Easy Costruction and Installation of a FUEL GAUGE

[theflyer]

Hi,

After my previous post where I described my simple circuit to display alternator charge status (see Gadgets & Farkles section) I will now present another of my achievements. This is an easy installation (without modify the bike tank) FUEL GAUGE.

(All new alert indicators are placed on a small additional panel with three high-brightness 3mm red LEDs, see image).
















Any comment is welcome !!!

[ImaginativeFig]

For anyone (like me) who didn't know what NTC was, it apperas to be a type of thermistor (that is, thermal resistor) with an inverse relationship between resistance and temperature (temp up, resistance down, the NTC stands for Negative Temperature Coefficient).
http://www.cornerstonesensors.com/Ab...hat&Print=Page
http://en.wikipedia.org/wiki/Thermistor#NTC
Googling, I can see that this is a fairly common use of NTC Thermistors, but why does it work? Is the liquid gas that much cooler than the air above it?
Also, the Flyer says that it's a 3.5 kOhm NTC... is that the R25 or what?
What ever happened to the Flyer? It looks like he never got around to posting the oil sensor instructions....

[Water Warrior 2]

It has been over 3 years so he likely found something else to ride or moved away. Riders come and go.

[5th_bike]

Originally Posted by ImaginativeFig

why does it work? Is the liquid gas that much cooler than the air above it? ....

No, but as long as the NTC is in the gas (fluid), it is liquid cooled. Once it is out of the gasoline, it is vapor cooled. The current going through it, warms it up. Since it is not cooled by liquid anymore, it becomes much warmer than when it would be submerged. Then, more current will flow through, it will become even warmer etc. and the LED lights up.

I wouldn't want some hot resistor in my gas tank, though...

And I have no idea what the resistors in series are for (parallel to the LED branch), and I wonder what the resistor parallel to the LED is for, that one is probably to keep the LED from gently shining when the NTC is still submerged.

[ImaginativeFig]

Oh, sure, (sort of) answer the easy question, but not the ones I'm really curious about. I guess I could do a more intensive googling for those though.

[ImaginativeFig]

Oh, that is an interesting consideration! Do you think it would get hot enough to cause potential ignition? Geez, I hope that's not why the flyer stopped posting.

Whenever I'm not in a class that actively uses circuits, I get a bit fuzzy on electrical stuff...

I found a post on another forum http://www.diystompboxes.com/smfforu...prev_next=prev that explains R4

I don't know what that specific circuit does, but I do know what mounting an LED on a voltage divider does.

Assume a circuit with two 1K resistors in series, and an LED parallel with the 1K to ground.

As you increase the voltage across the two resistors from 0V what happens?

Initially, the LED does not have enough voltage to conduct, so it's effectively not there. The voltage at the middle of the two resistors goes up linearly at 1/2 the voltage applied to the top resistor.

Then the voltage at the divider gets to the LED turn-on voltage. Now the LED starts sucking current. But how much?

The LED forward voltage is roughly constant. Not exactly, but close. So with a constant voltage across the lower 1k, its voltage remains constant by the same "roughly". Now as you increase voltage, all of the increased voltage appears across the top 1K resistor, and the current is ((Vsupply-VLED)/1K) + (VLED/1K) The first term being the variable current in the top 1K resistor, the second being the constant current in the bottom resistor.

It's a turn-on delayer. The LED turns on at twice the voltage it otherwise might.

I think that when the NTC is submerged (temp down, res up), the current will run primarily through the R1/R2 leg (and I figure there are two used just to get the right value?) so actually, that leg is also a turn on delayer (right?).
The combination of R3 and the unsubmerged NTC must be supposed to draw the appropriate amount of voltage for the LED, though I'm a bit confused why R3 wouldn't be before they LED, as that's where I understand voltage limiting resistors are supposed to go, but then then again, as I said, I'm pretty fuzzy on this. I did fail my circuits class the first time around.

[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...

[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?

[ImaginativeFig]

Ah! Clever!
Haha, my last post was more explaining for me than for you (since it didn't even really touch what you were talking about), originally I was going to post questions, but I figured out the answers to them which is what all of that post turned into, me answering my own questions.
And now you've answered my other question, as well as why R1 is shoved down there with the NTC. Cooool