120VAC relay to a 24VDC Allen-Bradley PLC card?

Hello all,
My customer wants me to monitor a 120VAC power source for failure. For this I am using a 120VAC relay (by Magnecraft). I want to take the contacts from this 120VAC relay to an Allen-Bradley digital input card, p/n 1756-IB16I.
Can I do this without damaging the digital input card? Do I NEED to use a 120VAC Allen-Bradley input card?
Perhaps I could use an interposing 24VDC relay between the 120VAC relay and the 24VDC input card?
Any thoughts on this?

Unless I'm not understanding correctly- all you would do was energize the 120VAC relay with the power power source you want to verify. One contact of that relay can be connected to 24dc supply with the other side of the contact directly to your PLC input module. If you the relay drops out because you lost your 120V source, then the contact to the plc opens as well.

rustydud said:

Hello all,
My customer wants me to monitor a 120VAC power source for failure. For this I am using a 120VAC relay (by Magnecraft). I want to take the contacts from this 120VAC relay to an Allen-Bradley digital input card, p/n 1756-IB16I.
Can I do this without damaging the digital input card? Do I NEED to use a 120VAC Allen-Bradley input card?
Perhaps I could use an interposing 24VDC relay between the 120VAC relay and the 24VDC input card?
Any thoughts on this?

Click to expand...

I'm just curious. How is it that you think that a 24 VDC signal switched through a relay contact could possibly damage a 24 VDC input?

On this relay.... Your 120 to monitor will land on 2 and 7 to pull in the coil, your input to your IB16 will land on 1 and 3, with power loss your input will open up

dginbuffalo said:

Unless I'm not understanding correctly- all you would do was energize the 120VAC relay with the power power source you want to verify. One contact of that relay can be connected to 24dc supply with the other side of the contact directly to your PLC input module. If you the relay drops out because you lost your 120V source, then the contact to the plc opens as well.

Click to expand...

So I CAN wire the contact from a 120VAC relay to a 24VDC PLC card. This answers my question. Thanks.

You sound happy with the answer you got, but do you understand it?

rustydud said:

...Perhaps I could use an interposing 24VDC relay between the 120VAC relay and the 24VDC input card?...

Click to expand...

You sound a bit confused here about what an interposing relay is, or does?

A rather simplistic explanation...

To monitor a signal of one type, such as 120VAC, on another type, such as a 24VDC input, you must interpose the 24VDC input signal with the 120VAC relay.

As has been demonstrated, the 120VAC signal is only energizing the coil of the 120VAC relay. So 120VAC is only present on the two terminals of the coil on the 120VAC relay.

More specifically, 120VAC is not present on any of the switching contacts on the 120VAC relay. The energizing of the 120VAC relay coil electro-mechanically switches these contacts in an indirect fashion. The 120VAC does not come in contact with the switching contacts.

When interposing, these switching contacts should be dry, or volt-free contacts i.e. no wiring on either side of the contact, so as to allow the interposing signal, such as 24VDC, freely switch through the contact and back to the PLC. This is interposing an input signal.

To interpose a 24VDC output signal, with say a 120VAC solenoid valve, you would energize a 24VDC relay coil from the output and switch 120VAC through a dry contact on that relay to energize the solenoid valve's coil. Again, in this case, the 24VDC on the relay coil has no direct contact with the switching contacts.

I'm not sure whether you knew, or not, how to wire this, but were more wondering can you, or should you do this? Either way I hope the above makes sense to "someone" who is not quite sure.

Regards,
George

rustydud said:

Hello all,
My customer wants me to monitor a 120VAC power source for failure. For this I am using a 120VAC relay (by Magnecraft). I want to take the contacts from this 120VAC relay to an Allen-Bradley digital input card, p/n 1756-IB16I.
Can I do this without damaging the digital input card? Do I NEED to use a 120VAC Allen-Bradley input card?
Perhaps I could use an interposing 24VDC relay between the 120VAC relay and the 24VDC input card?
Any thoughts on this?

Click to expand...

Actually, a relay may not be the best choice in this case, as the contacts could stick closed. It's unlikely, but possible.

A better choice may be a small 120VAC to 24VDC power supply. It's possible the the power supply could fail and not the 120VAC supply, but you will be in a failsafe situation.

rguimond said:

Actually, a relay may not be the best choice in this case, as the contacts could stick closed. It's unlikely, but possible.

A better choice may be a small 120VAC to 24VDC power supply. It's possible the the power supply could fail and not the 120VAC supply, but you will be in a failsafe situation.

Click to expand...

Well, you could use a safety relay with positive guided safety contacts. But figure in cost.

Standard relay $10 but not failsafe.
Safety relay $88-up.
Power supply $45-up.

Unless you already have a 24vdc power supply powered off the AC line to be monitored. Then just connect the DC power supply output to the input card.

John Soltesz said:

Well, you could use a safety relay with positive guided safety contacts. But figure in cost.

Standard relay $10 but not failsafe.
Safety relay $88-up.
Power supply $45-up.

Unless you already have a 24vdc power supply powered off the AC line to be monitored. Then just connect the DC power supply output to the input card.

Click to expand...

Or, you could use a plug-in transformer (if there's a receptacle in the enclosure)

The other thing to consider is the dropout voltage of the relay. I did a quick check of a Magnecraft relay (I just picked one) and its dropout voltage is 15% of the operating voltage so it would hold the contacts closed down to just over 100VAC. Other relays have a very wide operating range (Magnecraft makes some that will go from 90VAC to 280VAC). My question would be what exactly is the customer looking to know? Is it a simple “is the power out” or does he/she want to know that the AC voltage is where it should be and clean (or anything in between)? A relay is good for simple “on/off” but there are other conditions that can wreak havoc that a relay won’t tell you about. You can find AC current/voltage transducers that take AC voltage (and/or current) in and produce a 4-20mA signal out. You spend more money but you get more information.