Otis Singleton
In the context of electricity, COM stands for common terminal. It is the common connection for relay or transistor outputs. When the output is activated, the PLC energizes the relay or transistor, connecting the NO (normally open) pin to the COM (common) pin and completing the circuit. COM is also used in reference to DC power supplies, where it can mean common or DC ground.
| Characteristics | Values |
|---|---|
| Full Form | Common Terminal |
| Connection | COM is the common connection for relay or transistor outputs |
| PLC | When the output is activated, the PLC will energize the relay or transistor, connecting the NO (normally open) pin to the COM (common) pin |
| PLC with Form-C contacts | When the output is off, the COM and NC (normally closed) contacts are connected. When the output is on, the COM and NO contacts are connected |
| PLC with Form-C contacts and a light/buzzer/actuator | COM is connected to the power source, one leg of the device to the NO contact, and the other leg of the device to the supply return |
| PLC with multiple NO contacts | It is common for an I/O board to save space by having only one COM and a dozen NO contacts |
| Three-way switch | One common terminal, one position of the switch has one side open and the other closed, the other position has those reversed |
| DC COM | DC ground |
| DC COM | May be the same as V- |
| DC Power Supply | COM is the ground for the red connectors next to it |
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What You'll Learn
COM is an acronym for common
COM, in the context of electricity, is an acronym for ""common". In electrical engineering, the term "common terminal" is used to refer to a specific type of connection in circuits and power supplies. This "common" connection serves as a shared point that multiple components in a circuit can use as a reference or a return path for current flow.
The "common terminal" or COM is often found in various electrical and electronic systems, such as Programmable Logic Controllers (PLCs) and power supplies. In a PLC, the COM terminal is a crucial component that helps connect and control different devices. When dealing with transistor outputs, it is common to have a single COM rail, which acts as a central connection point for all the transistor outputs. This ensures that the outputs are not isolated from each other, and careful planning of the input/output (I/O) configuration is necessary.
In the context of power supplies, the COM terminal often represents the ground or return path for the electrical current. For example, in a DC power supply, the COM terminal is typically connected to the negative side of the power supply, serving as the common reference point for the voltages supplied by the power supply. This is similar to the concept of a "negative supply rail" in amplifiers.
It's important to note that the specific usage and interpretation of the COM terminal can vary depending on the system and context. In some cases, the COM terminal may be connected to the chassis ground, while in others, it may be isolated from the ground and earth, depending on the design and requirements of the electrical system.
Understanding the role of the "common terminal" or COM in electrical systems is essential for wiring, troubleshooting, and ensuring the proper functioning of devices. By providing a shared reference point for current flow, the COM terminal plays a pivotal role in the operation of various electrical and electronic devices, contributing to their control, activation, and overall functionality.
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It is the common connection for relay or transistor outputs
In electrical engineering, COM, or Common Terminal, is the common connection for relay or transistor outputs. When the output is activated, the PLC (Programmable Logic Controller) energises the relay or transistor, connecting the NO (Normally Open) pin to the COM (Common) pin and completing the circuit.
In a typical relay switch circuit, the coil is driven by an NPN transistor switch. The relay coil can also be connected to the Emitter terminal of the transistor to form an Emitter Follower circuit, which is useful for impedance matching applications.
Transistor outputs are electronic systems, allowing for an almost infinite number of switchings and almost instantaneous switching. They are, however, limited in the voltage they can switch, with practical levels around 30Vdc maximum. Transistor outputs are common for PID controls or to send coded binary signals.
Relays, on the other hand, are mechanical systems with an inherent activation delay and contact erosion on each switching cycle. They can handle higher voltages and currents than transistors, but they are more expensive and less dense, requiring more panel space.
When it comes to wiring, the COM terminal is connected to the common, return, or negative side of the power supply, while the device to be controlled is between the positive supply and the NO terminal. This allows for the option of using an automated relay or a manual button, as seen in a three-way switch.
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When the output is activated, the PLC energises the relay or transistor
COM stands for Common Terminal. When the output is activated, the PLC energises the relay or transistor. This means that the COM terminal should be connected to the common or return or negative side of the power supply. The device to be controlled is connected between the positive supply and the NO (Normally Open) terminal.
When the output is activated, the transistor is switched on, connecting the NO terminal to COM and completing the circuit. This type of transistor output is called an "open collector" output and cannot send a "high" signal.
PLC output modules can be either solid-state or relay. Solid-state modules include transistor and Triac types. Transistors can only control DC loads, while Triacs can control AC loads. Triacs are much faster than relays but are only suitable for low-power AC loads.
Relays are used to provide electrical isolation between the PLC and the external devices it controls. The PLC's internal circuits operate at low voltages, while external devices may operate at much higher voltages. Without isolation, a fault in the external device could damage the PLC. Relays also allow the PLC to control high-power loads indirectly. The PLC only needs to provide a small amount of current to energise the relay coil, and the relay's contacts handle the higher current needed by the external device.
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COM connects to the common or return side of the power supply
COM, or the common terminal, is an essential component in electrical circuits and power supplies. In the context of electricity, COM stands for the "common terminal", which serves as a common connection point for various electrical components. This terminal is also sometimes referred to as the "common connection" or simply "common."
In a power supply, the COM terminal connects to the common or return side of the power supply. This means that it is typically linked to the negative side of the power source, often represented as the "-" symbol. This connection is crucial for the proper functioning of the circuit and the activation of connected devices.
For instance, consider a scenario where you have a device controlled by a push button or a relay. In this case, the COM terminal would be connected to the power source, forming a crucial part of the circuit. The device being controlled would have one leg connected to the Normally Open (NO) contact, while the other leg would be linked to the supply return.
It is important to note that the specific wiring configuration may vary depending on the type of device and the circuit design. In some cases, a push button might be added across the COM and NO legs, providing the flexibility to use either an automated relay or a manual button to operate the device.
Understanding the role of the COM terminal is essential for electrical engineers and technicians. By correctly connecting the COM terminal to the common or return side of the power supply, they can ensure the proper functioning of devices and prevent issues such as frustration caused by the inability to supply a positive voltage in certain output configurations.
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COM and NO form a switch that closes when the output is turned on
COM stands for "common terminal" in electricity. It is the common connection for relay or transistor outputs. When the output is activated, the PLC energizes the relay or transistor, connecting the NO (normally open) pin to the COM (common) pin.
In a circuit with normally open contacts, electricity does not flow until a switch is activated. NO switches are used when it is necessary to detect a state change or control the electricity flow. They are often used in safety systems, requiring deliberate activation, such as a push-button mechanism.
NC (normally closed) switches, on the other hand, are used in scenarios where circuit interruption signals an abnormal condition. In its resting state, an NO switch keeps the circuit open, preventing electric current flow. When activated, the current can pass through by closing momentarily.
When the output is off, the COM and NC are connected. When the output is turned on, the COM and NO contacts are connected, forming a switch that closes. This can be visualized with a light switch: COM is the feed-in, and NO is the switch leg. The switch is normally in the off state, so it is normally open. When the trigger happens and the relay state changes, the normally open circuit closes, turning the power on.
An example of a device using normally open contacts is a doorbell. The doorbell does not ring on its own; one must push the button, closing the circuit and making the doorbell ring. Similarly, a push-button can be added across the COM and NO legs of a 12v load, allowing the option of using the automated relay or the manual button.
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Frequently asked questions
COM stands for "common" and is used to refer to the common terminal, common connection, or common contact.
A common terminal is a type of electrical connection that serves as a shared point for multiple circuits or components. It is typically used in wiring configurations where multiple devices or loads are connected to a single power source.
In a circuit, a common terminal is used to connect various components, such as relays, transistors, or switches. It acts as a common point of reference and provides a return path for the electrical current. When the circuit is activated, the common terminal helps complete the circuit, allowing the flow of electricity to operate connected devices.
NO stands for "Normally Open", COM refers to the common terminal, and NC stands for "Normally Closed." In a circuit, NO contacts are initially open, and the circuit is not complete. When the circuit is activated, the NO contact closes, connecting to the COM contact and completing the circuit. NC contacts, on the other hand, are initially closed and form a continuous path until the circuit is activated, at which point they open, disrupting the circuit.
COM and NO wires work in conjunction to control the operation of a device. The COM wire serves as the common connection or ground, while the NO wire is the switch leg or trigger. When the NO wire is activated, it closes the circuit, connecting to the COM wire and allowing electrical current to flow, which, in turn, activates the device.
