It is extremely simple and inexpensive to measure 4-20ma signal with a device that will measure only Voltage inputs. Most A/D boards will accept a 0-5Vdc signal, but may not accept a 4-20ma signal directly.
The solution to this problem will only take a few minutes. Basically, Ohms law is used to calculate a resistor value in order to convert the 4-20ma signal to a voltage.
A 250 Ohm resistor is the most popular resistor value for this purpose, since it will produce a 1-5Vdc when 4-20ma signal flows through it, and a 0-5Vdc input is very common for most
data acquisition systems or
Data loggers and other analog measurement devices.
However, there are cases where voltage inputs other than 0-5Vdc are desired, so the following example will demonstrate how simple it is to calculate the correct value resistor for any voltage input.
Practical example
For this example, we will assume that a 0-10Vdc input will be used to measure 4-20ma signal.
Ohms law states: R=V/I where V is the Voltage, I is the current and R is the resistance
R=10V/.020A = 500 Ohms
When 20ma flows through a 500 Ohm resistor, it will drop 10 volts
When 4ma flows through a 500 Ohm resistor, it will drop 2 volts
Therefore, 4-20ma signal through a 500 ohm resister will drop 2 to 10 volts
One more important thing to remember is that the resistor tolerance should be 1% or less; preferrably 0.1%, since errors in the resistance will result in errors in the voltage drop. You don't want a resister that fluctuates much with time or temperature, as it will effect your accuracy.
After you have selected a resistor value, you must verify your readings and make any fine tuning adjustments in your software to compensate for any errors in the resistor.
For example, a 500 ohm resistance may actually be 497.5 ohms, therefore your voltage output will actually be 1.99 to 9.95V and not 2 to 10V as we calculated.
You simply connect the resistor across the voltage input terminals for your data acquisition system, and then connect your 4-20ma signal to the same two terminals, so that as the current flows through the resistor, a voltage will be dropped and then measured by the data acquisition device.
Keep in mind that it may be necessary to ground the power supply if you are using it to
power a temperature transmitter or 2 wire sensor.
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