Measuring a 4-20mA Signal with a Voltmeter

Measuring a 4-20ma Input With a Voltage Input Device - While most PLCs are available with both voltage and current inputs options, many of your lower end PLCs will have built in 0-10VDC analog inputs.  I highly recommend you get the proper input for your analog device, but this is a very good exercise in how to use Ohms law to convert one unit measurement to another.  By connecting a 250 ohm resistor across a voltage input, a 4-20mA signal is converted to a 1-5VDC signal.  Here's how:

Ohms law: R=V/I where V is the Voltage, I is the current and R is the resistance.

4-20mA = 1-5VDC.  We want 5VDC to equal 0.020mA so if we plug that into ohms law R=5V/0.020mA=250 ohms. Then we need to calculate the voltage at 4mA using our 250 ohm resistor.  Rearranging our ohms law formula, V=R*I or 250 ohm*0.004mA = 1V

Why not use 2-10VDC?  Using our ohms law equation, if we wanted 10VDC to equal 0.020mA then it would be R=10V/0.020mA= 500ohms.  This is another common resistor so why not use it?  4-20mA transmitters have a maximum impedance which lots of times is as low as 500 ohms so the device can't regulate a current through a resistance that is greater than it's maximum impedance.

Next Steps

Go to the Analog Circuits Training Lesson Series to select your next lesson.  There are also many other Lesson Series on PLC Programming and Industrial Automation.

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