LED Constant Current

Summary of LED constant current

1. This circuit is really convenient to design as the components are easily available in the market.

2. Understanding the working of this circuit is very easy.

3. This circuit is used in the place where there is a need for constant flow of current from LED at a particular voltage.

4. With the help of this circuit you can drive any kind of LED. But the power consumption of the LED would still be 5W.

5. The voltage range that the circuit allows to work is between 2V to 24v.

This circuit is constructed using two transistors along with two LEDs.

Working ofLED constant current

In the circuit when the voltage goes above 2V, large current passes through the collector of T2. Then the base current of transistor T1 goes up so that it can bring this transistor in the conduction stage. However at T1 transistor creates more and more negativity. Exactly same thing occurs with the base of T2. In turn T2 gradually closes and acts against the starting growth of current. That’s how a steady effect is achieved and we get a fixed value of current that will flow from LED.

Rating of R in the circuit is equivalent with the LED current divided by 0.5

R (Ω) = I (mA)/0.5

The power dissipation of R is

P (W) = I² (A) x R (Ω)

For instance- For a normal LED with 20 mA value of current, 39E of resistor can be used.

For a normal red colour LED value of current depends upon voltage-

5V … 15mA

9V … 18mA

12V … 20mA

18V … 24mA

24V … 27 mA

Note- For the standard LEDs with current rate of approximately 20mA transistor (T2), BC547 can be used. However for large power usage it is advisable to utilize BD135, BD137 or BD139.

Components used in the circuit-

Components used in the circuit-

Transistor

T1, T2 BC547

Resistor

R1 2.2K

R2 39E

LED

D1 Red colour