resistance of the heater wire?
>
> Because the switches like that that I have seen are just that -
> switches, with 7 positions.  They go to a pair of relays, then to the
> three heater wires.  The six settings are the different combinations of
> the three different resistance wires...
>
> hth a little?  I don't have any heated seats around so I can't answer
> the other half of the question.

No.  There is only _one_ heater wire - the backrest and squab are wired
in series, and there's only one circuit back to the controller.

I suspect the switch presents on/off and six different resistances
to the controller.  The controller applies a voltage to the switch
resistance and to the thermistor in the seat element.  All it would
have to do is amplify the delta current and apply it to a relay via
a diode.

At least, that's the only mode of operation I can deduce from the
paucity of physical circuits present.