Loop powered device that won't power up???

boompy

Member
Join Date
Jul 2019
Location
Australia
Posts
6
Hi all

I'm attempting to use a Phoenix Contact signal conditioner - the device accepts mV signals and converts this to a 4-20mA output.

The device is meant to be 'loop powered', only when I connect it to a Wago 750-455 analog input card (4-20mA input module) the device is unresponsive.

I'm a bit new to all of this. Is this a limitation of the AI card?

I've connected the signal conditioner to my Fluke 789 4-20mA source output and it powers up the signal conditioner perfectly.

Am I doing something wrong, or is my AI card just incapable of powering the device?
 
It may be that the Wago input card does not provide power for the loop. Some analog devices are passive - they only read the signal.

You may have to add a separate 24VDC power supply inline with the loop to provide the power.
 
Hi Aabeck

Thanks for your reply.

Please excuse my ignorance - I'm a little bit green on this stuff.

So some AI cards are capable of sourcing and some are not?

Are you able to advise how I might add a 24V supply to the loop? Can I simply connect the 4-20mA output from the Phoenix device to the PLC AI input card and in addition to this, apply a 24V supply in parallel with this connection?

ie. wire 24VDC power source to the Phoenix device 4-20mA output terminals?
 
Just cut one into the positive lead in the loop and wire in the power supply, making sure the positive power goes to the positive input of the signal conditioner.

Also, it might not have to be 24 volts, some devices work on as low as 12 volts, but it seems everyone has a few 24V supplies on hand.
 
Sorry Aabeck, I'm not quite understanding.

At present, I have two wires leaving the Phoenix device and terminating on the AI card.

Can I just add another two wires to the AI card (the input terminals) in parallel and apply 24V to these?

My Phoenix card is happy to accept 24V as a supply.
 
You don't add wires, unless you need to lengthen them to reach your power supply.


Just cut one wire and connect the 2 new ends to the power supply, being careful that the positive power goes to the positive input of the signal conditioner.
 
Wow Aabeck

You've gone above and beyond!

Thanks so much for the detail you've provided, a circuit diagram speaks a million words and I couldn't ask for more.

I really really appreciate your help.

I'll wire it up on a test bench and see how we go.

Thanks so much!
 
Haha, well either way you've pointed me in the right direction and I greatly appreciate that.

Thanks for all your help!
 
Over here that's pretty much the standard pay rate too.


If you see an electrical utility crew in a boom truck they will do pretty much anything for a case of beer. A crew stopped by a garage sale and ended up installing an antenna on top of a light pole for me for a case.
 

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