Hi again (because of zone time difference),
Let us agree on some basics so that we can be surer on how to define our project here by real given data.
So far, we talked about a resistance and a voltage. I guess you like talking about voltages since we can measure them anytime by a voltmeter.
Also a resistance can be measured rather easily by an ohmmeter if the element (resistor) has no power applying on it (that is, there is 0 V across it prior to measurement). But what does the meter do in order to give the resistance reading? In case of a resistor, the simple formula could be applied:
V (DC voltage across the resistor) = I (DC current flowing in it) x R (its DC resistance value)
That is (if * means multiplication):
V = I * R
Therefore
R = V / I
So if the meter has an internal constant current (I) source (that is, its delivered current stays the same no matter if the voltage on its two terminals changes), measuring (V) then dividing it by (I) gives the value of (R). For instance, natural electric sources around us are close to the 'constant voltage source' instead, like batteries (that is, ideally, it applies a constant voltage on a load. like a resistor, no matter how much current the load takes). So to have a constant current source (within a limited range of voltage) we need to build an active circuit which could also be called a 'current regulator'.
Back to our formula, if I = 1 Amp (though this is a too high value for a meter, R would be equal to the measured V. So if V=1 V then R=1 Ohm, V=2 V, R= 2 Ohms... etc
But there is a fourth factor which is worth to always consider. It is the power (P); generated or dissipated. Also its calculation is simple:
P = V * I
or
P = V * V / R = I * I * R
Let us apply these two formulas on our project here.
We are talking about R in the vicinity of 40 Ohms and a voltage across it about 11 Volts.
I = 11 / 40 = 0.275 A
P = 11 * 11 / 40 = 3.025 W
Could those rather high values be real just for a sensor?
I am not familiar with car meters so I can’t give a practical comment on this.
On the other hand, should the car battery generator run before the meter and sensor can work properly?
You know, the nominal battery voltage is 12 V and it may be as low as 10.5 V (before a recharge is necessary).
For instance, how were you able to know about the sensor resistance?
Kerim