Diver300 said:The 9V zenner is a good idea. However, there are two possible problems with it.
Firstly, if the supply voltage changes, for example from 12V with the engine stopped to 14V with it running, then the voltage remaining after the zenner (12-9 = 3 up to 14-9 = 5) will change a lot.
How'd ya guessSecondly, the guage probably has resistance that is similar to the minimum resistance of the sender. For one thing, if the sender short out, it is a good idea not to have a lot of current and sparks inside the fuel tank.......
I like that last bit!So if the guage needs more than 3V accross it to read full you won't be able to get to the top of the scale.
You should try to keep the resistance in series with the sender the same as what it was before.
I suggest:-
View attachment 15102
R1, the zenner and the Base-emmiter drop of the transistor provide 3V to the sender. R2 is the fake guage, so that the sender has the same resistance in series with it as it was meant for. Try to find out the resistance of the original guage that the sender worked with before, and use a resistor of that value.
Because the pair is fed by 3V not 12V, the current should be 1/4 of original. The transitor has a current gain of 1 in this configuration so that current is fed to the guage.
This circuit will not be affected by the supply voltage.
You should check the collector voltage when the tank is full. If it is less than 4V the transistor is saturating and you might need a more complicated circuit.
You can adjust the zenner voltage and R2 to get the guage to read correctly over the whole range.
Norcal02 said:A couple of notes though, you mentioned R2, there are two R1's. Did you mean to label one of them R2?
Norcal02 said:What wattage would you suggest for the zener?
Any suggestions on a particular transistor that would be suitable? NPN right? Would any old NPN transistor work?
Norcal02 said:Last thing I can think of for now is should the ground on the sender be the same ground that is used for the zener, or should that not matter?
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