Four receivers from one 4-20mA Loop?

J

Thread Starter

jcook980

Need a 4-20mA loop to be read by two PLC analog inputs and two 4 1/2 digit panel displays. It would appear that an isolator/splitter is a
certainty. Are there any recommendations?
 
Wired in series, all four devices will see the same current flow through the loop. The current flows through dropping resistors at the load,
producing a voltage drop proportional to the current level. For example, a 250 ohm resistor in a 4-20 mA loop will produce a 1-5 volt drop.
If all four devices use 250 ohm resistors, the total load resistance is 1000 ohms (Plus the resistance of the wiring, if the distances are
significant). As long as the source device can output to a 1000 ohm load (and it should), wiring the four device in series should work.
 
D

david mertens

If you are not working in an Exi environment, you could simply put all receivers in series, however this would mean that if one instrument
fails, all will fail. A galvanic isolator (active if needed) is an option but introduces an extra element that can fail. In an Exi
environment, you don't have a choise unless you can afford to have your loop recertified. This only leaves the option to replace the current
receiver by an active Exi galvanic isolator. The four receivers can then be put in series on the output of the isolator like described
above, or they can themselves be supplied by an extra isolator for each receiver. My opinion is that the more isolators you add, the more
sources of failure you will have. So I would just put all receivers in series with a provision to short circuit every one of them so they can be removed in case of problems without disturbing the others.
 
P

Paul Dackermann

You are correct concerning isolation. A myriad of manufacturers have isolation/signal boosters for 4-20mA signals. RIS(Rochester Instruments), Dataforth, Action Instruments, Acromag, AGM to name a few. I had a similar situation where it was necessary to drive four signals. I used 2 signal boosters, a dedicated power supply, and the flow transmitter. The signal boosters were capable of driving 2 4-20mA signals. The 4 signals were easily achieved with the 2 signal boosters. Co-ordinate the output power to dedicated needs of the recieving device. Most of the signal boosters which drive 2 signals typically use separate supplies for each recieving device.

Regards,
Paul Dackermann
Whitman, Requardt and Assoc.
 
R
Several companies manufacture devices that will take an analog signal and split into four or less outputs. One manufacturer that comes to
mind is M-Sytems, 972-385-2277
 
W
Be very careful doing this.

Yes, the ANSI/ISA standard for current loop transmission is 4-20 mADC _isolated_ into 1000 ohms.

But if you look at the specs for many devices, sensors as well as receivers and transmitters, you will find that many devices do not meet the standard. Many devices are not isolated. Sometimes you must purchase an isolation option. Putting more than one device in a current loop is liable to present difficulties if one of the devices on the loop, or the receiver on the loop is not isolated.

Unless you can source 4-20 mADC isolated, and drive 1000 ohms, you will not be able to put 4 devices in series on the same loop. Most devices sink 250 ohms, as "Anonymous <?>" said below. Four such devices loads an ANSI/ISA
standard current loop to its maximum load.

I like belt-and-suspenders stuff, and I intensely dislike field-chasing ground loops. You have a PLC and an indicator pair, I presume, times two. I would take the original loop to two I/I converters, and source a PLC and an indicator from each converter. Yes, it costs more (about $600 for the two converters) but it will absolutely work no matter what. I don't know what field troubleshooting during startup costs you, but the length of time necessary to figure out what the problem is in a case like this probably costs you more than $600. If it does, doing it the belt-and-suspenders way is a no brainer.

Walt Boyes

SPITZER AND BOYES, LLC
"Consulting from the engineer to the distribution channel"

[email protected]
21118 SE 278th Place - Maple Valley, WA 98038
253-709-5046 cell 425-432-8262 home office
fax:801-749-7142
 
W
Well, I agree with you, except that in my experience I/I converters seldom fail, unless the loop is struck by lightning...and then the entire loop will fail, or the components will be so degraded that failure is immanent. Exi and IS I/I converters are commonly and inexpensively available.

Unless the source is isolated, and all the other devices are too, it will be hard to not get a groundloop with four devices on the loop.

Just my experience of many years of chasing isolation problems in the field.

Walt Boyes

---------SPITZER AND BOYES, LLC-------------
"Consulting from the engineer
to the distribution channel"

[email protected]ltboyes.com
21118 SE 278th Place
Maple Valley, WA 98038
253-709-5046 cell 425-432-8262 home office
fax:801-749-7142
--------------------------------------------
 
Moore Industries has signal splitters that will take one 4-20mA in and convert it to two identical 4-20mA outputs (www.miinet.com for main web site or http://www.miinet.com/products/sg_signal.shtml to go to the signal converters section). If you want assistance determining which model you need, give me a call or use our E-HELP EXPRESS to get a recomendation from an applications engineer.

Regards,
Steve Todd
Moore Industries
818-830-5502
[email protected]
 
G

George Robertson

However, as each device tends to have a resistance (typically 250 ohms or so), there is a corresponding voltage drop. This drop is typically considered to be as much at 5 V per device. You probably don't want more than three or so. Usually, by the time you have dropped the voltage to 12 V or so, some devices can no longer function correctly. Four devices is one more that I allow as a practice.

George G. Robertson, P.E.
Manager of Engineering
Saulsbury E & C
[email protected]
(915) 366-4252
 
Several responders have suggested simply connecting the four devices in series. Doing so invites at least two potential problems:

1. Ground loops - if there is more than one device with a signal
common that is connected to chassis/earth potential, depending on
the locations of the grounding points, the current may bypass one
or more of the devices.
2. A/D offset may exceed the maximum allowable for the input sensing
circuitry . If the circuit is grounded at the power source, at
full scale (20 ma) the voltage on the input of the first receiving
device in the circuit would be ~20 VDC referenced to earth. Such
value could easily exceed the capabilities of your input devices.

The surest way to accomplish your desired result, especially if the receiving devices are close to each other is to use devices which are high impedance voltage receivers (1-5 VDC) wired in parallel with a precision 250 ohm resistor placed across the input circuitry near the receiving devices. Use caution to make sure that one device does not place excessive load on the circuit. I have used this method successfully in several circuits in which the return terminals of two devices were at ground potential.

If you have further questions about this method, feel free to contact me off list.

John Beck
Control System Specialist, Retired.
[email protected]
 
W

Wayne Shimanis Vortex Technologies, Inc.

Mr. Cook:

You can wire each device in the loop in series. As long as you do not exceed the impedance rating of the device generating the 4-20 you should have no problems.

Wayne Shimanis
Vortex Technologies, Inc.
815-254-9400
 
P
I appreciate each receiving device will have something like 1-5 volts across their terminals but one end of the series chain could be up to 20 volts above the common ground. I don't think most industrial analog I/O modules will like this. Better check the common mode spec of your input module.

In my opinion you would be far better off using isolator/repeaters.

Good luck

Peter Green
Semi-retired paper industry control systems engineer
 
I agree. In my response, ' . . . same current. To analog inputs connected in series . . . ' was a typo and should have read ' . . . same current. Two analog inputs connected in series . . . ' (Two analog inpus connected . . . ) Thanks
for pointing this out.

[email protected]
 
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