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.
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