Current-limiting circuits restrict the current drawn from a power supply to a predetermined level, to protect the downsteam load from excessive current. These circuits are commonly found in many electronic systems and appliances. They have adjustable features to cut off the output current across the load; without them, excessive current could lead to a short circuit or system failure.
Current limiting circuits could be simple or more sophisticated, to operate at wider voltage fluctuations, or with greater temperature stability, or to suit specific applications. Nevertheless, they all function the same with different operating windows
To determine how such a circuit reacts to input voltage changes, we set up this experiment, consisting of a simple current-limiting circuit powered by 12Vdc; see Figure 1. This circuit is designed to deliver a maximum load current (through R3) of 7.3A when the R3 resistance is shorted (zero Ohm).
Figure 1: A simple current-limiting circuit with 12Vdc power supply and a maximum load current of 7.3A
To examine the circuit’s performance at various input voltages, I connected it to a variable power supply of between 1Vdc and 30Vdc. The current passing through the load was then measured. It’s worth mentioning that the maximum load current is measured by shunting the load resistance (load resistance at zero Ohms).
Figure 2 shows the maximum load current drawn from the circuit at various voltage levels. It can be seen that the load cutoff current is reversely-proportional in relation to the power supply voltage, changing at a rate of 0.13A for every 1Vdc change in voltage supply, for power supply voltages between 1.3Vdc and 3.2Vdc. The maximum load current changes by 2.8A for every 1Vdc, for supply voltages between 3.2Vdc and 30Vdc.
Figure 2: Maximum load current vs. power supply voltage
This leads to the conclusion that the selected current-limiting circuit maximum load current (cutoff current) is impacted by the power supply voltage at the point of short circuit (in the load).