How To Build an Optocoupler-Based 24V Relay Driver Using the TLP3120

 Introduction

When working with high-voltage or high-current systems, it is often necessary to isolate the control circuit from the load circuit to ensure safety and reliability. Optocouplers (or optocoupling devices) serve as an excellent solution for electrical isolation, enabling you to control high-power devices while protecting sensitive electronics from voltage spikes, noise, or ground loops.

One such optocoupler, the TLP3120, is a powerful component used for driving relays, controlling motors, or switching AC loads with the benefit of electrical isolation. The TLP3120 is a high-speed photorelay that can handle switching operations up to 1A of load current and 60V of voltage, making it ideal for controlling 24V relays or other medium-power loads.

In this DIY project, we will design and build a 24V relay driver circuit using the TLP3120 optocoupler. The circuit will allow you to control a 24V relay with a low-voltage signal (e.g., from a microcontroller or a control switch), providing the necessary isolation between the control side and the relay side.

The TLP3120 is especially useful for controlling devices that operate in industrial, automotive, or home automation systems. By using this optocoupler-based driver, you can efficiently switch relays while ensuring electrical isolation between your control logic and the high-power load, ensuring both safety and reliability.

Project Overview

In this project, we will design a relay driver circuit that uses the TLP3120 optocoupler to switch a 24V relay on and off. The optocoupler will serve as a signal isolator, allowing you to control the relay with a microcontroller, low-voltage logic, or any other low-power control device while keeping the relay and its high-power load electrically isolated.

Key components for this project include:

● TLP3120 Optocoupler: The central component used to drive the relay while providing electrical isolation.

● 24V Relay: The load that will be switched on and off by the driver circuit.

● Microcontroller or Switch: A low-voltage signal source to control the relay.

● Resistors: For current limiting and setting appropriate biasing levels.

● Diode: For flyback protection when switching inductive loads.

● Power Supply: To power both the control and relay circuits.

The design will also include some protective features to ensure the circuit operates safely, including a flyback diode across the relay coil to handle voltage spikes that occur when the relay is de-energized.

1. Understanding the TLP3120 Optocoupler

The TLP3120 is a photorelay optocoupler that consists of an infrared LED and a phototransistor, both integrated within the same package. It is designed to handle switching applications where electrical isolation is required, such as controlling relays, switching motors, or interfacing high-power devices with low-power logic systems.

Key features of the TLP3120 include:

● Output Current: It can switch up to 1A of current on the output side, which is ideal for driving medium-power relays.

● Operating Voltage: The TLP3120 can handle up to 60V of load voltage.

● High Isolation: The optocoupler offers 2500V isolation between the control and load sides, providing protection against voltage spikes and ground loops.

● Fast Switching: It has a switching speed of around 10ms, making it suitable for use in control circuits requiring moderate switching speeds.

● Compact Package: The TLP3120 comes in a 4-pin DIP package, making it easy to integrate into both breadboard and PCB designs.

The key advantage of the TLP3120 is its ability to provide isolation between the control side (low voltage) and the output side (high voltage), protecting sensitive components like microcontrollers from potential damage caused by high-voltage spikes, noise, or transient currents.

2. Designing the Relay Driver Circuit

The basic operation of the relay driver circuit will consist of the following steps:

● Triggering the TLP3120: A control signal, such as a 5V logic signal from a microcontroller or a manual switch, will be used to activate the infrared LED inside the TLP3120.

● Electrical Isolation: The LED side of the TLP3120 will be isolated from the phototransistor side, which controls the relay.

● Relay Activation: When the LED inside the TLP3120 is activated, it will turn on the phototransistor, allowing current to flow to the relay coil.

● Switching the Load: The 24V relay will switch its contacts to control the load (e.g., turning on a motor, light, or another high-power device).

Let's now break down the key steps involved in building the relay driver circuit using the TLP3120.

Step 1: Power Supply

The circuit requires two power supplies: one for the control circuit (typically 5V DC, which powers the microcontroller or logic signal source) and one for the relay circuit (typically 24V DC, which powers the relay and the load).

Ensure that both power supplies share a common ground. The 5V control circuit will drive the TLP3120's LED, while the 24V relay circuit will be controlled by the TLP3120’s phototransistor output.

Step 2: Connecting the TLP3120 Optocoupler

The TLP3120 has four pins:

● Pin 1 (Anode): Connect this to the control signal (e.g., microcontroller GPIO pin or a switch).

● Pin 2 (Cathode): Connect this to the ground of the control circuit.

● Pin 3 (Collector): This will be connected to the positive terminal of the 24V relay coil.

● Pin 4 (Emitter): This will be connected to the ground of the 24V relay circuit.

The LED inside the TLP3120 will be activated when a control signal is applied to the Anode (Pin 1), causing the phototransistor (Pin 3 and Pin 4) to conduct and energize the 24V relay coil.

Step 3: Current Limiting Resistor for the LED

To protect the LED in the TLP3120, you need to limit the current flowing through it. A current-limiting resistor is placed in series with the LED to control the current based on the supply voltage (usually 5V) and the LED's forward voltage (approximately 1.2V).

Assume the control signal is 5V, and the forward voltage of the LED is 1.2V. The remaining voltage drop will be across the resistor. A typical current for driving the LED might be around 10mA. Thus, using Ohm’s Law, you can choose an appropriate resistor value (e.g., 390Ω).

Step 4: Adding the Flyback Diode

Since we are driving a relay, which is an inductive load, there will be voltage spikes when the relay is de-energized. To protect the rest of the circuit (particularly the TLP3120 and other sensitive components) from these voltage spikes, a flyback diode is needed.

Place the diode in parallel with the relay coil, with the cathode connected to the 24V and the anode connected to the ground. A typical 1N4007 diode will work for this purpose. The diode provides a safe path for the inductive current to dissipate when the relay is turned off.

Step 5: Relay Contact Switching

The TLP3120 optocoupler is capable of switching up to 1A of current through its phototransistor, which is enough to control most 24V relays. When the phototransistor conducts, it will complete the circuit for the relay coil, energizing the relay and switching its contacts.

The relay can be used to control a high-power load, such as turning on a motor, light, or other devices, by connecting the relay’s NO (Normally Open) contact to the load and the COM (Common) terminal to the power supply.

Step 6: Testing the Circuit

Once the circuit is assembled, apply 5V to the control side (microcontroller or manual switch), and observe the relay activating. The TLP3120 will allow the 5V logic signal to switch the 24V relay, isolating the control side from the high-power relay circuit.

Test the relay by connecting a suitable 24V load to the relay’s switching contacts and verify that the load is controlled by the relay when the control signal is applied.

3. Conclusion

In this DIY project, we designed and built a 24V relay driver circuit using the TLP3120 optocoupler. The circuit is ideal for controlling high-power devices with low-voltage control signals while ensuring electrical isolation between the control and load circuits.

By using the TLP3120, we have created a safe, efficient, and reliable solution for switching relays and controlling medium-power loads. This project is useful for a variety of applications, including home automation, robotics, and industrial control systems, where isolation and protection are critical.

The combination of the TLP3120, a flyback diode, and a relay gives you the flexibility to control high-power devices with low-voltage logic, while protecting the sensitive control electronics from potential electrical hazards.