Controlling a 12V relay using a 5V signal is a common task in electronics projects. It involves using a transistor or other switching mechanism to amplify the low 5V signal to a level capable of driving the 12V relay coil. This technique is often employed in microcontroller-based projects where the microcontroller output is 5V but the device being controlled requires a higher voltage. There are several methods to achieve this, each with its own advantages and disadvantages.
Understanding the Challenge
The core challenge lies in the voltage difference between the control signal (5V) and the relay coil (12V). Directly connecting a 5V signal to the relay coil will not provide sufficient current to activate it. The relay coil requires a specific amount of current to close its contacts and allow current to flow through the load.
Common Methods to Power a 12V Relay with a 5V Signal
1. Using an NPN Transistor
One of the most popular methods is to use an NPN transistor as a switch. The transistor acts as an amplifier, allowing a small current from the 5V signal to control a larger current flowing through the relay coil.
Circuit Diagram:
!
Explanation:
- The 5V signal is applied to the transistor's base (B).
- The collector (C) of the transistor is connected to the positive terminal of the 12V power supply.
- The emitter (E) of the transistor is connected to the negative terminal of the 12V power supply.
- The relay coil is connected between the collector (C) and the emitter (E) of the transistor.
When the 5V signal is HIGH (logic 1), the transistor turns ON, allowing current to flow through the collector-emitter path, energizing the relay coil. When the 5V signal is LOW (logic 0), the transistor turns OFF, and the relay coil de-energizes.
Components:
- NPN transistor (e.g., 2N2222, BC547): The transistor is the main switching element.
- Relay (12V): The relay is the device that controls the larger current flow.
- Resistor (e.g., 220 ohms): The resistor limits the base current of the transistor, protecting it from damage.
2. Using a MOSFET
A MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) can also be used to switch the relay coil. MOSFETs have a lower on-resistance compared to transistors, resulting in less power dissipation and a faster switching speed.
Circuit Diagram:
!
Explanation:
- The 5V signal is applied to the gate (G) of the MOSFET.
- The drain (D) of the MOSFET is connected to the positive terminal of the 12V power supply.
- The source (S) of the MOSFET is connected to the negative terminal of the 12V power supply.
- The relay coil is connected between the drain (D) and the source (S) of the MOSFET.
When the 5V signal is HIGH (logic 1), the MOSFET turns ON, allowing current to flow through the drain-source path, energizing the relay coil. When the 5V signal is LOW (logic 0), the MOSFET turns OFF, and the relay coil de-energizes.
Components:
- MOSFET (e.g., IRF520, IRF530): The MOSFET acts as the switching element.
- Relay (12V): The relay is the device that controls the larger current flow.
- Resistor (e.g., 10k ohms): The resistor is used to pull the gate of the MOSFET low when the 5V signal is not present.
3. Using a Logic Level MOSFET
A logic level MOSFET is specifically designed to be switched by a low voltage signal. These MOSFETs have a gate threshold voltage (Vgs(th)) that is typically lower than 5V, making them suitable for direct control from a 5V microcontroller.
Circuit Diagram:
!
Explanation:
- The 5V signal is applied directly to the gate (G) of the logic level MOSFET.
- The drain (D) of the MOSFET is connected to the positive terminal of the 12V power supply.
- The source (S) of the MOSFET is connected to the negative terminal of the 12V power supply.
- The relay coil is connected between the drain (D) and the source (S) of the MOSFET.
When the 5V signal is HIGH (logic 1), the MOSFET turns ON, allowing current to flow through the drain-source path, energizing the relay coil. When the 5V signal is LOW (logic 0), the MOSFET turns OFF, and the relay coil de-energizes.
Components:
- Logic Level MOSFET (e.g., IRLML6401, IRLZ34N): The MOSFET acts as the switching element.
- Relay (12V): The relay is the device that controls the larger current flow.
4. Using a Relay Module
A relay module is a pre-assembled circuit designed to control a relay using a low voltage signal. It typically includes a transistor, a diode, and a resistor, along with the relay itself.
Features:
- Simplified wiring: The module provides input and output terminals, eliminating the need to connect individual components.
- Protection: Some modules include built-in protection features, such as reverse polarity protection and overcurrent protection.
- Variety: Relay modules are available in various configurations, including different relay contact ratings and voltage inputs.
Choosing the Right Method
The best method for powering a 12V relay with a 5V signal depends on the specific requirements of your project.
- Cost: Transistors are generally the most cost-effective option.
- Efficiency: MOSFETs offer higher efficiency due to their lower on-resistance.
- Switching Speed: MOSFETs, particularly logic level MOSFETs, have faster switching speeds than transistors.
- Simplicity: Relay modules provide a simple plug-and-play solution.
Considerations
- Relay Specifications: Ensure the relay you choose is rated for the voltage and current requirements of your load.
- Heat Dissipation: Transistors and MOSFETs can generate heat during operation. Use a heat sink if necessary.
- Protection: Consider using a diode across the relay coil to prevent voltage spikes from damaging the transistor or MOSFET.
- Reverse Polarity Protection: Use a diode to protect the circuit from damage if the power supply polarity is reversed.
Conclusion
Using a 5V signal to power a 12V relay is a common technique in electronics projects. Various methods, including transistors, MOSFETs, and relay modules, provide different trade-offs in terms of cost, efficiency, and simplicity. The best choice depends on the specific requirements of your application. By carefully considering the factors outlined above, you can choose the most suitable method for your project and reliably control the relay using a low voltage signal.