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Join Date: Apr 2012
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### 4-20mA Simulator

04/02/2012 3:53 AM

Hi

I am trying to build a simulator for a Data Logger that accepts a 4-20mA input measuring a current between 0-100 Amps by using a LEM transducer. Is there a simplistic circuit I can build that has a variable resistor to simulate the current varying between 0-100 Amps with a 4-20 mA output?

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Guru

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#1

### Re: 4-20mA Simulator

04/02/2012 4:14 AM

24VDC into a 10kΩ variable resistor would do it simplistically.

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#2

### Re: 4-20mA Simulator

04/02/2012 4:55 AM

Many Thanks

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#3

### Re: 4-20mA Simulator

04/03/2012 8:45 AM

You will burn out your variable resistor if you put it straight across 24V and run it down to zero resistance. Better to use a higher voltage and fit a safety resistor on each side of the variable resistor.

Alternatively use a bench top power supply and vary the voltage into a selected resistor read your Ma output on the PS or an external DMM.

Or use a 3 terminal voltage regulator chip wired as a variable constant current scource this could be run from a 9V battery if you need portability, a very simple circuit and it would fit in a small box used in conjunction with a DMM to read the current.

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#4

### Re: 4-20mA Simulator

04/03/2012 8:58 AM

Send me an email and I can send you a circuit diagram of one we used to build at MTL many years ago

Steve

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#5

### Re: 4-20mA Simulator

04/03/2012 9:28 AM

You can also use a LM317 (3-terminal voltage regulator), wired as a constant current source through a potentiometer. I believe I saw the wiring diagram in the Texas Instruments datasheet for the device. If you use discreet resistors with a multi position switch, you can set the resistor values for exact values for test.

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#7

### Re: 4-20mA Simulator

04/03/2012 11:33 AM

I had forgotten this solution, it is a far more practical than what we developed but didn't exist at the time

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#6

### Re: 4-20mA Simulator

04/03/2012 11:14 AM

Why don't you use a DRUCK, FLUKE, TRANSMATION, or other manufacturer 4-20mA source and scale the output?

4 = 0

20 = 100

These sources are variable, rather inexpensive especially when purchased used on EBAY, AMAZON,etc. , and they are very accurate.

You can monitor and manipulate the source output to whatever value needed by simply rotating the dial (old units) or using the up-down function buttons.

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#8

### Re: 4-20mA Simulator

04/03/2012 11:41 AM

Omega,

I may have misunderstood what you are needing. My apologies.

Here is a link that should help.

The current measurement output should be a mV signal. Measure it and scale accordingly.

Simply place a 10K multi-turn variable resistor pot in the IC feedback loop to get control of the output.

You will have to do calculations to determine the current limiting resistor sizes for the inverting and non-inverting IC inputs based on your ciruit voltage values.

http://en.wikibooks.org/wiki/Circuit_Idea/Op-amp_Inverting_Voltage-to-Current_Converter

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#9

### Re: 4-20mA Simulator

04/03/2012 12:55 PM

I just use a 9V battery and a pot. Inherently current limited by virtue of the capacity of the battery.