I have a load that has varying resistance and want to know the change of resistance by measuring the voltage with constant current. The figure below is known to provide constant current. But I have no idea what transistor, resistor, and zener diode I need to use. Assuming that 5V will be supplied, and the load will be in approximately range of 2.5 - 15 ohms, what transistor, resistor, and zener diode will I need to use? At least, would you please inform me the equations that i need to set up for the resistor, and etc.?
Okay look the circuit you have provided is the constant current circuit. Now to find the constant current you first need to find the voltage drop across Re (emitter resistor) and voltage across zener diode (suppose if the zener is of 2vV and supply i.e Vcc = 5 V then drop across zener will be Vz=2 V).
Now if Vz=2 V then Ve (i.e voltage drop across Re) is Ve= 2 V-0.7 V=1.3 V (considering Vbe=0.7 V).
Now that you have Re and Ve you can find Ie (emitter current) this is the constant current that will flow through the load.
So if the load is 2.5 ohm then load voltage VLoad=(2.5*Ie) and if the load is 15 ohm then the load voltage VLoad=(15*Ie).
Note: Here Re=R2