More DIY EGOR

[steve ravet]

A constant power source won't put constant power to the heater.  Bruce's
email showed that the resistance varies with temp.  Remember that
power=I^2R (current squared times resistance).  In this case current is
constant, so power is proportional to resistance.  And, since resistance
increases with temp, power to the heater will also increase with temp,
which seems like the opposite of what you want.  A constant voltage
source will provide power proportional to 1/R (P=V^2/R), which means
that as temp goes up, resistance goes up, and heater power goes down.

Hope I didn't screw up something simple there...

--steve

RDwoo10 at aol.com wrote:
> 
> Maybe i am missing something but i think that is the opposite of what you
> want to do.  if the temp is moving up & down then you want the current
> (Power, heaters are power devices) to change as well.  that is why the
> feedback is in the heater circuit to continually monitor the feedback off of
> the heater to "figure out" what the temp of the sensor is.  this way if the
> temp goes up the resistance goes up.  so we need to drive the heater less to
> keep the temp of the heater @ the running temp.  if we provided it with a
> constant current source it would always use the same power & the temp of the
> sensor would go up & down with the exhaust temp.
> 
> any of the real EE's can jump in here, if i am all wet.
> 
> Bob Wooten
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-- 
Steve Ravet
steve.ravet at arm.com
ARM,Inc.
www.arm.com
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