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Adjust the potentiometer to vary the load.
An Electronic Load is a test equipment that is used to test the capabilities of a power source. An Electronic Load has different modes of testing: Constant Current (CC), Constant Voltage (CV, an odd mode), Constant Resistance (CR) and Constant Power (CP). This circuit is an Electronic Load in Constant Resistance mode. This mode is used to test power sources such as batteries.
VALUE OF CONSTANT RESISTANCE:
Let's call the voltage appearing at the non-inverting terminal of the op-amp V+ and the voltage appearing at the inverting terminal V-.
By voltage divider theorem,
V+ = V∙(R2/(R1+R2))
By ohm's law,
V- = I∙Rs
Where V and I are the voltage and current from the power source under test, R1 is the 100k resistor, R2 is the 100k potentiometer and Rs is the current sense resistor.
For an ideal op-amp,
V- = V+
Therefore,
V∙(R2/(R1+R2)) = I∙Rs
Rearranging the equation,
V/I = Rs∙(R1+R2)/R2 = Rconst
Where Rconst is the Constant Resistance offered by the Electronic Load. So the formula to calculate the Constant Resistance value is
Rconst = Rs∙(R1+R2)/R2
Since the numerator is a constant, Rconst varies as R2 is varied.
Putting the values of the resistances used in this circuit,
Rconst = 1∙(100k+100k)/50k = 4ohms
This can be confirmed from the I-V curve. At the lower end, current is 1.25A and voltage is 5V. Ohm's law tells us that the resistance is 4ohms. Taking values from the upper end (3.75A and 15V), we again see that the resistance is 4ohms.
Note: those 10nF capacitors don't have any real use. I have put them there because they keep the simulation from crashing. In real life, you can build the circuit without them.
Source: A PDF from keysight. Google electronic load keysight technologies.
MOSFET used: IRF540N
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