Analog signal
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Why isn’t the voltage of the valve positioner 24 volts, but only around 5V, while pressure transmitters or flow meters have 24V? Aren’t all of them supposed to use 4-20mA? Why is there still a difference? Why is that? I’m a student who has just started working; I would appreciate some advice from those with more experience! By the way, I’m talking about 2-wire analog signals!
The 5V or less you mentioned refers to the voltage under load, right?
For a positioner, this 4–20 mA current serves as the current source output by the DCS. Since the impedance of a positioner is typically 250 Ω, the measured voltage drop is 1–5 V； For the transmitter, these 24V represent the voltage source provided by the DCS, which serves as its operating voltage; a 4–20mA signal is generated and added to this 24V circuit, a configuration commonly referred to as two-wire wiring. It’s that the power supply and signals share these two wires. For transmitters, there is also a four-wire connection method, in which the power line and the signal line are separate. This is the case for electromagnetic flowmeters, for example, as they consume a large amount of power during operation; the two-wire 24V/4–20mA system is not sufficient to drive them. Please be careful in the future: at this time, when a 4–20mA signal is connected to the DCS channel, the voltage drop across that channel is also 1–5V (assuming the channel impedance is 250Ω). Similarly, this 4–20mA signal represents the current source that the transmitter outputs to the DCS.