when we are using HART COMMUNICATOR why we are using 250 ohm
resistance while using HART COMMUNICATOR.
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Answer Posted / vinodkumar
when calibrating an hart transmitter in lab, we need a loop resistance between 250 ohms & 600 ohms, because without a resistor the HART communicator did not communicate with the transmitter. Because transmitter is a high impedance circuit with respect to the HART communicator. When HART communicator try to communicate with transmitter by producing a digital signal, the signal gets grounded through power supply and thus the signal didn't enter the transmitter without the resistor. When resistor is added, the transmitter become a low impedance circuit with respect to the resistor, so the signal produced by the HART communicator SHOULD have pass through the transmitter and so does the communication between HART communicator and HART Transmitter. Without the resistance between 250 ohms and 600 ohms the HART communicator did not communicate with Transmitter.
But in analog transmitters exact 250 ohms is used to convert 4 - 20mA to 1 - 5V respectively. For this only and exact 250 ohms is needed
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