Circuit design using LED to indicate current amplitude

The luminous intensity of the LED in the picture is proportional to the load current . The circuit is designed to provide a very compact circuit to replace the meter on the 12V power line in some astronomical equipment. This equipment contains low-power heating elements (anti-exposure elements) whose operating conditions are not visible. However, when the heaters are switched on, the LEDs emit visible light, clearly indicating that they are connected and working.

Circuit analysis is very simple. The voltage across the 22 ohm resistor is the same as the voltage across RS ENSE. The current flowing through the 22 ohm resistor is the same as the current flowing through the LED. Therefore, for the parameter values ​​given in the figure, the LED current is equal to 0.05/22 of the load current. When the load current changes from 200mA to 6.6A, the LED changes from weak light emission to full brightness (limited by the 680 ohm resistor).

If desired, another LED with a fixed resistor can be added for comparison. Alternatively, the histogram display can be obtained by constructing different examples of this circuit in which different resistors are used instead of the 22 ohm resistor and RS ENSE remains the same.

The op amp must have inputs and outputs with amplitudes up to the positive voltage rail and can withstand supply voltages of slightly more than 12V. TI's TLV2731 is ideal for meeting these requirements.

When the load current is 200mA, the LED emits weak light; when the load current increases to 6.6A, the LED emits the brightest light.