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Associate

Join Date: Sep 2009
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### PLC 4-20mA Output

04/27/2012 3:08 AM

Hi all.

In a PLC analogue output, how does the PLC vary the 4 - 20mA. The voltage is constant 24 Vdc...

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Guru

Join Date: May 2007
Location: Harlow England
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#1

### Re: PLC 4-20mA output

04/27/2012 3:36 AM

It can't until a load is connected and then it will use current sensing and feedback to control the current (probably by adjusting the voltage).
I suspect you are measuring it off load.

Del

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Good Answer (Score 5)
Participant

Join Date: Nov 2011
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#2

### Re: PLC 4-20mA Output

04/27/2012 7:55 PM

A PLC can only work with digital number and to convert a 4-20 Ma signal for a PLC you install a 100 Ohm precision resistor across the two 4-20 ma wires. This will convert the 4-20 ma signal to a 1-5 vdc signal. The 1-5 signal is proportional to the 4-20 Ma signal

Power-User

Join Date: Jul 2010
Posts: 400
#3

### Re: PLC 4-20mA Output

04/27/2012 9:21 PM

250 ohms to get 1-5 volts, not 100 ohms.

Guru

Join Date: Feb 2006
Location: middle of nowhere, Pennsylvania
Posts: 618
#4

### Re: PLC 4-20mA Output

04/30/2012 8:50 AM

Timing is everything - just got an email from EDN.com that has an article on how to build a circuit to convert 1 to 5 v signal to a 4 to 20ma output - if you understand electronics, the circuit explains everything.

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