another thermistor hack

[ken1784]

Originally Posted by Dan.

I also have no clue what value R1 is either.

My experiment tells me the R1 value is approx 2.6kΩ.

Ken@Japan

[Dan.]

Originally Posted by Dan.

Anyway, I got the range of the Thermistor (R2) from Hobbit's graphic as between 79Ω (@+140°C) and 156KΩ (@-40°C), but I don't know what it's "operational" range is at temps between 40C and 95C. I also have no clue what value R1 is either.

My complete and WAG puts R1 between 7K-10KΩ. That would make the current range between 0.03mA and 0.70mA. Does this sound right for this type of curcuit? Seems kinda low for curent.

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[Dan.]

Originally Posted by Dan.

5V-----R1-----a------R2------b------GND

R1 = Internal resistor
R2 = Thermistor
a = Sample Point
b = Sample Point

Now the ΔV between "a" and "b" is I*R2, and 5V=I*R1 + I*R2, so I = 5V / (R1 + R2)

so the ΔV between "a" and "b" is 5V * [R2 / (R1 + R2)] or

5V * α where α = [R2 / (R1 + R2)]. This makes since since as R2 approaches ∞, α approaches 1, and as R2 approaches 0, α approaches 0.

First guess was way off. From hobbit's page on the subject sounds like R1 is known to be 2500Ω and R2 is VERY non-linear. So by plugging some numbers, I'm thinking a 5K POT in series with a 500Ω resistor may be a better way to move between 40°C 70°C. If my math is right, at 40°C if you turn on the circuit you will blink 5500Ω in parallel. This should pop your temp up to about 46°C (1.30V). Now if you dial the POT all the way clockwise you should be at 75°C (0.60V). On the other end... when the thermistor is registering 70°C and your POT is dialed as close to off as you can get it, your will be down to 500Ω of resistance, giving you an "adjusted" temp of about 72°C (0.64V).

As a side note, looks like you could dial in 70°C in as soon as ~27°C (but you really should wait for 40°C).

Any Thoughts?

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[Dan.]

Originally Posted by ken1784

My experiment tells me the R1 value is approx 2.6kΩ.

Thanks Ken... sorry I missed your reply.

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[TheForce]

Dan a 1k linear POT and 250ohm resistor works just fine. If you want a little safety in the winter use a 520ohm resistor. I find that the 520ohm resistor will give you about 160F when the actual coolant temp is around 100F. This will tell you if you need to start the engine you might want to turn off the fake temp so you dont run the chance of an engine stalling or getting error codes. If you want to stay in S4 in the winter for long periods of time like me since I have the extra battery the 240ohm will keep you there for ever. But if your actual temp drops way below 100F and you try to start the engine with the fake temp you might run in to trouble.

For my commute 240ohm would not help since when I get off the highway I'm traveling below 35MPH and dont really need S4.

[Dan.]

Very cool. Thanks,

Think I'm gonna go with a 1k linear POT with a 240 or 520 resistor. The idea of going with a 5k POT looked bad once I graphed it. Got real non-linear. Solution would be a 5k reverse-logarithmic POT (left side of the dial has a bigger effect than the right side). Finding a 5k reverse-log POT is not as easy as getting a 1k linear, so decision made.

I'm also probably gonna get a POT with a built in switch. Just one component to install.

As a after thought, I was considering putting an LED or something in the circuit, but since the current is so dang low, I doubt the thing would glow at all. Neat idea though.

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[TheForce]

I had a hard time finding a 1k POT with switch. I just checked and the place I bought mine no longer has them.

This place looks to have them. Dont know how long the shaft is but you can always cut it if its too long. Plus you will need a knob if you dont want it to look like a hack job.
Potentiometers
You have to buy $10.00 worth before shipping.

If you look at their catalog you might be able to get everything you need and come up with $10.00. I had to do that with the other place and ended up buying a few extra to sell.


I've been in contact with jstcd on his automatic design and I under stand it more now. I have parts on order to make a few and I should have a protype built hopefully in the next few weeks.

[hobbit]

I also found that the internal ECU resistor is about 2.5K.
.
For my fakeout I went with 470 ohm in series with a 5K linear pot,
which gives me a little more range but "safeties" the thing so it
doesn't drive the temp way up into fan-running territory. And a
separate toggle switch to cut the whole thing out once things are
genuinely warmed up. It'll get a mention in the next biggish
maintenance writeup I'm working on.
.
_H*

[mrbigh]

I'm including the last post I just made on the Coolant Temperature Hack with some updated information.

Happy PHEVing