Fields of Application
- Industrial Automation: The Didactum 4-20mA connector is ideal for monitoring processes that use current sensors, such as in automated production lines or machinery.
- Environmental Monitoring: It can be used in environmental measurement systems that rely on current-based sensors to record parameters like temperature, humidity, or pressure.
- Building Management Systems (BMS): The converter can be integrated into BMS to monitor and control heating, ventilation, and air conditioning (HVAC) systems as well as other critical infrastructure.
- Diesel Emergency Tanks: Monitoring diesel level sensors with current output.
Examples of use:
The Didactum 4-20mA connector for water level measurement is monitored by the Didactum monitoring unit.
Sensor data are transmitted to the network monitoring center via SNMP traps.
The monitoring unit features integrated logic circuits that send e-mail and SMS notifications in case of an emergency.
Monitoring the diesel level in the diesel tank
The Didactum monitoring unit is capable of simultaneously monitoring multiple Didactum 4-20mA connectors by continuously reading the values from the current loops.
These values are then integrated into the Didactum monitoring system, which provides real-time data and triggers alarms in case of threshold violations.
This allows, for example, the precise monitoring of pressure in industrial systems or pipelines, enabling timely response to changes or potential problems.
Installation Guide (Hardware)
Connection of Two-Wire Sensors and Transducers with 4-20mA Signal Input
- Assignment of the 4-20 mA Signal
The standard 4-20 mA DC signal is widely used in measuring instruments and industrial automation:
- For interfacing with sensors and transducers to perform parameter measurements
- For transmitting information between devices
The analog signal is represented as a direct current in the range of 4-20 mA, where 4 mA corresponds to the minimum signal level and 20 mA to the maximum level. - Advantages of the 4-20 mA Signal
The 4-20 mA current loop signal offers several advantages:
- Two-wire connection
- Ability to monitor short circuits and line interruptions. The “zero” current loop of 4-20 mA corresponds to the “zero” of the operating device, enabling reliable fault detection of devices as well as short circuits, disconnections, or line breaks
- High noise immunity: The 4-20 mA loop has low resistance and is therefore less susceptible to interference than voltage signals. - Wiring Diagrams for Connecting Two-Wire Sensors and Transducers
The two-wire circuit is the simplest and most reliable for operating the sensor (transducer).
The sensor (transducer) is insensitive to reverse polarity during power input, provides protection against short circuits, and is less susceptible to interference (especially at low load resistance). The two-wire connection also facilitates the implementation of measures to reduce electromagnetic interference (industrial or radio disturbances).
3.1 Connecting the Sensor to a 4–20 mA Sensor or Transmitter with Integrated Power Supply
The right-hand image shows a sensor that uses a 24 V power supply.
- R420 – Resistance of the Didactum 4-20mA connector (load), measured in ohms.
- Rline1 and Rline2 – Wire resistance of the connection cable, measured in ohms.
- Vpower – Voltage of the sensor power supply, measured in volts.
- Arrows indicate the current direction of the 4–20 mA loop.
- Rballast – Ballast resistor (optional), measured in ohms, used to limit the power consumed by the sensor.
- AO – Analog output.
- AI – Analog input.
3.2 Connecting the Sensor to a 4–20 mA Sensor or Transmitter without Integrated Power Supply
If the 4–20 mA sensor or transmitter does not have an integrated power supply, or if the available power is insufficient to operate the sensor, use an external power source. The analog input 4–20 mA AIine2– is passive.
The image on the right shows a sensor that requires a 24 V power supply.
- R420 – Resistance of the Didactum 4-20mA connector (load), measured in ohms.
- Rline1 and Rline2 – Wire resistance of the connection cable, measured in ohms.
- Vpower – Voltage of the sensor power supply, measured in volts.
- Arrows indicate the current direction of the 4–20 mA circuit.
- AO – Analog output.
- AI – Analog input.
3.3 Connecting Multiple 4–20 mA Sensors or Transmitters to the Converter
The wiring diagram for connecting multiple sensors using a single power supply is shown in the image on the right.
The analog inputs of the converter AIine1 to AIine3 are passive. The power supply (UDP) must provide the total current required to power all connected sensors (converters).
Example:
The maximum current of one sensor is 24 mA; therefore, the power supply for three sensors must deliver at least 72 mA.
The image on the right illustrates the circuit for connecting multiple sensors to the converter with a single power source:
To avoid additional errors caused by the sum current of the sensor outputs, the load combination must be made at a single point. To minimize interference in the power supply lines, the connection of the sensor power cables should be implemented directly at the positive terminal of the power source. The cable from the negative pole of the power supply to the common system point should be kept as short as possible.
3.4 Power Supply Calculation
The minimum required supply voltage (Vpower min) is calculated using the following formula:
Vpower min = Usens min + U420 min + (R420 + Rline) × Imax / 1000
Where:
- Vpower min: Minimum supply voltage in volts
- Usens min: Minimum sensor voltage according to the sensor documentation
- U420 min: Minimum voltage of the VT420 converter (5 V)
- R420: Resistance of the VT420 (24.95 ohms)
- Rline: Cable resistance in ohms
- Imax: Maximum current (24 mA)
Example calculation with a CAT5e cable length of 100 m (Rline = 2 × 10 ohms) and a minimum sensor voltage of 8 V:
Vpower min = 8 V + 5 V + (24.95 Ω + 20 Ω) × 24 mA / 1000 = 8 + 5 + 44.95 × 0.024 = 14 V
This calculation ensures that the power supply provides sufficient voltage to properly power both the sensor and the converter. It takes into account voltage drops caused by cable resistance and combines the requirements of both the sensor and the converter. A supply voltage below the calculated value may result in measurement errors or failure of the sensor power supply.
Recommendation:
Use this formula to dimension the power supply while taking into account the cable length and sensor requirements to ensure stable and reliable measurements.
3.5 Maximum Sensor Power Consumption
The maximum power consumption of the sensor (Psens max) is calculated using the following formula:
Psens max = Imax × [Upower - U420 min - Imax × (R420 + Rline) / 1000] / 1000
Example calculation with a 19 V power supply:
Psens max = 24 mA × [19 V – 5 V – 24 mA × (24.95 Ω + 20 Ω) / 1000] / 1000 = 0.31 W
The calculated power consumption should not exceed the maximum value specified in the sensor’s operating manual. This is important to prevent overloading or damaging the sensor.
This calculation helps ensure proper power supply sizing so that the sensor operates within its specified power limits. Excessive power consumption may indicate wiring errors or incorrect voltage levels and should be avoided.
4. Recommendations for Selecting and Connecting a Cable
For the connection of sensors and converters with a 4–20 mA output, it is recommended to use a shielded twisted-pair cable with a minimum cross-section of 0.5 mm² (multistrand). The cable shield should be connected to protective earth (PE) to minimize electrical interference and signal noise.
If the converter is installed in a metal enclosure, the shield should be connected to the enclosure’s ground to effectively dissipate electromagnetic interference.
The use of copper wires with 16 to 22 AWG (0.205 mm to 0.823 mm diameter) is recommended. The cable length should be kept as short as possible to minimize voltage loss due to line resistance. It is also important to ground the shield at only one end to avoid ground loops.
These measures ensure stable signal transmission and prevent measurement errors caused by interference or insufficient shielding. Adhering to these recommendations is especially important for long cable runs and in environments with high electromagnetic interference.
5. Recommendations for Selecting a Power Supply
If the converter allows the same poles of the analog inputs to be combined, a multi-channel power supply is not required. This simplifies installation and reduces costs.
The advantage of multi-channel power supplies is that they typically provide a low short-circuit current. If a wiring connection is accidentally short-circuited, this protects the analog input from damage.
The main purpose of multi-channel power supplies is to galvanically isolate all signal circuits without significant additional costs. Such galvanic isolation prevents potential differences and interference that could otherwise cause measurement errors or device damage.
For reliable 4–20 mA sensor operation, the power supply should be designed to provide sufficient voltage and current based on the number of sensors and the cable length. The typical voltage for 4–20 mA loops is 24 V, which usually provides enough headroom to compensate for voltage drops.
Summary:
- For combined analog inputs, a single power supply is sufficient.
- Multi-channel power supplies offer short-circuit protection and enable galvanic isolation.
- The power supply must provide adequate voltage and current for all sensors.
- 24 V is a common value for 4–20 mA loop power supplies.
Monitoring system setup:
Sensor Settings in the Didactum Monitoring System
After the Didactum 4-20mA connector has been connected to the sensor according to the instructions above and integrated into the Didactum Monitoring System, the sensor will automatically appear in the user interface. You can find it in the system tree of the interface under: Interface > System Tree.
The sensor is initially displayed with the designation "fA". To configure settings, click on the sensor with the mouse. In the settings window, the following parameters are available:
- Name: Automatically assigned, but can be changed freely (e.g. "Pressure sensor").
- ID: System ID of the sensor element.
- Type: Displayed as “fcurrent” (current function) for 4-20mA converter.
- Custom type: Select an icon for easier identification (e.g. temperature, voltage, humidity).
- Class: Analog.
- Hardware port: Shows the physical connection label on the device.
- Current status: Displays Normal, Warning, or Alarm.
- Current value: Sensor value as reported by the converter.
- Additional fields: Contains the linear formula for converting sensor current values.
- Alarm levels: Settings for threshold values (low alarm, low warning, high warning, high alarm) for alerting and automatic actions.
- Expression: Allows adjustment of the linear conversion formula for the sensor signal.
These configuration options enable precise adjustment and monitoring of the sensor in the Didactum system, including alarm management, value display, and symbol selection for clear visualization.
Additional Settings in the Didactum Monitoring System
When you click on the sensor in the Didactum Monitoring System, the following settings are available:
- Name
The name is automatically assigned by the system but can be changed as needed, for example from "Pressure sensor" to "Pressure". - ID
System ID of the element. - Type
The Didactum 4-20mA connector appears as an fcurrent (Function of current) sensor. - Custom type
You can choose an icon for the sensor, used only for visual identification. Available symbols include: none, current, factor, frequency, humidity, power, temperature, vibration, voltage. - Class
Analog. - Hardware port
Name of the physical port on the device (read-only). - Current status
Possible states: Alarm, Warning, Normal. - Current value
The current sensor value provided by the converter. - Additional fields
Displays the linear formula for calculating the sensor value from the current signal. - Alarm levels
Thresholds for low alarm, low warning, high warning, and high alarm; these can be used for notifications and automated actions in logical circuits. - Expression
The linear conversion formula of the sensor signal can be customized here.
This user-friendly configuration helps you monitor sensor values precisely and manage alarms effectively.
Example for Determining the Expression Formula Using the PD-39 X Low Pressure Sensor
The Didactum 4-20mA connector operates only with sensors that use linear functions within the 4–20 mA current loop.
As an example, we take the pressure sensor “PD-39 X low pressure,” which is available in standard pressure ranges of 3, 10, and 25 bar. We use the model with a measurement range of 0 to 25 bar.
The sensor characteristic is defined by the points within the 4–20 mA range:
- A(xA, yA) = A(0, 4 mA)
- B(xB, yB) = B(25, 20 mA)
To determine the linear equation passing through points A and B, the following formula is used:
\[ y - y_A = \frac{y_B - y_A}{x_B - x_A} \times (x - x_A) \]
Substituting the values:
\[ y - 4 = \frac{20 - 4}{25 - 0} \times (x - 0) \Rightarrow y = 0.64x + 4 \]
This equation describes the relationship between pressure (x in bar) and current (y in mA).
Fine Adjustment via “Expression” in the System
Assume you have an expression for converting the current value to pressure: y = 2 * (x - 4).
Proceed as follows:
- Name the sensor, e.g., “Pressure”.
- Select “current” as the custom type icon.
- Enter the formula
2 * (x - 4)in the “Expression” field. - Save the changes.
This ensures that the conversion of the current signal is correctly implemented within the Didactum Monitoring System and that the pressure value is accurately displayed.
Changes in the “System Tree” after Adjusting Sensor Settings
After editing and saving the sensor settings in the Didactum Monitoring System, you will see the following visible changes to the sensor in the System Tree:
- Name changed: The new name, e.g. “Pressure,” is displayed instead of the automatically assigned name.
- Icon changed: The selected custom icon, e.g. “current,” is shown as the symbol in the system tree.
- Value calculated: The sensor value is no longer displayed as a raw reading but calculated according to the defined expression formula (for instance, converting 4–20 mA current signals into physical units).
These changes enhance system clarity and ensure accurate value representation according to the sensor’s function. This makes different sensors easy to identify and interpret correctly.