The issue of connecting a 250-ohm resistor in series in the circuit?
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Connecting a 250-ohm resistor in series in the circuit actually converts a 4–20mA signal into a 1–5V voltage signal – is this understanding correct? ? ?
Each case needs to be analyzed on its own; it can’t be understood in just this way. If you want to obtain a standard voltage signal from this standard resistor, then its function is to convert a 4–20mA signal into a 1–5V voltage signal. However, in some circuits, a 250-ohm resistor is connected in series to meet the requirements of a minimum load; for example, when using HART375, it is required that the circuit resistance be at least 250 ohms. Therefore, in certain instrument circuits, a 250-ohm resistor must be inserted in series in order to be able to communicate with the 375 device.
More or less, that’s what it means
I agree with what was said on floor 4, but I would like to add something further. If this resistor is connected in series within the circuit, it is there to increase the load on the circuit, which is used for online communication. If it is connected in parallel within the circuit (across the input terminals), it is used to obtain 1-5V signals
This post was last edited by myrhythm on 2009-5-28 at 11:20. I remember that for Honeywell’s ST3000 smart meters, a 250-ohm resistor also needed to be connected in series in the circuit during calibration! It’s the same as what was mentioned by the person on floor 4, but in that case HP’s PDA and MCTOOL software were used. Are there any other scenarios? Please share them too!
A 250-ohm resistor is connected in series in the circuit – how to understand this depends on the purpose of inserting this resistor. I think the user from floor 4 explained it very well and clearly!
I don’t think so~ It’s because of the HART protocol; although voltage is measured, it shouldn’t be 1 to 5 volts~
Series 250-ohm resistors are sometimes used for signal conversion; in some cases, when primary and secondary instruments feeding into a DCS can only output 4-20MA, resistors can be used for conversion.
This understanding is incorrect. The 250-ohm resistor is necessary in the circuit so that a resistance of 250 ohms is present in order to be able to communicate with 375
My understanding is as follows: Digital signals that comply with the HART communication protocol are superimposed on the 4-20mA analog signal. To add or retrieve these digital signals, a handheld device is used to apply pulse digital signals to the 250-ohm resistor in the circuit, or to retrieve such pulse signals, thereby enabling communication with the transmitter. Due to the DC-blocking capacitor on the controller side, pulse digital signals are allowed to pass through while DC analog signals are blocked. It can be seen from this that what the hand controller requires is not a 1-5V DC analog voltage signal generated by 4-20mA across a 250-ohm resistor.
Has anyone considered the load capacity of the instrument? Adding a 250-ohm resistor increases the circuit resistance
Increase the loop load, but ensure a voltage of 10.5V is maintained