Slim Relay (PLC Relay): Function, Applications, Fundamentals

A slim relay is a thin-structured, highly insulated relay type used in PLC applications. It is an interface between controllers and the loads.

A slim relay’s operation is the same as a normal control interface relay. However, there are differences between the two relays in terms of usage and features. Let us explain it as follows.

PLCs are one of the main components of today’s automation systems. PLCs connect sensors and actuators to processes. However, PLCs are not completely isolated from the industrial environment. Therefore, overvoltage impulses and transient currents can affect their operating functions. Also, the application areas are generally limited to 24 VDC / 100 mA.

Therefore, it is recommended to use an interface device to adapt the application voltage and/or current and provide correct electrical isolation to the PLC.

This process is possible thanks to slim relays (PLC relays). Slim relays provide wide compatibility in voltage, current, and high isolation between inputs and outputs.

Slim relays are used between the output of PLCs and the input of the contactor. They provide electrical isolation and increase the signal level.

I can hear you ask the question:

“What if the slim relay is not used? And wouldn’t it be okay if the contactor is controlled directly from the PLC output? Why is it needed?

We can answer as follows:

The output power of the PLC is not sufficient to energize the contactor coil at the desired level and to activate the contactor contacts. For this, a slim relay is used between PLC and contactor.

In the first step, the coil of the slim relay is energized and its contact is switched from the PLC output. Then, the contactor coil is connected with slim relay contact. Thus, the contactor coil is energized and the contacts of the contactor are closed.

So how do we know that the power of the PLC cannot directly switch the contactor’s contacts?

For the answer, we have to look at the power consumption values ​​of the contactor coil and slim relay coil.

While the coil power consumption of the 24VDC contactor is 3… 4 W, the coil consumption of the 24 VDC PLC relay is 0.17 – 0.30 W. So almost one in 10.

For this reason, the slim relay’s coil power is enough to be switched by PLC.

Thus, the signal level is increased gradually. In other words, a high-power motor in the field is controlled by a low-power PLC in this way. Slim relay allows a small PLC to control a large motor.

Advantages of slim relays

• Its dimensions are considerably thinner than normal plug-in relays.
• Modular and pluggable.
• They don’t require much space in the panel.
• Coils can be energized with AC and DC.
• Contacts: 1 n / c, 1 n / o, 1 o / c and 2 o / c.
• They do not have 3 or 4 contact outputs like plug-in relays.
• They have LED indicators.
• Jumper bars and separators can be used between each other.

Applications of slim relays

• Building automation systems.
• Transportation.
• DCS and PLC systems.
• In all kinds of industrial electrical panels.
• On cabinets of all kinds of machines.
• Wherever there are motors and automation.