I have an LEL sensor which is giving out a 4-20ma signal. Someone else is currently reading that signal using the two 4-20ma signal terminals. I also want to read in the 4-20ma signal. Can I put another set of wires on those signal terminals? Will it screw up the signal?
Read one 4-20ma signal with two different receivers
- Thread starter jump3mb
- Start date
- Status
petersonra
Senior Member
- Location
- Northern illinois
- Occupation
- Semi-retired engineer
You cannot just put another set of wire on those terminals.
There are several ways to accomplish what you want to do. One is to wire in an isolator. You can usually get one that has a 4 to 20 mA coming in and two 4 to 20 mAs going out.
You might also be able to wire The two places that you want to send the 4 to 20 milliamp signal to in series.
There are other options as well but really the isolator is the simplest solution for most people because it is fairly obvious how it works.
Thanks! I was reading that you can't put the signals in parallel, only in series. However doing that doesn't necessarily work all the time depending on what you are connecting it to. An isolator or a splitter definitely seems like the safest, easiest way to go to make sure the signal doesn't get screwed up.
zbang
Senior Member
- Location
- Roughly 5346 miles from Earls Court
Series wiring only works if there's only one current source and it has enough voltage to drive both sinks; I've seen a few setups where the combination of low voltage and long & small leads meant we barely got 20ma at the one receiver (sensor at the end of lots of 24g wire, I don't remember most of the other specifics, it was 35 years ago).
- Location
- Bremerton, Washington
- Occupation
- Master Electrician
AGM is a company that has a great line of signal converters and isolators, I had hundreds in service in the past. Give them a call and if they don't have what you want they will make it, modules are small, easy to install and have 7 year warranty
Just piling on to the above ideas as well. A lot of times you'll see that a device that is outputting 4-20mA (like a sensor or analog output card) specify that it can support a 'max impedance'. Often you'll see something like 500-ohms or 750-ohms. And this translates to how many devices can read the signal in series. Most devices that read signals (like an analog input card) have an input impedance of 250-ohms (though not always, so check the datasheet). So if the sensor supports 500-ohms, and your sensing devices are 250-ohms each, you can have 2 devices read that same signal in series.
Bonus round: 250-ohm sensing is very common. Why is that? Well a little V=IR action.
At 4mA, this is 1v.
At 20mA, this is 5V.
A chip to read 1-5V across a 250-ohm resistor is pretty easy to do with common components.
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
Don't forget the devices that ground one of their leads, this definitely makes a series connection difficult.
gar
Senior Member
- Location
- Ann Arbor, Michigan
- Occupation
- EE
210812-1334 EDT
jump3mb:
Your thread title does not correlate with your first post.
.
jump3mb:
Your thread title does not correlate with your first post.
.
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
- Status