Just think a water hose filling a bucket.
A normal circuit would be like setting the water pressure voltage and hoping the bucket fills perfectly.
But with LEDs, a special circuit acts like a fancy nozzle that keeps the water flow current steady, even if the pressure changes a bit.
This ensures the LED lights up properly and lasts longer.
Circuit Working:
Parts List:
| Type | Value | Quantity | Note |
|---|---|---|---|
| Resistors | 2.2k | 1 | |
| R (see the text) | 1 | ||
| Semiconductors | Transistor BC547 | 1 | |
| Transistor BD139 | 1 | ||
| Other | LED any 5mm, 20mA | 1 |
This LED constant current circuit is designed with simplicity in mind using just 2 transistors, 2 resistors and an LED.
It can accommodate input voltages ranging from 2V to 24V making it suitable for driving almost any type of LED with a power consumption of up to 5W.
For the T2 transistor, a BC547 can be used with standard 20mA LEDs, but it is recommended to use BD135, BD137 or BD139 for LEDs with higher power dissipation.
When the voltage exceeds 2V the collector current of T2 increases causing an increase in the base current of T1 which in turn brings T1 into conduction.
This causes the collector potential of T1 and the base of T2 to become more negative gradually “closing” T2 and counteracting the initial current increase.
This stabilizing effect ensures a constant current through the LED.
For example, for a 20mA LED the value of R would be approx 30Ω.
The following are the current values for a normal red LED depending on the voltage:
Formulas:
- The value of resistor R is calculated as the LED current divided by 0.5:
R (Ω) = 0.6 / I (mA)
where,
- R(Ω) is the resistance, expressed in ohms, that we are computing.
- I (mA): represents the known or given current in milliamperes (mA).
- 0.6: in the formula, this constant denotes 0.6 milliamperes (mA).
Example:
Assume you have a circuit with a resistor receiving a current of 10 mA (milliamperes).
To ascertain that resistors resistance:
R(Ω) = 0.6 / 0.02 = 30 ohms
In this case, the resistors R resistance would be 30 ohms.
This formula, which is particularly helpful in circuit analysis and fundamental electronics, is simple to use and may be quickly calculated when working with current and resistance.
2) The below formula determines a resistors power dissipation P in watts W given its resistance R in ohms Ω and the current I flowing through it in amperes A.
The power dissipation of resistor R is given by:
P(W) = I2 (A) x R(Ω)
where,
- P(W) stands for the resistors power dissipation in watts.
- The quantity of electrical power that is transformed into heat as current flows through the resistor is referred to as power dissipation.
- I(A) is the current, expressed in amperes A, that is passing through the resistor.
- In an electric circuit, it refers to the speed at which electric charge moves past a point.
- R(Ω) is the resistors resistance expressed in ohms Ω.
- The opposition of an electric current flowing through a conductor is known as resistance.
Note:
When building circuits, this formula is essential for ensuring that resistors can withstand power dissipation without overheating, breaking down or harming other components in the vicinity.
It draws attention to the connection between resistance, current, and the subsequent heat dissipation a link that is crucial for choosing the right resistors for electrical circuits.
More power dissipation results from higher currents or resistances, highlighting the necessity of appropriate heat management in electronic systems and devices.
How to Build:
To build a Simple LED Constant Current Circuit you need to follow the below mentioned steps:
Select the LED:
- Choose an LED based on the desired power consumption and forward current rating.
- Ensure that the LEDs forward voltage drop is within the range of 2V to 24V.
Calculate the resistor value R:
- Determine the LEDs forward current I rating.
- Calculate the resistor value using the above formula.
- For example, for a 20mA LED, R would be around 39Ω.
Assemble the circuit:
- Connect the components as shown in the schematic below:
- Connect the positive terminal of the power supply to the collector of T1.
- Connect the emitter of T1 to the anode of the LED.
- Connect the cathode of the LED to the collector of T2.
- Connect the base of T1 to the junction of resistor R.
- Connect the base of T2 to the junction of 2.2k resistor.
- Connect the emitter of T2 to the negative terminal of the power supply.
Power up the circuit:
- Apply the desired voltage between 2V and 24V to the circuit.
Test the circuit:
- Verify that the LED is lit and the current through it remains constant as the input voltage varies within the specified range.
Note:
- Please note that proper care should be taken while handling electronic components and ensure that the circuit is built and tested in a safe manner.
Conclusion:
A LED constant current circuit is a crucial component in LED lighting systems ensuring that LEDs receive a stable and regulated current for optimal performance and longevity.
By maintaining a constant current these circuits help prevent LED damage and ensure consistent brightness, making them essential for a wide range of lighting applications.
Whether through simple resistor based designs or more complex regulator circuits LED constant current circuits play a vital role in the efficient and reliable operation of LED lighting systems.