4-20 Loop Splitters in Series

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shoon

Member
Location
Canada
Is it possible to wire loop splitters in series?
I have a couple weidmuller wavepak loop splitters. There's 2 outputs for each splitter and I need 3x outputs so was wondering if it was possible to wire one of the two outputs on the first card to the second giving me 3x independent / usable outputs.

My concern is if the single 4-20 output from our AB analog output card is going to provide a strong enough signal to drive the 2x splitters. Product datasheet states they use optical isolation to minimize signal loss, but just trying to get some definitive info before I pull the pin on this project.

Datsheet

Background:
The issue is we have a single 4-20 output (fan speed control) driving 3 VFD's for glycol cooling fans;
The first VFD gets the signal and simulates an output based on it's operating speed to VFD #2, VFD #2 is controlled by the simulated output from VFD #1 and simulates an output to the VFD #3.
It's functional but the issue is if the first fan in the bank fails we lose 3x fans instead of 1x. And as luck would have it, it's always the first fan in the bank that does fail.
 
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Ingenieur

Senior Member
Location
Earth
you want to drive 1 splitter with the output of another
can't see an issue
everything is isolated

you could also possibly series the splitter inputs of off the plc output and have 4 outputs
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
The outputs of the spliters are "loop powered" so if you want to drive the second splitter from the first, you will need to add a DC power supply between the two splitters. You will also need a DC power source from between the splitters and the VFDs. (many VFDs have internal DC supplies that can be used for this purpose)

If this was mine project, I would connect the inputs of the two splitters in series with the AB output. Most analog outputs drive 3 devices.
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
Depends entirely on the voltage drop. Most 4 to 20 milliamp outputs will drive at least 750 ohms. You need to look at what the input impedance of the Splitters is plus whatever other loads there are that it has to drive and then see how much of a load the output can drive.
 

Ingenieur

Senior Member
Location
Earth
Depends entirely on the voltage drop. Most 4 to 20 milliamp outputs will drive at least 750 ohms. You need to look at what the input impedance of the Splitters is plus whatever other loads there are that it has to drive and then see how much of a load the output can drive.

Max drop is < 3.5 volts from 4-20 mA
175 to 875 Ohm
they don't give a specific value
but in series max power required 0.02 x (2 x 3.5) = 0.14 W

using your 750 Ohm at 20 mA
= (0.02^2) x 750 = 0.3 W >> 0.14

looks like it should be fine?
 

MrJLH

Senior Member
Location
CO
Have you tried looking at a Moore STA?

I used something similar for a shut down on a fuel gas burner shutdown system.

4-20mA comes in from a pressure transmitter which in turns goes to a PLC as one loop, then the 4-20mA output goes to a DCS.

You might be able to daisy chain these.
 
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