420mA potentiometer
How to use a potentiometer for simulating a 420mA signal. The use of this document is at your own risk, errors in the calculation or wrong selection of components can lead to damage to the equipment.
Current loop tester panel mounting
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Connecting the potentiometer.
In this example we are using an A/D converter which operates at a maximum input voltage of 5 volt. In order to transfer the current signal from 420mA to a voltage signal we need a resistor of 250 ohm (5 volt / 20 mA = 250 ohm) The potentiometer shall be connected as indicated in the diagram below.
Calculation of resistance for potentiometer
When the power supply voltage is known, the resistance for the potentiometer and the series resistance can be calculated. The series resistor is necessary for limiting the current to 20 mA. In many control systems 24VDC is used as supply voltage. To limit the current to 20 mA a resistance of (24V/ 20 mA = 1200 ohm) is necessary. This current will flow when the potentiometer is at its limit and has a resistance of 0 ohm, so the the total resistance is the resistance of the measurement resistor (Rm) and the series resistor, the measurement resistor is 250 ohm, so the series resistor shall be 1200  250 = 950 ohm. A resistor of 950 ohm is not available in the E12 selection. We use a resistor of 820 ohm, which is a standard value from the E12 selection. Next, we need to calculate the value of the resistance for the potentiometer. We need to calculate the resistance to create the 4 mA current. To create the 4 mA signal we need a resistance of 24V/ 4mA = 6000 ohm. The resistance of Rm is 250 ohm, the series resistance is 820 ohm, so the potentiometer needs a resistance of 6000  820 250 = 4930 ohm. A potentiometer with a resistance of 4930 ohm is not easily available, this is not a standard value for potentiometers. Standard values for potentiometers are 4700 ohm and 5000 ohm. With a value of 4700 ohm the current of 4 mA can not be reached, so we choose a potentiometer of 5000 ohm.
The following currents can be set:
The minimum current is: 24/(5000+820+250) = 3,96 mA
The maximum current is: 24/(820+250) = 22,4 mA.
With this potentiometer and series resistor, the current from 420 mA can be simulated by turning the potentiometer.
Disadvantage of use of a potentiometer for 420mA simulator.
A potentiometer is a adjustable resistor, this can not be used to regulate a current, therefore, this method has the following disadvantages:
 The graph shows the position of the potentiometer vs the current. As shown in the graph, turning the potentiometer from 0 to 50% results in a current change of 2,7 mA while turning from 50 to 100% results in a current change of 15,7 mA. Precise setting of a current between 18 and 20 mA is very difficult.
 The value for the resistors will vary by +/ 10% due to tolerance in production. This will influence the current, which therefore can vary from the calculated current.
 The power dissipated in the series resistance is 0,33 watt at 20 mA, the power is 0,01 watt at 4 mA. This change in power dissipation will influence the temperature of the resistor and therefore the resistance. This change in resistance value will influence the current. The current will not be stable over time due to heating and cooling of this resistor.
 Calculating errors can cause damage to the connected 420 mA apparatus.
Better alternative for 420mA simulator with potentiometer
The DIVIZE variable current source does not have these disadvantages. The diagram shows a simplified diagram of the DIVIZE current source.
The current is set with the potentiometer (RP). This potentiometer is supplied with a constant voltage by the zener diode (Z1). The voltage from the potentiometer is transferred by amplifier (U1) and transistor (T1) in a current. The current is measured with resistor (RM) and feed back to amplifier (U1). This amplifier controls the transistor in order to make sure the set current flows independent of the supply voltage, temperature and load on the circuit.
Because the potentiometer is supplied by a constant voltage, the output voltage is linear to the position of the potentiometer axle.
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