HOW THE 4–20 mA CIRCUIT WORKS
Most 4–20 mA control circuits have a device that outputs a control signal and another that receives the signal.
Figure 1 shows a very basic 4–20 mA circuit. It shows an output device and shows a 4–20 mA display as the receiving device. Notice that the circuit also includes a precision 250 ohm resistor, which we will discuss later.
The circuit also shows wire resistance, which is a concern when long runs of wire are used. A voltmeter is shown connected across the precision resistor for reference purposes in our discussion. (It is not actually included in 4–20 mA circuits.) All devices in the circuit (except the voltmeter) are connected in series.
The key to understanding how the 4–20 mA circuitworks is knowing how the output device works. Remember, it is wired in series with the other devices in the circuit. That means that the number of mA through it is the same as in all other parts of the circuit. So the output device “reads” the number of mA to see if it is the predetermined value needed. If not, it changes its output voltage to achieve the predetermined value.
For example if 20 mA is required for 100 percent output, the output device generates the amount of voltage needed to cause a 20 mA current through the circuit. If 4 mA is required for zero percent output, the output device generates the amount of voltage that causes 4 mA. And so on.
See Figure 1 for a comparison of mA and voltage values and how to calculate them.
AN ANALOGY
You can compare the way the output device works with driving your car. You press on its accelerator to change its speed. You “read” the speedometer to see if the speed is what you want. If not, you change pressure on the accelerator to achieve the speed you want.
Similarly, the output device can put out various amounts of voltage, similar to the way the accelerator changes the speed of your car. The device then “reads” the current to see if the needed number of milliamps was achieved, similar to the way you read your speedometer. If the number of milliamps is not the amount needed, the device readjusts the voltage to attain the milliamps needed, similar to the way you change pressure on the accelerator until the speedometer shows the speed you want.
WHY A PRECISION RESISTOR IS USED
A precision 250 ohm resistor in the 4–20 mA circuit (Figure 1) enables the output device to work with virtually no error. The resistor is connected in series with all devices in the circuit, which ensures that the amount of current through the resistor is exactly the same as the current through all other devices in the circuit.
Accordingly, when the voltage across the 250 ohm resistor is 1 volt, the current through the resistor is always exactly 4 mA. Note: Figure 1 shows how a voltmeter has to be connected to read the voltage across the resistor. When the voltage across the resistor is 5 volts, the current through the resistor is always exactly 20 mA. And when the voltage across the resistor is somewhere between 1 and 5 volts the current through the resistor is exactly in direct proportion to that voltage.
So the precision 250 ohm resistor ensures that a predetermined current in the circuit will always have a predetermined voltage across the 250 ohm resistor. Accordingly, a 4 mA current will always have 1 volt across the resistor even though the voltage in other parts of the circuit may be different due to wiring resistance or other variables. Likewise a 20 mA current will always have 5 volts across the resistor even though the voltage may be different in other parts of the circuit.
ADVANTAGES OF A 4–20 mA CIRCUIT
A question that often arises is, “why not just use predetermined voltages instead of predetermined currents to provide the control signals?” The answer is that current is always exactly the same in all parts of a series circuit. But voltage may vary throughout a circuit according to resistance in the circuit. Thus, actual voltage at the receiving device would depend on resistance in another part of the circuit and would be less reliable.
For example, consider how resistance of wiring in the circuit could affect voltage at the receiving device. The resistance of wiring depends on wire size and length. Thus variances in wiring would produce variances in the voltage values for control. And this would produce control errors.
Using a precision resistor and an output device that varies its output voltage to produce predetermined currents eliminates error due to resistance. The output device simply outputs enough voltage to overcome wiring resistance and produce predetermined mA currents. Consequently, voltage measured at the output device and voltage measured across the resistor may be significantly different. But the voltage across the precision resistor depends entirely on the predetermined current. It will always match the mA values shown in Figure 1.
USE OF 4 mA FOR ZERO PERCENT
Another question that is sometimes asked is, “why is 4 mA used instead of 0 mA to produce zero percent or to shut off a device?” The answer is that a loss of power would result in 0 mA and the device being controlled could not sense the difference.
