ECT and voltage divider help?

[Daniel Houlton]

Mike R Brown wrote:
>
>Bernd Felsche wrote:
>> 
>> Daniel Houlton writes:
>> 
>> >So, my question is, how do I switch something based on this voltage?
>> >Basically, I want to monitor the voltage to the ECT sensor and when it
>> >drops to around .4 or .5 V I want to trigger an output (that eventually
>> >drives a relay).  Then, while the output is triggered and the voltage
>> 
>
>	Don't know what kind of thermistor is in your sensor but these guys can
>be (are) very nonlinear.  The common GM sensors used in the system I'm
>running have very little delta_R/delta_T at the very temperature range a
>person is most interested in.  You can see some graphs here where I put
>sensors in a thermal chamber and plotted their resistance:
>http://www.sonic.net/~mikebr/plots/mat_vs_res_adc.html

That plot looks to match my readings.  I'm getting 2.77 kOhms cold 
(about 70 deg) and 270 ohms hot when the ECM is about to kick up the
rpm for the engine driven fan.


>What this boils down to is that the voltage ranges you are most likely
>dealing with can very small.  The difference between the fan on and fan
>off voltages can be very small (in the order 10-100mv).  Before I got

Yeah, very small, but I was considering now to instead of turning it
on and off between voltages, to turn it on at a certain voltage and
let it run a minimum amount of time (say 30 - 60 seconds) using a 555
timer.  In testing, 60 seconds will raise the voltage from .6v to 
around .85v.


>into reverse engineering the EFI system in my car I built an analog
>circuit that would turn the fan on and off at certain set points. 
>Problems you will run into is that a car is a very electrically noisy
>environment.  You need to pay attention to your input and supply
>filtering.  Also watch out for ground loops.  My battery is in the rear
>and the frame is the common return.  When the fans came on (about 30
>amps inrush current) a few millivolts would develop across the frame

Yeah, I noticed there was a large current at startup too.  I noticed
the fan came with a self-resetting circuit breaker instead of a fuse
in the main power supply wire.  I'm guessing that this is because a
fuse would blow on startup unless you made it so big it didn't really
protect the wiring system any more.


>are out there looking for you.  Knowing what I know now I would use the
>filtering and ADC in the ECM to control these things.  A small PIC or
>something like Bernd suggest might be the way to go. 

I would like to do that as well, but I would like to complete this.  
I've been working on an Atmel controller for an additional injector.
It's been a year now and I'm still nowhere close.  I'm afraid this 
would turn into the same thing.


thanks again,

--Dan
houlster at inficad.com

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