AC to DC Interposing Relay

I read the following post:

http://www.plctalk.net/qanda/showthread.php?t=49630&highlight=interposing

Kid mentioned the 700HL interposing relay that I am using for my 24VDC outputs to drive 120VAC solenoids:

700-HLT1Z24

Can I use the 700HL interposing relay for voltage transformation where I'm using 120VAC pushbuttons that drive 24VDC I/O card inputs?

700-HLT1U1

Thanks in advance...

I don't think so. Remember the "input" to a relay is the coil and its voltage will be 24vdc when driving 120VAC loads from a 24vdc signal and 120VAC when driving 24vdc signals from a 120vac pushbutton.

Choose the same type solution, just change the coil voltage of the relay to suit. Just a different part number. I'd be willing to bet Finder makes those relays for Allen-Bradley to private label. You could save some money (and get the same hardware) by consulting a Finder distributor.

Here is a picture of what I mean...

That should work just fine, this solution is used all the time. I did not verify your relay part number but I'm sure you understand the coil voltage must match what is applied - your drawing states 120vac coil.

But why don't you just use the contact of the pushbutton as the input of the I/O card. If you have to have a 120VAC for some other reason, many pushbutton have more than 1 dry contact available. You will eliminate a relay (and a potential failure point) this way.

brucechase, 120V control is often used if the wire lengths are long or if the pushbuttons are located such that the wires are in conduits mixed with other 120vac conductors. 120v control gives you a level of noise immunity not easily acheived with 24vdc control.

But, if those things are not issues, then you are exactly right. Simple is good and going directly with the button contact to the drive input terminals utilizing 24vdc would be a better plan.

rlmunoz said:

Here is a picture of what I mean...

Click to expand...

I assume this is to give us an idea only. Don't forget to put some sort of over current protection in your application.

DickDV said:

brucechase, 120V control is often used if the wire lengths are long or if the pushbuttons are located such that the wires are in conduits mixed with other 120vac conductors. 120v control gives you a level of noise immunity not easily acheived with 24vdc control.

But, if those things are not issues, then you are exactly right. Simple is good and going directly with the button contact to the drive input terminals utilizing 24vdc would be a better plan.

Click to expand...

I'm not sure what you mean by long, but I have gone almost exclusively to 24VDC on all my controls. I have photoeyes, solenoid valves and pushbuttons that easily exceed 750 feet and close to 1000 feet from the plc. I've never had any problems with any of my controls. If you keep the 24VDC separate from AC - run in it's own cable trough or conduit - then there really is no noise issues.

I initially used four Allen Bradley 24VDC pushbuttons in a Hoffman pendant box. However, my client wants add four more pushbuttons to the pendant to add more functionality. Adding a total of eight buttons meant that a Hoffman pendant box would go to 2 rows. My client wants a single row pendant. So I suggested the Schneider Electric pushbutton station type SKYP80 that has KA1 contacts on the pushbuttons.

Reading the Scheider Electric catalog, the KA1 contact looks to require approximately 5 amps of current at 24VDC to make or break. It looks like I would have to add a power supply just for this pendant due to these contacts. If you could give advice on whether the KA1 contacts can easily pass the 24VDC, then that would be great. Seems like a lot of current.

In summary, in this unusual situation I chose the quick way that I had more confidence would work. I prefer 24VDC for safety, however when it comes to low confidence in current and voltage requirements of contacts and switches...then 120VAC will never do you wrong.

Fuse and surge protection would be added to the diagram...I was just trying to get the point across that I was unsure of. Thanks...

I think what you are seeing is the maximum rating of the contacts. These are dry contacts that are spring operated so there is no reason to have a rating of 5amps to make/break. I have used plenty of the KA1 contacts on 24VDC. You will be fine putting 24 VDC across the contact in the pendant station - therefore not needed any interposing relay or additional wiring.