Why should I use relays with PLC DI/DO cards

crevans2025

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Jan 2018
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What is the benefit of using external relays when you wire a DI signal to a PLC I/O card. For example, why would I install an external relay in my flow switch circuit.:eek:
 
Isolation..

Keep whatever happens in the outside world (anything outside the PLC cabinet) away from your PLC.
 
The most common use on a DI is to protect the PLC input from voltage spike or change voltage levels such as using a 120 volt device on a 24vdc input. For a DO the external relay is generally used to protect the PLC output from overcurrent or to change voltage levels.
 
The main benefit is isolation, especially if another circuit needs to use the same contact or if there is field wiring with a high risk of noise or transient voltages.

Also some devices use transistor outputs. If the device shares a common power supply with your PLC and the output of the device is not isolated from the supply rail then you might have trouble interfacing to your PLC without a relay or pull up resistor.
 
Even if your solid state PLC output can handle the load of the field device, using isolating relays can offer protection against a failure of the device or wiring. It is much easier for an end user to replace a relay that goes poof than a whole PLC or output module. Even though we know that the relay has a finite life, I like to use isolation relays on solid state PLC outputs most of the time, and leave the end user with a few spare relays.

If the frequency of operation is on the high side, I may skip the relay. Most of what I control now (water/wastewater stuff) is not in that category.

I almost never isolate PLC inputs with relays...only where required to match incompatible signals.
 
Last edited:
Also, if the circuit is switching on and off a lot, it's far far cheaper to unplug a small relay and replace it, than to replace an entire Input or Output card in a PLC.
 
I design with forcable DI relays when the operators must be able to run in case of a broken sensor or other invalid state. Then it is uncomplicated to generate a good signal.
 
Are relays the only ways to isolate electrical signals? I use A.B. cat 700-TBR60 relays with the cat 700-HLT1U1 base. Is there any other way to isolate various signals without using relays? Just curious if this is possible. Thanks.
 
spaderkung said:
I design with forcable DI relays when the operators must be able to run in case of a broken sensor or other invalid state. Then it is uncomplicated to generate a good signal.
Click to expand...

No need to do that with relays, can also be done in the PLC code. I do that all the time with non-critical sensors where I want the process to be able to continue by overriding a missing input signal. It helps in this context to have a decent HMI (where I typically show a warning for any active overrides).
 
Mickey said:
Isolation..

Keep whatever happens in the outside world (anything outside the PLC cabinet) away from your PLC.
Click to expand...

Agree with this. Just make sure your application won’t suffer from the delayed response times because of the relays switching.

Something like a water treatment application would probably be ok but a high speed machine control application may have a problem.
 

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