Constant Current Source with BJT Transistor

Summary

This video demonstrates how to design simple transistor circuits for creating a constant current source or sink, ideal for driving LEDs. The speaker explains the challenges of using resistors for current limiting and introduces transistor-based circuits that stabilize current even with varying supply voltages or LED configurations. The video also covers using diodes for more stable voltage regulation, ensuring consistent current through LEDs. With basic calculations and easy-to-implement components, the tutorial shows how to create cost-effective and reliable LED drivers, offering a practical solution for non-precise, low-cost applications.

Takeaways

• 😀 Using a simple transistor circuit can be an affordable and low-precision option for creating a constant current source or sink.
• 😀 A common use case for a constant current source is driving LEDs, especially when resistors alone aren't sufficient due to varying current requirements.
• 😀 A typical LED circuit with a resistor requires recalculating the current-limiting resistor whenever you switch LEDs or add more in series, but a constant current circuit can solve this problem.
• 😀 Transistor-based circuits can provide a more stable current for LEDs without needing to adjust the resistor when changing LEDs or adding more in series.
• 😀 A simple NPN transistor with a voltage divider and an emitter resistor can be used to control the current flowing through the LED.
• 😀 The transistor’s base voltage can be set using a bias voltage divider to ensure the desired current flows through the emitter resistor.
• 😀 The beta value of the transistor should be considered when designing the circuit to ensure consistent operation across different transistors.
• 😀 A common rule for resistor values is that the voltage divider resistors (R1 and R2) should be much smaller than the emitter resistor to ensure proper operation.
• 😀 Diodes, including Zener or regular silicon diodes, can replace the resistor divider for a more stable constant current setup by maintaining a fixed voltage drop across them.
• 😀 Using diodes in place of resistors for voltage regulation simplifies the setup, as the current remains stable even with varying supply voltage, as long as there is enough current flowing through the diodes.
• 😀 The circuit setup with diodes and resistors helps ensure a stable current for LEDs in varying supply conditions, with only minor deviations in current when the number of LEDs changes.

Q & A

What is the main purpose of using a constant current source for LEDs?

The main purpose is to provide a stable current through the LEDs, which is important for consistent brightness and avoiding damage to the LEDs. A constant current source helps ensure the current remains steady, even if the voltage supply or the LED characteristics change.

Why can't a simple resistor be used to drive LEDs in all cases?

A simple resistor doesn't provide a stable current if the supply voltage or the number/type of LEDs changes. The current will vary significantly depending on the LED's forward voltage or the number of LEDs in series, making it unreliable for consistent performance.

What is the limitation of using a resistor-based current-limiting circuit for multiple LEDs?

When using a resistor to limit current for multiple LEDs, the voltage drop across each LED adds up. This can reduce the voltage available for the resistor, causing the current to decrease as more LEDs are added, which affects the performance of the circuit.

How does a transistor-based constant current source improve the situation compared to a simple resistor circuit?

A transistor-based constant current source provides more stability because it adjusts based on the emitter resistor and the transistor’s behavior. It can regulate the current more precisely, allowing for multiple LEDs to be driven in series without needing recalculations of the current-limiting resistor value.

What is the significance of the beta (β) of a transistor in the constant current source circuit?

The beta (β) of a transistor is crucial in determining the current gain. It affects the consistency of the current in the circuit. To ensure stable operation despite variations in beta between different transistors, the circuit design accounts for this by ensuring the emitter resistor value is large enough relative to the voltage divider resistors.

Why is the emitter resistor crucial in a transistor-based constant current source circuit?

The emitter resistor sets the current through the circuit. By calculating the appropriate value for this resistor, the current can be controlled independently of the transistor's variations, providing a stable current for the LED regardless of other factors.

What role does the voltage divider play in the transistor circuit?

The voltage divider helps bias the base of the transistor, ensuring it turns on properly. By adjusting the resistors in the divider, you can set the base voltage and consequently control the emitter current, which determines the current flowing through the LED.

How do diodes improve the transistor-based constant current source design?

Diodes, such as silicon or Zener diodes, can be used to set a fixed voltage drop across them, stabilizing the base voltage of the transistor. This provides a more stable operation for the constant current source, especially as the supply voltage changes.

What is the advantage of using multiple diodes in the current source circuit?

Using multiple diodes allows for more precise control over the voltage drop across the base of the transistor, which can be adjusted to meet specific requirements, such as maintaining a particular current or voltage level without needing to adjust other circuit components.

How does the supply voltage affect the transistor-based current source circuit?

The supply voltage influences the voltage divider and the transistor’s operating point. If the supply voltage changes significantly, the current may fluctuate unless the circuit is designed with components (like diodes) that stabilize the transistor’s base voltage, ensuring the current remains relatively constant.