Simple Current Limiter Circuit using Transistors

This article shows you how to build a circuit that limits how much power goes to other electronics.

It uses two transistors and a special resistor to act like a guard and make sure things do not get overloaded.

What is a Current Limiter Circuit using Transistors:

A current limiter circuit using transistors is designed to limit the amount of current flowing through a load providing protection against overcurrent conditions.

This type of circuit is commonly used in power supplies and electronic devices to prevent damage to components due to excessive current.

Circuit Description:

Parts List:

Circuit working process:

T1 operates as the main current controller.

The current flowing through T1 is determined by the load and is limited by the characteristics of T1.

The base of T1 is connected to the positive supply allowing it to conduct and pass current through to the load.

When the load current exceeds a certain threshold the voltage drop across the current sense resistor increases.

This increased voltage drop is fed to the base of T2 through the emitter of T1.

T2 responds to this increased voltage and starts conducting.

The conduction of T2 effectively shunts the base current of T1 to the ground limiting the current through T1.

Formulas and Calculations:

To ascertain the value of R1 you can utilize the following equation.

Here we will take the example of a 100 Watt LED as the load.

R1 = (Us−0.7) × Hfe / Load Current

where,

• Us represents the supply voltage Hfe is the forward current gain of T1 and the load current is equivalent to the LED current, calculated as:

100W / 35V = 2.5 amps

Substituting the values we obtain:

R1 = (35 − 0.7) × 30 / 2.5 = 410 Ω

For the corresponding resistor the wattage can be determined using:

P = V2 / R = 35×35 / 410 = 2.98 or approximately 3 watts

The calculation for R2 is as follows:

R2 = 0.7 / LED current

where,

the LED current is 2.5 amps. Therefore,

R2 = 0.7 / 2.5 = 0.3 Ω

The wattage for R2 can be computed as:

0.7 × 2.5 = 2 watts

Note:

T2 acts as a current limiter by monitoring the voltage drop across the current sense resistor.

When an overcurrent situation occurs T2 limits the base current of T1 reducing the collector current through T1 and, consequently limiting the current to the load.

This type of current limiting mechanism is commonly used in power supply and electronic protection circuits to prevent damage due to excessive current.

How to Build:

To build the simple current limiter circuit, you would need to follow these steps.

• Identify the pinouts of transistors T1 and T2.
• Usually, transistors have three pins: collector, base, and emitter.
• Connect the collector of T1 to the positive side of your load e.g. LED.
• Connect the emitter of T1 to the ground.
• Connect the collector of T2 to the base of T1.
• Connect the emitter of T2 to the ground.
• Connect the base of T2 to the junction between the emitter of T1 and the current sense resistor R2.
• Connect one end of R2 to the emitter of T1.
• Connect the other end of R2 to the ground.
• Connect the positive terminal of the power supply to the base of T1.
• Connect the negative terminal of the power supply to the ground.
• Calculate the values of R1 and R2 using the provided formulas.
• Connect R1 between the power supply and the base of T1.
• Connect R2 between the base of T2 and the ground.

Test the Circuit:

• Apply power and observe the LED or load.
• Test the circuit with varying loads to ensure that the current limiting mechanism functions as expected.
• If needed adjust resistor values or transistor choices to fine tune the current limiting behavior.

Important Considerations:

• Choose transistors with suitable voltage and current ratings for your application.
• Use standard resistor values closest to the calculated ones.
• Ensure that the power ratings of resistors are sufficient to handle the calculated power dissipation.
• Work with the circuit cautiously especially if you are dealing with high currents or voltages.

Conclusion:

Transistors are used in current limiter circuits, which provide a flexible and manageable method of shielding delicate components from high current.

This simple current limiter circuit using transistors secure electronic devices against harm by limiting current to a predetermined amount by leveraging the transistors capacity to control current flow.

The working characteristics of the current limiter may be tailored to different applications based on the particular transistor arrangement and component choice.

References

Current limiting