PLC Output Units

PLC Output units can be:
 Relay,
 Transistor, or
 Triac.

Check the specifications of load before connecting it to the plc output.
Make sure that the maximum current it will consume is within the specifications of the plc output.

Relay Outputs
One of the most common types of outputs available is the relay output.  Existence of relays as outputs makes it easier to connect with external devices. A relay is non-polarized and typically it can switch either AC or DC.

Transistor Outputs
Transistor type outputs can only switch a dc current. The PLC applies a small current to the transistor base and the transistor output “closes”. When it’s closed, the device connected to the PLC output will be turned on.

A transistor typically cannot switch as large a load as a relay. If the load current you need to switch exceeds the specification of the output, you can connect the plc output to an external relay, then connect the relay to the large load.

Typically a PLC will have either NPN or PNP transistor type outputs.  Some of the common types available are BJT and MOSFET. A BJT type often has less switching capacity than a MOSFET type. The BJT also has a slightly faster switching time.

A transistor is fast, switches a small current, has a long lifetime and works with dc only. A relay is slow, can switch a large current, has a shorter lifetime and works with ac or dc.

Triac Output
Triac output can be used to control AC loads only. Triac output is faster in operation and has longer life than relay output.

Inductive loads have a tendency to deliver a “back current” when they
turn on. This back current is like a voltage spike coming through the system. This could be dangerous to output relays. Typically a diode, varistor, or other “snubber” circuit should be used to protect the PLC output from any damage.