What is a transmitter?
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In a measurement circuit, such as a pressure monitoring circuit, the 4–20 mA signal output by the transmitter is determined solely by the actual pressure level at the site; it is not affected by the voltage supply of the circuit (which can be 24 V or around 22 V). Does the transmitter function in such a circuit as a variable load, or something else? Some say it’s equivalent to a constant current source, right?
1. It’s not that no one has replied; it’s just that a few days ago, when people came in, they couldn’t see the content of this post — there was an issue with the website’s system. 2. The transmitter acts as a constant current source in the circuit. . . The voltage applied to the transmitter terminals by the power supply changes as the output current varies. .
Is there any theoretical support for this?
There’s no need for any theoretical analysis for this. . . Have you studied amplification circuits? Take a look at instrumentation amplifiers and related topics
A transmitter is a controlled constant current source. . . The constant current value is controlled by the sensor output value. . . Going further, it is controlled by the parameter being measured that is input to the sensor
When the voltage is higher than the operating voltage of the pressure transmitter (usually above 12V), the current value of the pressure transmitter depends only on the measured pressure value.
You are probably puzzled about the working principle of two-wire pressure transmitters. That’s right; a two-wire pressure transmitter is connected to a 24V circuit. The sampling circuit of such a pressure transmitter relies on operational amplifiers, and these amplifiers require a certain operating voltage (which can be floating, as long as it remains within a certain range). This op-amp circuit typically consists of two stages: the first stage, which is an amplification circuit whose main function is to amplify the millivolt signals (these millivolt signals are proportional to the pressure signal, as generated by the front-end pressure sensor). The second stage is a voltage-controlled current source, which converts the voltage amplified in the first stage into 4-20mA in proportion. Two-wire pressure transmitters are designed to consume less than 4 mA of current. During operation, the loss in the wire plus the voltage drop across the sampling resistor must not exceed 12V; in other words, it must not be greater than 12/20mA = 600 ohms.