What you need to know about PLC Outputs

PLC Outputs are the control circuits of the PLC and also refers to the devices controlled by the PLC. Be aware when talking about PLCs the devices like motors and lights are also referred to as PLC Outputs. Devices called actuators convert the electrical signal of the PLC to a physical movement for instance a valve solenoid stoke or a motor contactor. With regards to the variable output, the I/P (current to pneumatic) actuator, is an example.

There are two types, the ON/OFF output and the variable output. Digital output (DO) are for the ON/OFF in your control scheme. Some examples are motors that need just be ON or OFF, Lighting, solenoid valves, door locks. Analog output (AO) are for variable level or range of output between OFF or stopped and ON or full speed as for an electric motor for instance. Examples of analog outputs are a VFD (Variable Frequency Drive), a valve position actuator, and a industrial variable power supply.

Now lets touch on the types of control circuits within the PLC Output Module. PLCs have four typical output types. Three are DO and the other is AO.

For the Digital Outputs they are transistor, relay, and the triac. Relay dry contacts are the quick choice since they are voltage independent and they are a easy interface to a customer's system. Relays generally have a higher current rating than transistors, but have a mechanical life span that has to be considered.

Transistor types are for DC applications. They are smaller and thus offer higher I/O count per unit of circuit board real estate. You may also choose them for faster switching speeds and longevity over relays.

Triacs are the solid state choice for AC and may require additional circuity called snubbers. Also keep in mind to check the leakage current spec of the transistor or triac to make sure it will not have the the possibility of turning on your output when it is OFF!

The PLC Analog Output is usually configurable for loop or internally powered and externally powered, a voltage (typically 0 to 10VDC) or current (typically 4-20ma). In this case the PLC uses a DAC (Digital to Analog Convertor) to drive the output. Other available Analog outputs supply these typical ranges: -5 to 5 vdc, -10 to 10 vdc, or 0-5 vdc.

You must be aware of the inductive loads that are switched by the PLC outputs. These include, motor starters, solenoids, and relays. If these devices are too large for direct connection a interposing relay will be be required. These inductive loads will produce a sizable reverse voltage, known as Back EMF. This Back EMF and corresponding current can damage the PLC outputs and therefore has to be redirected.

For protection, devices like MOVs (metal Oxide Varistors) and diodes are installed as close to the inductive device as possible to suppress or divert these voltages. Without surge suppression relay contacts can pit from arching, generate electrical noise and may weld closed! Manufacturers often provide guidelines for choosing and even supplying suppression devices, but others may already have the suppression built-in and will not need a separate component.

As I mentioned above, inductive devices produce voltage spikes and snubbers are used for thyristor (Triac ) PLC outputs. Snubbers for this use are RC circuits that reduce the voltage rate of change as not to falsely trigger the triac .However, most industrial applications have low enough voltage and current ratings they can be connected directly to the PLC outputs and some have built-in protection anyway. Most industrial solenoids are energized by 24Vdc and consume only about two to three hundred mA.

Finally, we must talk about sinking and source outputs. A source output will connect the voltage to the load, its return or ground is always connected. A sinking output will connect the load to its return or ground, it is always connected to its voltage source. Okay, do you see a potential problem?! What happens in this case if the circuit is made complete by a short to ground?! That is right! Unintentional turning on of the output! For this reason and the fact that I just consider connecting a circuit to power to energize more logical, I prefer sourcing outputs.

Remember that outputs come in two main flavors, DO and AO. You have to consider what you are going to control and to choose the right combination of PLC output module and supporting components such as interposing relays. Oh, one more tip, create a spreadsheet of all your inputs and outputs and all their requirements before you buy anything!


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