Easy Costruction and Installation of a FUEL GAUGE

[spldart]

Originally Posted by 5th_bike

Aha, I think I figured it out.

You see, R1 is only rated 1/8 (=0.125) watt. And the current through R and R2 is about 12.6 V /(330+22) ohms = 0.0341 A. The wattage through R1 and R2 is then 0.0341 A x 12.6 V = 0.45W. Which is more than what R1 is rated for (0.125W), therefore R1 will run hot. And R1 will heat up the NTC much more when it is not submerged, and then the LED will go on.

So, it is not the NTC that heats up by itself, but R1; and R1 heats up the NTC.

I was kind of on the right track in my previous post, but I hadn't figured the function of R1 and R2 out. You see, once I used a thermistor to measure the temperature somewhere, with a 24V PLC; it didn't work and wondering why, I almost burned my fingers on it, because the current going through it was too much. duh. So that's why I thought the NTC would heat up by itself.

Yes R4 is to keep the LED off, to have a low voltage across the LED until the NTC conducts more.

And you can swap R3 and the (LED+R4), they are in series, R3 can also go between 12V and the (LED+R4). R3 is the current limiter for the LED.

Lastly, I'm not afraid the tank would explode, because there is no oxygen in there, just gas vapors. Still, I wouldn't like a hot resistor in my tank...

You did the math I just did now as I just wandered onto this thread.
Everyone needs to know that that uncovered 330 ohm resistor WILL smoke.
And if you happen to open your tank when it does oxygen can find it.

What I was typing up...
I'm gonna guess no one has tried this circuit.

R1 330 ohms 1/8 watt (in gas tank I presume)
in series with
R2 22 ohm 1/4 watt (out of gas tank I presume)
Combined value is 352 ohms.
Ohms law
I=e/r
e is volts. Running voltage would be 13 +
R is 352 ohms
So I, amps, is .037
p=i*e
p=power equals i=current in amps times e=voltage
p=.037 X 13
p = .48 watts dissipated on that portion of the circuit
R1 is rated for .125 watts
R2 is rated for .25 watts
R2's lesser value will shift most of the vdrop on that portion of the circuit to R1

I would want that in my gas tank.... If I had a death wish.

The other leg of that circuit could work if this NTC devise had the right characteristics.

[Zjajo]

Any confirmed Darwin Awards yet?