Simple Current Limiter Circuit using Transistors

Designing a circuit that limits the amount of power that is supplied to other electronics is addressed in this tutorial.

Working as a protection to avoid overload it makes use of two transistors and a particular resistor.

What is a Current Limiter Circuit using Transistors:

The current flowing through a load is limited by a transistor based current limiter circuit to avoid overcurrent problems.

This kind of circuit uses electrical devices and a power source to protect against component damage from high voltage.

Circuit Working:

Parts List:

The primary current controller is T1.

The load is used to measure the current passing through T1 which is limited by T1s qualities.

T1 can conduct and transfer current to the load because its base is connected to the positive supply.

Once exceeding a specific threshold the voltage drop across the current sense resistor rises.

Through T1s emitter the base of T2 receives this higher voltage drop.

In reaction to this higher voltage T2 begins to conduct.

The base current of T1 is simply directed to the ground by the conduction of T2 which limits the current flowing through T1.

Formulas and Calculations:

The below mentioned formula is used to find the value of R1.

For example a 100 Watt LED is used as the load.

R1 = (Us−0.7) × Hfe / Load Current

where,

• Us indicates the supply voltage
• Hfe is the forward current gain of T2 and
• load current is equal to the LED current which is calculated as:

100W / 35V = 2.5 amps

Substituting the values we get:

R1 = (35 − 0.7) × 30 / 2.5 = 410 Ω

The power rating of the connecting resistor is calculated by 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 power rating for R2 is measured as:

0.7 × 2.5 = 2 watts

Note:

By keeping an eye on the voltage drop across the current sense resistor T2 acts as a current limiter.

In the case of an overcurrent T2 limits T1s base current which lowers the collector current flowing through T1 and as a result reduces the current flowing to the load.

Power supply and electronic protection circuits often use this kind of current limiting system to protect themselves against damage from excessive current.

How to Build:

To build the simple current limiter circuit one would need to follow the following steps.

• Identify the pinouts of transistors T1 and T2.
• Normally transistors have three pins: collector, base and emitter.
• Connect the collector of T1 to the positive side of the load e.g. LED.
• Emitter of T1 is connected to the ground.
• The collector of T2 should be connected to base of T1.
• Connect the emitter of T2 to the ground.
• The base of T2 to the connected to junction between the emitter of T1 and the current sense resistor R2.
• Join one end of R2 to the emitter of T1.
• The other end of R2 should be connected to the ground.
• Connect the positive terminal of the power supply to the base of T1.
• And the negative terminal of the power supply connect to ground.
• Calculate the values of R1 and R2 using the provided formulas.
• Resistor R1 must be connected between the power supply and the base of T1.
• And connect R2 between the base of T2 and the ground.

Test the Circuit:

• Once the power ON one should keep an eye on the LED or load.
• Test the circuit with different 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 carefully especially if you are dealing with high currents or voltages.

Conclusion:

Using transistors in current limiter circuits provides a powerful and adjustable way to protect sensitive parts from excessive current.

This simple transistor based current limiter circuit protects electronic devices from damage by using the transistors capacity to regulate current flow to limit current to a set level.

Depending on the specific transistor configuration and component selection the current limiters functioning features are modified for various purposes.

References:

Current limiting