VT420 / Converter 4-20mA

Physical description

Product page: https://vutlan.com/4-20ma-sensors/13-vt420-converter-4-20ma.html

Side showing output (RJ11 / RJ12) :	Side showing input (terminal plug 2 pins 5mm):

Side showing output (RJ11 / RJ12) :	Side showing input (terminal plug 2 pins 5mm):
Open	Open

Function and purpose

The converter of direct current 4-20mA is used for measurement of constant current on 4-20mA from different sensors and converting data into the format of an IP monitoring system. The system interface allows you to assign the sensor and introduce the function data conversion.

Package includes

Package content can be found at VT420 package content

How do I know that my sensor can be used together with VT420 / 4-20mA converter?

Open the documentation of your sensor to see if it has the following "output signal":

If it does, then it can be used.

Installation instructions (hardware)

Connection of two-wire sensors and measuring converters with 4-20mA signal power supply

1. Assignment of 4-20 mA signal
The standard 4-20 mA direct current signal is widely used in measuring instruments and industrial automation means:

- to interface with sensors and converters, to perform parameter measurements;

- for information communication between devices.

The analog signal is represented as a direct current with a range of 4-20 mA, where 4 mA corresponds to the minimum signal level, and 20 mA to the maximum level.

2. Advantages of 4-20 mA signal

The 4-20 mA current loop signal has several advantages:

- Two-wire connection scheme;

- the ability to control short circuits and line breaks. The "zero" current loop of 4-20 mA is the "zero" of the operating device, which makes it possible to reliably detect a malfunction of the equipment, as well as a short circuit, a detachment, or a line break;

- a high degree of noise immunity. The current loop 4-20 mA has a low resistance and therefore is more resistant to interference than voltage signals.

3. Diagrams for connecting two-wire sensors and converters
The two-wire circuit is the simplest and most reliable for the operation of the sensor (converter). The sensor (converter) does not fail if it is switched on incorrectly (in case of the incorrect polarity of supply), in case of short circuits, and is less sensitive to interference (especially at low load resistances). When two-wire switching is easier to implement measures to reduce the effects of electromagnetic interference (industrial interference, radio interference).

3.1 Connecting the sensor to a 4-20mA sensor or transducer, that has a built-in power supply

Where:

R420 - resistance of VT420 (load), measured in Ohm.
Rline1 and Rline2 - resistance of the wires of the connecting line, measured in Ohm,
Vpower is the voltage of the sensor power supply, measured in V.
Arrows indicate the direction of the current 4-20mA.

Rballast - ballast resistor (it is not necessary), measured in Ohm, for power limitation consumed by the sensor

AO - analog output
AI  - analog input

3.2 Connecting the sensor to a 4-20mA sensor or transducer, that does not have a built-in power supply

If the 4-20mA sensor or transducer does not have a built-in sensor power supply or its power is insufficient to connect the sensor, use an external power supply. The analog input of 4-20mA AIine2- passive. The connection diagram is shown below:

Where:

R420 - resistance of VT420 (load), measured in Ohm.
Rline1 and Rline2 - resistance of the wires of the connecting line, measured in Ohm,
Vpower is the voltage of the sensor power supply, measured in V.
Arrows indicate the direction of the current 4-20mA.

AO - analog output
AI  - analog input

3.3. Connecting several 4-20mA sensors or transducers to VT420 converters
The scheme for connecting several sensors using a single power supply is shown in the figure below. The analog inputs of the converter AIine1-AIine3 are passive. The UDP power supply must provide a current for supplying all the sensors (converters). For example, the maximum current of one sensor = 24mA, in this case, the power supply must provide a current for three sensors, at least 72mA.

The picture below shows the scheme of connecting several sensors to the converter, using a single power supply:

To exclude the appearance of an additional error from the flow of the sum of the output currents of the sensors, the combining of the loads must be performed at one point. To minimize feedback on the power wires, the combination of the sensor supply wires must be performed directly on the positive terminal of the power supply. The wire connecting the negative terminal of the power supply to the common point of the system should be of minimum length.

3.4 Power Supply Calculation

The minimum required power supply voltage is calculated by the formula (1):

Upower min, V = Usens min + U420 min + (R420 + Rline) * Imax / 1000

Where:

VT420 data:

Example:

CAT5e cable length 100m: Rline = 2 * 10 Ohm
The minimum voltage of the sensor Usens min = 8V

Uppower min = 8V + 5V + (24.95 Ohm + 20 Ohm) * 24mA / 1000 = 8+5+(24.95+20)*24/1000 = 14V

3.5 Maximum sensor power consumption

The maximum power consumption of the sensor is calculated by the formula (2):

Psens max, W = Imax * [Upower - U420 min - Imax * (R420 + Rline) / 1000] / 1000

Example:
Let's say that we use a 19V power supply, then:

Psens max = 24mA * [19V - 5V - 24mA * (24.95 Ohm + 20 Ohm) / 1000] / 1000 = 24 * (19-5-24 * (24.95 + 20) / 1000) / 1000 = 0.31W

The calculated power consumption should not exceed the power indicated in the operating manual for the connected sensor (or converter).

4. Recommendations for selecting and connecting a cable
To connect sensors and converters with a 4-20 mA output, it is recommended to use a cable that consists of a shielded twisted pair with a cross-section of multicore wire of at least 0.5 mm. The shield of the cable is connected to the protective earth (PE). If the converter to which the sensor is connected is installed in a metal shield, the screen should be connected to the shield ground.

5. Recommendations for choosing a power supply
If the converter allows the same poles of analog inputs to be combined, then there is no need for a multichannel power supply.
The advantage of multi-channel units is that they tend to have a small short-circuit current, and if the communication line is accidentally closed, damage to the analog input is eliminated.
The purpose of using multi-channel power supplies is to galvanically unbind all signal circuits without high costs.

Connecting the sensor to the Vutlan monitoring system

Using included cable RJ11 / RJ12 connect VT420 to any analog port of the Vutlan monitoring system A1...A8. The system uses plug&play, the sensor will be detected automatically and will appear in the system.

Sensor settings

After VT420 has been connected to the sensor following instructions from above and back to the Vutlan monitoring system, it should appear in the interface, you will find it in the "system tree" of the interface: Interface > System tree

Initially, VT420 is denoted by "fA":

Mouse-click on the sensor to edit it. You should see:

Where:

Finding the expression formula

For example, we have a pressure sensor with a linear function. Let's take the pressure sensor "PD-39 X low pressure" as an example.

If we look at the documentation of the sensor we shall see the following:

From the table above we see that there are 3 sensors with standard pressure of 3, 10, and 25 bars. Let's say that we use a sensor with a standard pressure range of 25 bars. This means that the sensor can measure from 0 to 25 bars. Thus we get the following characteristics for the 4-20 mA current loop:

A(xA,yA) = A(0, 4)

B(xB,yB) = B(25, 20)

To find the equation of the line passing through points A(xA, yA) and B(xB,yB) ( xA≠xB ):

we use the formula:

y−yA=(yB−yA) / (xB−xA) * (x−xA)

we get:

y-4=(20-4) / (25-0) * (x-0)

y=0.64*x+4

Using the expression formula to tune the sensor

Let's say that we have a pressure sensor with an expression formula for the 4-20mA current loop y=2*(x-4).

1. Let's name the sensor: "Pressure".

2. Choose "current" for the "user-defined type".

3. We put this formula inside the field "Expression".

4. Press "Save" to save changes.

Inside our "System tree" we shall see new changes for the sensor:

a) The name changed

b) the Icon of the sensor has changed.

c) The value is calculated with the use of our expression formula.

Specifications

Usage example 1: 

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Remote Infrastructure Monitoring and Control

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Slovak Republic

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