Hello -- question regarding a circuit component that I have been asked to source 1

[camfromvan]

Hey Guys,

I just started to work for the electrical department at a place that I work.
I was asked to source a device that is able to monitor a 4-20mA signal (24VDC), and it will be able to be hooked up to an alarm that will sound when the signal skews out of that range.
Does anybody have any recommendations for where I could go about finding something like that?
My supervisor says that it needs to be pretty high quality.
Thanks very much

Cam

 

[MikeHalloran]

This search will get you there:

https://www.google.com/search?q="4-20mA"+alarm&ie=utf-8&oe=utf-8

You do understand how 4-20 signaling works, right?
To review just for fun,
A power supply makes voltage available to a loop.
A sending unit changes its impedance in such a way that the loop current is limited to a range of 4mA to 20mA.
No particular association is defined between the current transmitted and the quantity it represents; it could be anything.
A loop current below 4mA suggests that the loop is broken or the power supply is off or dead.
A loop current above 20mA suggests an overrange or other error condition.
A receiving unit measures the loop current and displays it, possibly after converting it to some other set of numbers, like 0...100.

I did not find a single unit that did exactly what you were asked for, but there are a bazillion single setpoint loop alarm modules available, any of which can be set to alarm above some threshold. Of course you can put two in the loop, and invert one's output to alarm below some threshold.

Note that a strictly loop powered unit may not behave in a well defined way at 0mA, because that transmits no power to the module. Also, an actual alarm annunciator of almost any sort will require a power source other than the loop itself.

The whole point of the exercise may be to get you familiar with current loop signaling and the ways in which it can go wrong. Do you have an inventory of standard parts used in your workplace, or a small budget that you can blow on some parts to play with? That's where I'd start.


Mike Halloran
Pembroke Pines, FL, USA

 

[ScottyUK]

Google the term "trip amplifier" and you'll find countless examples. I like those made by Pepperl & Fuchs but there are loads of good brands.

 

[camfromvan]

Great, thanks very much guys!