# Adding Two 4 -20 ma Signals to Equal One Output Signal

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#### Christian

How can I add two 4-20 ma signals to equal one 4-20 ma output signal without a plc?

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#### Carl Burgess

Check out:
http://www.status.co.uk/files/Products/121.pdf
- This has an 'adder' circuit using their SEM 1015 unit.

They also have some more sophisticated 'MEDACS' units which will do the trick.

(I have no connection with Status Instruments)

> How can I add two 4-20 ma signals to equal one 4-20 ma output signal without a plc?

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#### W.L. Mostia

There are a number of manufacturers that make calculation modules that can add two 4-20 mA signals and output 4-20mA. Here is a sample:

http://www.acromag.com/computation

http://www.fwkirkco.com/math-function-modules.html

http://www.prelectronics.com/filearkiv/PDF/5100 series/5115/Manual/5115V104_UK.pdf

http://www.miinet.com/Portals/0/DataSheets/MDS_Datasheet_Moore_Industries.pdf

William (Bill) L. Mostia, Jr. PE
ISA Fellow, SIS-TECH Fellow,
FS Eng. (TUV Rheinland)
SIS-TECH Solutions, LP

Any information is provided on Caveat Emptor basis.

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#### Curt Wuollet

The word add is misused here and if I get what you want to do, you will need some sort of electronics. You could sum with a resistor and the voltage would would correspond to the sum of the currents. You would then need to translate that voltage to a 4-20 ma signal. If you used a 125 ohm resistor, you would get 1-5 V then you could use a signal conditioner to convert. That is _if_ the current sources behave in that connection.

Regards
cww

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#### Curt Wuollet

Actually, on reflection there is a cheap simple way if the input you are driving is the typical 250 ohms across a voltage input, Simply place another 250 ohm resistor across it making it a 8 to 40 ma input.

Problem solved.

Regards
cww