Useful Information | Another Design Scheme for Constant Current Circuit

When it comes to constant current circuits, hardware R&D engineers are no strangers. When designing LED driver-related projects, they often encounter such circuit problems. For constant current circuits, the general method is to use a mutual clamping circuit of two triodes or a precision constant current circuit built with an op amp. Today, I will briefly introduce the schematic diagrams of these two constant current circuits.

Constant current circuit design:

1. Transistor constant current circuit:

Transistor constant current circuit

The constant current circuit of the transistor mainly utilizes the characteristic that the base conduction voltage of the Q2 transistor is 0.6~0.7V; when the Q2 transistor is turned on, the base voltage of the Q1 transistor is pulled down and cut off, and the load R1 does not work; the current flowing through the load R1 is equal to the current of the R6 resistor (ignoring the base current of the Q1 and Q2 transistors), and the current of the R6 resistor is equal to the 0.6~0.7V voltage across the R6 resistor divided by the resistance value of the R6 resistor (fixed), so the current flowing through the R1 load is constant, even if the VCC voltage at the power supply end of the R1 load is variable, the constant current circuit effect can be achieved;

2. Operational amplifier constant current circuit:

Operational amplifier constant current circuit

The constant current circuit of the operational amplifier mainly utilizes the "voltage following characteristic" of the operational amplifier, that is, the circuit characteristic that the voltages of the two input pins Pin3 and Pin2 of the operational amplifier are equal; when Vin is input into the resistor R4 to stabilize the power supply voltage, the voltage across the resistor R7 is also unchanged at Vin, so no matter how the external circuit changes, the current flowing through the resistor R7 is unchanged; the same as the principle analysis of the transistor constant current circuit, the current of the R2 load is equal to the current of the R7 resistor, so even if the power supply of the R2 load is a variable voltage power supply, the current of the R2 load is also kept fixed, achieving the effect of constant current;

In addition to the constant current circuit designed with triode and op amp, Chip Brother introduces another constant current circuit design scheme, which mainly uses the voltage stabilizing characteristics of the voltage stabilizing diode.

Zener diode constant current circuit

In the constant current circuit of the Zener diode, the base voltage of the transistor Q4 is limited to the stable voltage Uzd of the Zener diode. Therefore, the voltage of the resistor R10 is equal to Uzd minus the conduction voltage drop of 0.7V between the base and emitter of the transistor, that is, U=Uzd-0.7 remains constant. Therefore, the current flowing through the resistor R10 is fixed even if the VCC power supply is variable, that is, the current of the R8 load remains unchanged, achieving a constant current effect.

The three different constant current circuits can meet the design requirements of the project well. I wonder if you have used the constant current design scheme of the Zener diode introduced above?

Source: Toutiao Chip Brother