Voltage regulators are essential in electronic circuits, ensuring a stable output voltage despite fluctuations in input voltage or load current. However, standard three-terminal regulators like the LM7805 often have limitations in the maximum current they can supply. Current boosting techniques enable these regulators to deliver higher currents to the load, making them more versatile for various applications. Here we explain how current boosting works, using an example circuit with an LM7805 3-terminal +5V DC output voltage regulator and a 2N2955 PNP power transistor.
Voltage regulators are essential in electronic circuits, ensuring a stable output voltage despite fluctuations in input voltage or load current. However, standard three-terminal regulators like the LM7805 often have limitations in the maximum current they can supply. Current boosting techniques enable these regulators to deliver higher currents to the load, making them more versatile for various applications. This article delves into how current boosting works, using an example circuit featuring an LM7805, a 2N2955 PNP power transistor, and a 1N4007 diode.
The LM7805 Voltage Regulator
The LM7805 is a popular three-terminal linear voltage regulator that provides a fixed 5V DC output. It is simple to use, requiring only a few external components. However, the LM7805 typically has a current limit of about 1A. For applications requiring more current, a current-boosted configuration is necessary.
Current Boosting with a Power Transistor
By adding a power transistor, a resistor, and a diode to the LM7805, the regulator's output current can be significantly increased. The transistor shares the load current with the LM7805, allowing the circuit to provide higher currents without overloading the regulator.
Circuit Description
Below is the schematic for a current-boosted positive voltage regulator:
LM7805: The main voltage regulator that ensures the output remains at 5V.
2N2955 Power Transistor: A PNP transistor that provides additional current to the load.
R1 (100Ω): A current-sensing resistor that directs excess current to the transistor.
1N4007 Diode: A general-purpose diode connected between the emitter and collector of the 2N2955 to prevent reverse voltage damage to the transistor.
How It Works
When the load current is within the LM7805’s capability, it supplies all the current directly.
As the load current increases beyond the LM7805’s limit, the voltage drop across R1 increases.
This voltage drop activates the 2N2955 transistor, which starts conducting and supplies the additional current required by the load.
The 1N4007 diode ensures that any reverse voltage across the transistor (e.g., during transient conditions) does not damage the transistor.
The LM7805 continues to regulate the output voltage, while the 2N2955 handles the excess current.
Circuit Design Considerations
Current-Sensing Resistor (R1): The value of R1 determines when the transistor starts conducting. A value of 100Ω is used in this example to balance current sharing and activation sensitivity.
Power Transistor (2N2955): The 2N2955 must have a current rating sufficient to handle the additional load current. Ensure adequate heat sinking to prevent overheating.
1N4007 Diode: This diode protects the transistor by blocking reverse currents that could occur during transient conditions or shutdown.
Thermal Management: The LM7805 and 2N2955 can generate significant heat under high currents. Use heat sinks to dissipate heat effectively.
Related Resources
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Check out how to build an inverter with Arduino for converting DC to AC power efficiently.
Important Notes
No Overcurrent Protection: This circuit does not provide overcurrent or overtemperature protection. Use fuses or other safety measures to protect the circuit and connected devices.
Negative Regulators: For negative voltage regulators, the power transistor changes to an NPN type, but the principle and equations remain the same.
Applications
Powering high-current devices such as motors and relays.
Extending the current capabilities of existing power supplies.
DIY projects requiring more current than standard regulators can provide.
Conclusion
Current boosting with a power transistor and a diode is a straightforward and effective way to enhance the current-handling capabilities of three-terminal voltage regulators like the LM7805. By understanding the design and operation of such circuits, you can tailor your power supply to meet the demands of high-current applications. However, always ensure proper thermal management and consider safety features for robust operation.
