Norcal02

Alright, this is what I think I've found. Correct me if I'm wrong please
Edit-I hope I'm not too far off track, by the end of this post I think there's a solution that might work with an extra resistor. What do you think? I have a knack for popping zeners so I cant test my idea right now LOL

As I was going lower on R1 values, the difference on the gauge got smaller and smaller. Reason being, the lower the resistance, the higher the mA at the base of TIP31, but as the mA's get higher, the change of the collector voltage gets smaller as the curve flattens out.

To get to the point where I need to be to get the gauge to read full, I've figured the mA input needs to probably be about 10ish mA to get a low enough output voltage at the collector to read the tank full. I hope that doesnt screw up the empty reading too badly, but I'll figure that out as I go.

This is how I've sort of gathered the ~10mA figure.
Two resistors I've tried-
a)3900hm:
b)560hm:
Two zener's I've tried-
a)3.6v
b)3.9v
Results for each-
aa)0.923mA
ab)1mA
ba)6.43mA
bb)6.96mA

Gathering that with the AB(1mA) combo went to about exactly 3/8'ths tank, and BB(6.96~7mA) was dead on at 3/4, what would I do to find what would give me a full tank considering the collector emitter voltage graph on the datasheet is a curve, so I dont think I can just make a straight line from 3/8 to 3/4 to 1 and change the numbers at the same ratio. If I did though, I would probably need that ~10mA base current to read a full tank on the gauge. If that's right, looks like it's close to something like .16-.18 base/emitter voltage.

Couldn't I just use the 3.9v zener, 560hm: R1, and put in an extra resistor between the gauge and collector to lower the mA to the gauge that extra little bit? Like 1hm: or less?

Perhaps there is a better suited transistor all together?


Edit 2-The difference between collector emitter voltages between what I have now and what I think would be closer, is so close even a 1hm: resistor would overshoot it depending on the real world resistances. If in a perfect world I have 0.19v now, and .5hm: resistance, the mA is .38 and reads 3/4 tank. I guess I should measure that to be sure next time, then I can have this for sure, I think. If I want to get the mA that 0.17v with .5hm: resistance would make (.34), I'd only need a difference in resistance of +.56hm:... umm but that doesn't seem right...? Of course this is still all speculation on what it looks like on the graph, and that a straight up 10mA base current is what the gauge needs to read full.

Im just trying to figure out some stuff on my own, right or wrong, but need to get pushed in the right direction again I hope I didn't write too much to confuse the situation. It's what I get for thinkinga bout it at midnight!

Norcal02

To let you know, I dont expect you to respond to all of this, most of it is just for record keeping and posting just in case any of it is useful.

Update, I found the obvious, putting a resistor between the gauge and collector is just like increasing R2, which only moves the sweep up and down on the gauge, but does not adjust the range larger or smaller. Undecided whether to use 39hm: or 36hm: at R2 quite yet. 39 reads exactly an empty tank when decreased from full to E, but turned on at E, rises to just below E, its maximum sweep up is to dead on 3/4 tank. 36 turns on when at E just a half needle below E (better), rises to 7/8 tank (better), but reduced to E from Full, it doesn't quite reach (badd), indicating I still have gas left when I wouldn't. I think I'll keep the 39hm: for now.

To reach the readings above I was using a 3.9v 500mW zener and 220hm: resistor.

Diver300

That sounds like the zenner was doing what it should. R1 should provide enough current to keep the base of the transistor at 3.9 V. Reducing R1 below what is needed will just result in extra heating.

The current required by the base of the transistor depends on the gain of the transistor, and the value or R1 that I suggested was based on typical figures for the TIP31, so it could have been wrong.

Diver300

That shouldn't be the case. The collector current shouldn't vary much at all if the collector voltage changes, unless the resistance is so big that there is no voltage left across the transistor.

It could well be that a TIP31 has less gain than would be useful.

If you put aside everything except the guage, and find out what resistance between the guage and ground makes it read full, and what voltage there is across it at that point, it would help me work out what is happening.