Simple constant current source circuit diagram analysis

This circuit provides a constant current to the LED. The current through the LED depends on the value of resistor R2. Assume R2 is 560Ω. When 1 mA current flows through R2, a voltage of 0.56V will be generated across the resistor, turning on the BC547. This will shunt the base current of the BD679, causing it to turn off.

If the supply voltage increases, this will cause the current through the circuit to try to increase. If the current tries to increase, the voltage across R2 increases and the BD679 closes more, which in turn causes the voltage across R2 to decrease, thus clamping each other to keep the circuit at a constant current.

Constant current source circuits 2 and 3

By rearranging the components in the circuit above, it can be designed to control the constant current source circuit to be turned on or off by an input level.

The value of R determines the current through the light emitting diode (LED).

5 mA R = 120R or 150R

10mA R = 68R

15mA R = 47R

20mA R = 33R

25mA R = 22R or 33R

30mA R = 22R

Constant current source circuit 4

With the BC328 circuit, the output will be limited to 100mA by using a red LED and a 10Ω resistor, and to 500mA-800mA by using a red LED and a 2.2Ω resistor. By replacing the BC328 with a BD140, the current will be limited to 1A by using a red LED and a 1Ω resistor.

The second constant current source circuit has an output current of 1.5A, which can drive a 5-watt LED, which has a voltage drop of about 3.2V. This circuit can also change the number of diodes in series to change the output current.