I m going to design a circuit to measure a resistor,R n display to LCD. this circuit will using a constant current. by using voltage drop trough the R, I will programming the PIC to measure the resistance of the R using formula:

R =

I(constant)

however the circuit is using voltage source. And my problem is how to make the current flow trough the R always constant although the change of value of R. Because I have simulate a circuit that I get from internet, the current still changing when I change the value of R...WHY?

R =

__Va-Vb__I(constant)

however the circuit is using voltage source. And my problem is how to make the current flow trough the R always constant although the change of value of R. Because I have simulate a circuit that I get from internet, the current still changing when I change the value of R...WHY?

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