What is the purpose of this 250 ohm resistor?

What is the purpose of this 250 ohm resistor here? please anyone explain 1

In instrumentation and control systems, impedance matching plays a crucial role in ensuring efficient signal transmission and minimizing signal distortion. When interfacing between different types of signals or devices, such as converting a current signal (4-20 mA) to a voltage signal (1-5 V) for HART (Highway Addressable Remote Transducer) protocol communication, impedance matching becomes particularly important.

  1. Impedance Matching for HART Communication:
  • In HART-enabled devices, a 250 ohm resistor is commonly used for impedance matching when converting the 4-20 mA current loop signal to a voltage signal suitable for digital communication.
  • This resistor allows the voltage drop across it to correspond to the current flowing through the loop, enabling accurate conversion to a voltage signal in the range of 1-5 volts, which is typically used for digital communication in HART devices.
  1. Current-to-Voltage Conversion:
  • The resistor facilitates the conversion of the 4-20 mA current signal to a voltage signal, which can be measured by the receiving device for digital communication or processing.
  • This conversion is essential for interfacing sensors or devices that produce current signals with control systems or instrumentation devices that require voltage inputs.
  1. Signal Conditioning and Protection:
  • In both HART and general control loop applications, the resistor may serve in signal conditioning or protection roles.
  • It could be used for biasing, calibration, or establishing reference voltages or currents within the circuit.
  • Additionally, it may provide protection against excessive current or voltage levels, ensuring the integrity and safety of the circuit.
  1. Voltage Divider and Calibration:
  • The resistor might be part of a voltage divider circuit used to scale down higher voltage signals to levels suitable for input to control systems or instrumentation devices.
  • It could also play a role in calibration procedures, ensuring accurate measurement or control within the system.
  1. Alternative Communication Protocols:
  • In scenarios where HART protocol or 4-20 mA current loop communication is not utilized, the purpose of the 250 ohm resistor may differ.
  • For instance, in PSK multidrop versions of HART or other digital communication protocols, the resistor might not be necessary for current-to-voltage conversion.

If this resistance is connected to the analog input, it appears to be a 250Ω “shunt” resistor with an accuracy of 0.05%.

This resistor is used to create a voltage drop in the current circuit generated by the 4-20mA transmitters, which corresponds to 1-5vdc.

In other words, it plays a crucial role in converting current to voltage, enabling analog signals to be interpreted by electronic systems.

1). Resistance Value

The mentioned resistance is 250 ohms. This means that when an electric current passes through it, it creates a certain resistance to the flow of that current.

2). Shunt Resistance

The term “shunt” refers to a configuration in which a component is connected in parallel with another component. In this post, the 250-ohm resistance is being used as a “shunt resistance,” meaning it is connected in parallel with some other component or circuit.

3). Accuracy

The accuracy of this resistance is specified as 0.05%. This indicates how close the actual value of the resistance is to the nominal value of 250 ohms. An accuracy of 0.05% suggests high reliability and consistency in resistance values.

4). Purpose

This resistance is being used to create a voltage drop. When an electric current passes through it, due to its resistance, there is a voltage drop proportional to the current passing through. This is useful for measuring current in a circuit.

5). 4-20mA to 1-5VDC Conversion

The 4-20mA of 1-5VDC. The 250-ohm resistance is part of the circuit that performs this conversion. The relationship between current and voltage is linear in this circuit, where 4mA corresponds to 1VDC and 20mA corresponds to 5VDC.

Therefore, the 250-ohm resistance plays a crucial role in measuring and converting current to voltage in a circuit using 4-20mA transmitters. Its accuracy is essential to ensure the accuracy of measurement and conversion.