Efficient Current Monitor

R1 is a very low value resistance in the milliohm range. It could be a PCB track or a short length of (thin) wire. In normal circumstances Q1 and Q2 act as an approximate current mirror. The small current flowing through R2 will be reflected at the collector of Q2. As the load current increases Q1 begins to turn off and all the current flowing through R1 ends up in the base of Q2. Lighting LED1 strongly.

Since R1 is such a low value resistor the losses are low and the circuit is efficient.

The LED could be replaced by a relay or some other circuitry to stop the power supply if an overload occurs.

An efficient current monitor circuit based on a current mirror is useful in a variety of applications where it is necessary to measure current with a low voltage drop. For example, in battery-powered devices, every millivolt of voltage drop counts, so it is important to use a current monitor circuit that is as efficient as possible.

Another example of an application where an efficient current monitor circuit is useful is in power electronics circuits, such as motor controllers and solar inverters. In these circuits, the current levels can be very high, so it is important to use a current monitor circuit that can handle the high currents without overheating.

Here are some specific examples of how an efficient current monitor circuit based on a current mirror can be used:

Battery monitoring: An efficient current monitor circuit can be used to monitor the current flowing into and out of a battery. This information can be used to estimate the state of charge of the battery and to prevent the battery from being overcharged or discharged.
Power supply monitoring: An efficient current monitor circuit can be used to monitor the current flowing through a power supply. This information can be used to ensure that the power supply is operating properly and to prevent it from being overloaded.
Motor control: An efficient current monitor circuit can be used to monitor the current flowing through a motor. This information can be used to control the speed and torque of the motor and to prevent it from overheating.
Solar inverter monitoring: An efficient current monitor circuit can be used to monitor the current flowing from a solar panel to an inverter. This information can be used to maximize the efficiency of the solar power system and to prevent the inverter from being overloaded.