# Two 4-20mA Signals from a Single 4-20mA Signal

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#### pneumatics

I'm a newbie. Please don't get me wrong!

The basic idea of current loop is that you can connect n number instrument in a loop (as long as voltages are maintained). Whereas in a voltage system loading will be there. Here, why we need to split the loop? Can't we just increase the loop voltage and connect all in the same loop?

#### David_2

> Can't we just increase the loop voltage and connect all in the same loop?

Sometimes it works to run two loads (analog inputs, AI) in series in a loop, but many it doesn't work. When it doesn't work it's because there can be multiple grounds between the power supply, and the multiple analog inputs. When the AI's lack sufficient isolation to overcome the common mode voltage in a ground loop, then the signals offset, many times off-scale.

Two AI's in series can work, I've done it, but sometimes it's just easier to buy a splitter and get the job done, than to experiment.

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#### Roy Matson

> The basic idea of current loop is that you can connect n number instrument in
> a loop (as long as voltages are maintained). Whereas in a voltage system loading
> will be there. Here, why we need to split the loop? Can't we just increase
> the loop voltage and connect all in the same loop?

It depends as you say if one of the inputs is isolated from ground (example a panel indicator) you can connect the loads in series but if they are both grounded cannot.

If the loads are close together in one panel you can run to the first load as 4-20 mA and parallel the other load as 1-5 Volts by removing the 250 Ohm resistors. For example you have a DCS where the inputs are referenced to a grounded 24 VDC you can also wire to the 1-5 Volt inputs of a PLC or chart recorder that is also referenced to ground.

On the other hand where you have a SIL rated safety PLC you are better off with a signal isolator, the safety PLC gets the signal in series with the isolator and the other device gets the re-transmitted signal. There are lots of variations.

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#### pneumatics

Thank you for the information. Never thought of grounded power supplies. I should read more about grounding and common mode voltages.

If we ground DC supplies how will be noise levels. Just curious!

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#### Roy Matson

> If we ground DC supplies how will be noise levels. Just curious!

Over the years I have seen it done in several different ways
Ungrounded

With a pair of lamps in series with the center grounded

I have come to the conclusion that grounding is best, if you get a ground fault it will blow a fuse or trip a breaker whereas if it's floating you wouldn't know if the positive was grounded somewhere.
I have seen situations where the positive was grounded pushing the negative to -24VDC which the analog inputs were unable to handle as the electronics for those was powered from a separate grounded supply.

I can't really answer the noise question, each circumstance is different.

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#### Dan

you can use something like the following converter

http://www.clrwtr.com/PDF/ABB-Controls/ABB-Analog-Signal-Converters.pdf

> The basic idea of current loop is that you can connect n number
> instrument in a loop (as long as voltages are maintained). Whereas
> in a voltage system loading will be there. Here, why we need to
> split the loop? Can't we just increase the loop voltage and connect
> all in the same loop?