As a PLC professional technician, understanding the various output units and output circuits of programmable controllers is crucial for successfully designing and troubleshooting automated systems. Output units, also known as output modules, are responsible for converting digital signals from the PLC processor into physical signals that control actuators and other devices. In this article, we will explore the different types of output units and output circuits commonly used in PLCs.
Relay Output Units
Relay output units are the most commonly used type of output unit in programmable controllers. They use electromechanical relays to switch the output signals, providing a physical connection between the PLC and the controlled device. These units are ideal for applications that require high voltage and current ratings, as relays can handle a wide range of loads. However, they are limited in speed and may not be suitable for high-speed applications.
Transistor Output Units
Transistor output units use solid-state switching devices, such as transistors and thyristors, to turn the output signals on and off. Compared to relay output units, they offer faster switching speeds, higher reliability, and longer lifespan. They also consume less power and are smaller in size, making them suitable for applications with limited space. However, they have lower voltage and current ratings and may require additional circuitry to handle higher loads.
Solid-State Relay Output Units
Solid-state relay (SSR) output units combine the advantages of both relay and transistor output units. They use solid-state components to switch the output signals, providing the speed and reliability of transistors and the high voltage and current ratings of relays. SSR output units are commonly used in applications that require fast switching speeds and high load capacities, such as in motor control and power distribution systems.
Triac Output Units
Triac output units are similar to SSR output units, but they use triacs as the switching elements. Triacs are bidirectional devices, meaning they can switch AC signals in both directions. This makes them suitable for controlling AC devices, such as lamps and heaters. However, triacs may cause interference in sensitive electronic devices due to their switching characteristics, and additional precautions may be needed to reduce EMI (electromagnetic interference).
Analog Output Units
Analog output units are used to control devices that require varying levels of voltage or current, such as motor speed controllers and valves. They convert digital signals from the PLC into analog signals, which can then be used to control the output devices. Analog output units can be either voltage or current output, and the accuracy and resolution of the output signals depend on the PLC's analog-to-digital converters (ADCs). These units are essential for precise control of analog devices in automated systems.
Output Circuits
The output circuits of programmable controllers are the external connections between the output units and the controlled devices. They provide the physical path for the output signals to reach the actuators, sensors, and other devices in the system. Depending on the type of output unit, different output circuits may be used. For example, relay output units require a switching circuit with a relay coil, while transistor output units may use a transistor array IC (integrated circuit) to switch the outputs.
Relay Output Circuit
The relay output circuit consists of a relay coil, a diode, and a load connected in series. When the PLC output is energized, the relay coil is also energized, closing the contacts and allowing current to flow through the load. The diode is used to protect the PLC from voltage spikes that occur when the relay is de-energized. This circuit is simple and reliable, but it may be limited in speed due to the mechanical nature of relays.
Transistor Output Circuit
The transistor output circuit uses a transistor to switch the output signal. The transistor acts as a switch, allowing current to flow through the load when it is turned on by the PLC output. The base of the transistor is connected to the PLC output through a series resistor, and a flyback diode is used to protect the transistor from voltage spikes. This circuit is faster and more reliable than the relay output circuit, but it may require additional components to handle higher loads.
SSR Output Circuit
The SSR output circuit is similar to the transistor output circuit, but it uses an SSR instead of a transistor to switch the output signal. The SSR is a solid-state device with an optocoupler and a triac or SCR (silicon-controlled rectifier) that provides electrical isolation between the input and output sides. This circuit is highly reliable and fast, but it may require additional circuitry to handle high inrush currents or inductive loads.
In Conclusion
In summary, programmable controllers offer a variety of output units and output circuits to meet the needs of different applications. As a PLC professional technician, it is essential to have a thorough understanding of these components to ensure the efficient and reliable operation of automated systems. By selecting the appropriate output unit and output circuit for each application, you can design and troubleshoot PLC systems with confidence and professionalism.