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Industrial Relay Control System | Wiring a 24 Volt DC Relay Circuit

Vladimir Romanov
September 20, 2019
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Industrial Relays have been used in Automation for decades. These fundamental building blocks of electrical circuits allowed the first automated systems to function without the need of modern PLCs and computers. Although you won’t find any relay based logic circuitry today, they still play an important role in modern control systems.

A mechanical relay has a major advantage over a solid-state contact: it’s able to conduct large currents & supply loads which would require a much larger and expensive semiconductor. They do have some drawbacks; one of which is the fact that they break down much faster due to repetitive motion. Although a relay isn’t recommended for many cases, it should still be used for loads which require a high amperage: motors, heaters, actuators, etc.

In this article, we will explore a simple “ice-cube” or industrial relay, go over the basic functionality and explore the wiring process.

Industrial Mechanical Relays

A mechanical relay will contain two main components: a coil & one or multiple sets of contacts. As the coil is energized, the normally open set of contacts are closed and the normally closed are opened. It’s important to know the terminology as well as the difference between the two. Furthermore, it’s important to quickly determine the configuration of a specific relay and circuit based on the diagram on the front of a specific relay.

Here’s an example:

Industrial Relay Control System | Wiring a 24 Vold DC Relay Circuit

The relay above features a 24VDC coil between A and B contacts. Note that a DC relay will have a polarity assigned to the terminals while an AC relay will not. In this case, the positive terminal is terminal A and the negative one is terminal B.

The contacts are labeled from 1 to 9. By following the diagram, we can identify the contacts as follows:

Normally Open

  • 4 – 7
  • 5 – 8
  • 6 – 9

Normally Closed

  • 1 – 7
  • 2 – 8
  • 3 – 9

A normally open contact will not conduct any electricity while the coil is de-energized. In other words, you may measure an infinite resistance across any of the terminals listed within the “Normally Open” list above while the relay coil is not powered. Once there’s a current drawn by the coil and the relay is energized, the contacts will conduct current.

The opposite is true about normally closed contacts. They will conduct current in the de-energized state and stop conducting when power is applied.

Wiring an Industrial 24 VDC or 110 VAC Relay in Control Systems

An output from a PLC or an auxiliary device such as Point IO or Flex IO may be used to power the coil of a relay. By programming the coil to turn ON and OFF, the contacts of the relay will transition from de-energized to energized and back. This action will allow the current to circulate. By creating this loop, we may build a circuit which will power a load based on the state of the relay.

Using the example above, we will land the positive terminal on a PLC based output. The negative terminal is landed on the ground of the 24VDC power supply.

Now that we can control the relay, we may use the other terminals to build auxiliary circuits. A relay contact is an electrical switch the behavior of which can be compared to that of a light switch. As the switch is pressed, the circuit is either turned ON or OFF. By combining multiple relays in series or parallel, one can create complicated logic which would require

Practical Uses of Relays

There’s a time and place to use any piece of technology. A mechanical relay has many downsides which make it the non-ideal choice in most cases. However, it’s a required component of many circuits I can think of.

Avoid using relays in circuits which can be driven through a solid state output. In other words, use a standard output tied directly to a load instead of a relay if you can. The issue with using a mechanical relay is that it will breakdown after a certain number of uses. A solid-state component will last much longer.

Do use a relay on loads which exceed the current requirements of a standard input/output. This includes heaters, valves, motors, etc. In certain circumstances, these components will include an on-board relay and thus will not require a separate component. An example of this would be an SMC valve which has an internal relay and can be driven by a standard output. No relay is required in this case.

Lastly, relays are particularly helpful in separating logical areas of circuits. An example of this would be a “Ready” signal of a specific machine. As a machine builder, you can provide the customer with a signal which tells them when the machine is “ready”, “running”, “starved”, etc. By using a relay, you allow the plant to use their schema, voltage, etc. You don’t need to be pre-occupied with what’s going to be installed in the field.


Relays play an important role in modern control systems despite being a foundational block a few decades ago. Although they aren’t used as excessively as they were in the past, relays are capable of running large loads and to separate logical areas of circuits.

In many plants, relays are used to run motors, heaters, valves and more. It is thus important to understand the functionality of a relay in order to be able to troubleshoot and install such circuits.

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