Relay Output Module vs. AC Output Module

smohamed

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Join Date
Mar 2008
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ON
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108
Hey everyone,

I'm new to posting on this website, however I've taken in lots of information previously thanks to a lot of knowledgable people on this site posting great information.

My question is, as a n00b to all of this, is that what is the real difference between say an AC Output module and a Isolated Relay Output module? From what I can understand, there is a "dry contact" within the relay output module, such that you can drive the output directly (ie. a solenoid), while with an AC Output module, you may have to place an interposing relay within the output to drive the same device.

Any advice?

Thanks!
 
Advantage - A relay output module can switch a variety of voltages at typically higher voltages and higher currents. If the outputs of the relay module share a common then all of those must be the same voltage.

Disadvantage - the cycle rate and expected cycle lifetime for a relay output are usually much lower than a solid state (either AC or DC) module. Use it only for occasional switching. (E.G. machine turn on then stay on for hours)
 
besides the differences listed by my distinguished colleague, Bernie, many solid-state outputs allow a small trickle of "leakage" current to flow - even when they're turned OFF ... the relay type outputs normally don't do that ... this is an issue for some "light-duty" field devices (small Radio-Shack type whistles, neon lamps, etc.) because the leakage current may keep the device ON even when it should be turned off ...

also ... some (but not all) relay type outputs have a Normally-Closed contact section ... so when the PLC "dies" for any reason (fault, etc.) those contacts will automatically close ... this can be used to energize an alarm, etc. ...

and welcome to the forum ...
 
Last edited:
Great. Thanks to both of you for the advice. I assume that I was correct in my assumption as well in terms of how the relay operates too, just with the dry contact.

Thanks!
 
A 'dry contact' typically refers to an operating contact provided such that neither side is pre-connected to an existing voltage. With shared commons on some relay output cards you have to carefully analyse the configuration.
 
I never use AC outputs and rarely use relay outputs. All mine are transistor driving an an external relay with a flywheel diode fitted - easy to replace an external relay in the middle of the night - not so an internal relay.
 
I never use AC outputs and rarely use relay outputs. All mine are transistor driving an an external relay with a flywheel diode fitted - easy to replace an external relay in the middle of the night - not so an internal relay.
I second that.
And it's a shame to junk an entire micro PLC just because one of the relay outputs is shot.
 
BobB said:
I never use AC outputs and rarely use relay outputs. All mine are transistor driving an an external relay with a flywheel diode fitted - easy to replace an external relay in the middle of the night - not so an internal relay.

I hear this quite a bit on this site and understand the reasoning behind it but in the 10 years I have been in the business I can only think of 1 machine that this was done on.

The only time I use a relay output is if I need isolated outputs for some sort of voltage reason or if a Triac can't drive the load. The only reason I will use an external relay is if I can't get an ouput card that can handle the load.

Not trying to argue, just a different point of view. Maybe I should try and figure out how to do a poll.
 
I have found that PLC Relay Output Modules and Isolated Relay Output Modules to be useful for projects where I had no idea what field devices had been purchased or will be purchased.

That sounds unbelievable, but it is amazing how many times the project manager, the chemical engineer, and the mechanical engineer go off and design things and buy things without ever checking with the electrical department to see what voltage range would be best.

With a few PLC relay outputs, I can usually accomodate whatever they throw at me. True, relays don't last as long, use more power, and cost more than transistors, but I can make them work with nearly any field device.

For the devices that I do have good info on, I do buy output modules to match, depending on the voltage and current requirements.
 
My two cents...

I prefer relay outputs for my projects.

The systems that I have installed have held up well over the years as I always install a shunt diode or R/C snubber to prolong the life of the contacts during the load switching.
 

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