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In today’s video we are going to be talking about relays, but more specifically we want to talk about “interposing relays”. What does that mean? In what circumstances are they used and why are they used?
It is not uncommon to here the term “interposing relays” being used as it pertains to industrial automation. It is important to understand that an interposing relay not some special kind of relay that is being used, but instead, it is the way in which we use a particular relay that will determine if it is in fact an “interposing relay”.
So, to illustrate this in a more basic form, an interposing relay is simply an auxiliary relay that is used to isolate two different systems or devices from one another. Now that we have a basic explanation what an interposing relay is, let’s explain why this isolation might be necessary and in what scenarios we might see this method being used.
So why do we need to isolate different devices in the first place. Let’s consider a scenario where we have the need operate a device that requires 120VAC, but the PLC in which we need to control this device can only output 24VDC. There is no way to accomplish this output control without the use of a “interposing relay” The relay… in this case, would us a 24VDC coil that would open or close a contact on that relay that is rated for the 120VAC needed for the device we want to operate. Simply put, we use a small amount of voltage to control a high voltage device. Another benefit of using a relay in this manor, is that if a spike in voltage or current occurs on the device we are operating, the relay will act as a “buffer” between the PLC and the higher voltage device. Because of this, the relay also provides some protection to the PLC. So, the worst case scenario is… we may have to replace a relay that has become damaged, but the PLC will remain undamaged. This logic can also apply to inputs that need to be read by the PLC, but don’t input at the proper voltage or current.
So what other scenarios do you see this type of isolation method being used? It is very common to see interposing relays being using in the automotive industry. One of the most common applications is the starter motor in our vehicle. In order to start our car, we need to initiate some kind of start command or control, which is why we turn the key… this is our start command. However, the starter motor in our cars draws a lot of current. It would be unsafe and very impractical to have the key switch that amount of current. Instead, a solenoid relay (which acts like an interposing relay) is used to switch a relatively safe of amount of current, that in turn switches a much higher amount of current in a safer location within the vehicle to provide the amperage that is needed to crank the starter motor.
The examples we have discussed in this video are just a few of the many different scenarios in which we see interposing relays being used. It is one of they easiest ways to isolate different devices while giving you the control and monitoring ability that is needed many different industries.
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