Lets try this again. The sensor is 24 ohms around the hot end, but it's
in series with the gage, which is about 60 Ohms cold (it may get a bit
higher when the heater gets hot, but for this lets call it 60 Ohms).
That's a total of 84 Ohms, with an average of 10 volts across it.
Do the math, and that's around an average of about 119 mA running
through the string. So you can calculate the max power dissipated in
any added series resistor based on this, P = I*I*R. Add 10 ohms, for
instance, that makes the string 94 Ohms, that gives a current of 106mA
at 248 degrees. The dissipation works out to about 0.112 watt. As you
can see from my data, lower tehperatures give higher resistances, and
therefore lower currents and power dissipations. So in this case, a 1
watt or even a half watt resistior in series would leave plenty of
margin.
It was said elsewhere, but this fixes it at one temperature, but throws
it further off elsewhere.
Think about this: What info are you REALLY looking for from your
temperature gauge? As your car warms up, the temp gauge comes up to a
certain point and stabilizes there, for a while anyway. That's the temp
your thermostat regulates at. As long as your gage is pointing around
there, all's well. If it goes a little higher, and circumstances
explain this, stuck in traffic on a hot day, that's ok, but worth
watching. If it goes way higher, then you have a potential problem. Do
you really need numbers for this? This is LUCAS stuff, for heavens
sake, not NIST traceable instrumentation.
Stu
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