If there are any experts in instrument types out there, could you please clarify how loop powered instruments function? Your insights on these circuits would be greatly appreciated. Thank you in advance!
Hey Marc, your image didn't display properly on Sparky.
Marc, you sure are fast to get things done! Everything should be in good shape now.
There are essentially two main types of analog transmitters commonly used in industrial settings. The first type takes in a +24VDC signal on the + terminal and outputs a 4-20 mA signal on the - terminal, which is then sent to a receiver such as a PLC or chart recorder. This type of transmitter is unpowered in the system diagram. The second type of analog transmitter has separate +24VDC and 0VDC terminals, using this power supply to generate a separate 4-20 mA signal (or 0-10V signal) sent to the PLC or chart recorder. This type of transmitter is powered in the system diagram. In the event of a transmission wire being shielded, the second type of transmitter can still function if it has a local display, while the first type will shut down if either the + or - wire is removed or broken due to lack of power source. I hope this explanation clarifies the difference between the two types of analog transmitters. Please let me know if you need further information. -Marc
To ensure clarity, it is important to have some type of power supply connected to the "powered current loop input" diagram. The diagram should include lines running to the transmitter box labeled "power", whether it is DC or AC. The assumption is that the transmitter is connected to a power source in order to power the output, as indicated by the term "powered current loop input", also known as an "active output". In the lower section of the diagram, "unpowered" signifies that the instrument does not internally provide the power needed for its 4-20mA output. Instead, the power comes from an external source, typically referred to as the loop power supply. In a loop powered transmitter, a portion of the current running through the loop is utilized to power the instrument's internal components, while the output circuitry controls the current to produce the desired 4-20mA output. Another scenario not depicted is a powered instrument with a "passive" output, requiring power from a loop power supply. This setup helps to avoid ground loop issues that can arise from multiple non-floating power supplies. Additionally, there are loop powered instruments that rely on an external power supply for both instrument operation and 4-20mA output power. These instruments feature a power supply within the output loop (2-wire loop powered instrument). Furthermore, there are powered instruments with a loop powered output, necessitating two power sources - one for the instrument itself and another for the loop. This setup differs from instruments where the output is powered by the instrument's power source, with separate connections for power and output (4-wire instrument, active output).
Thank you for the responses! Can you suggest any online resources or websites for learning the fundamentals of instrumentation? Cheers, Sparky.
Loop powered instruments, often using a 4-20 mA current loop, operate by drawing power from the same current that carries the measurement signal. Essentially, they require minimal external power because they utilize the current traveling through the loop, making them very efficient and reliable for long-distance signal transmission. When the sensor detects a variable (like temperature or pressure), it modulates the current between those two levels, enabling the receiving end to interpret the measurement without needing additional power sources nearby. This also enhances noise immunity, which is crucial in industrial settings. Hope that helps!
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Answer: 1. What are loop-powered instruments and how do they work? - Loop-powered instruments are devices that operate using the current loop principle, where the instrument is powered and the signal is transmitted through the same 2-wire loop. The instrument draws power from the loop and sends back the measured signal using variations in current.
Answer: - Loop-powered instruments are known for their simplicity, as they require only a two-wire connection for both power and signal transmission. They are also cost-effective and easy to install compared to traditional instruments that require separate power and signal wiring.
Answer: - Loop-powered instruments are commonly used in industrial settings where running separate power lines for each instrument may be impractical or costly. They are ideal for measuring variables such as temperature, pressure, and flow in process control systems.
Answer: - When working with loop-powered instruments, it is important to ensure that the power supply voltage and current ratings match the requirements of the instruments. Proper grounding and shielding practices should also be followed to prevent interference and ensure accurate measurements.
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