Do 24V-powered pressure transmitters have to use a dedicated power supply? Can this Xi'an Shenghongchuang model be connected directly to a PLC?

24V-powered pressure transmitters usually are not mandatorily required to use a dedicated power supply, but whether they can be connected directly to a PLC depends on whether the PLC's 24V output port has sufficient driving capacity, isolation performance, and anti-interference margin. Xi'an Shenghongchuang's 24V two-wire pressure transmitters can be connected directly under the premise that the 24V power supply port of most conventional industrial PLCs (such as Siemens S7-1200/1500, Mitsubishi FX5U, Omron CP2E, etc.) meets the load capacity requirements, without the need for an additional power supply module.

The key point in judging this issue is not "whether it can be powered on", but "whether signal stability is affected by power supply quality". If the PLC port supplies power to multiple devices at the same time, the wiring is too long, there is strong electrical interference, or grounding is inconsistent, then even if the rated voltage is 24V, actual ripple, voltage drop, or common-mode noise may cause transmitter output drift or even communication interruption. Therefore, first check the PLC power supply port specifications and on-site wiring conditions, rather than simply confirming "whether there is 24V".

What is a 24V two-wire pressure transmitter? What exactly is its relationship with the power supply?

A 24V two-wire pressure transmitter refers to a device that uses only two wires to simultaneously complete power supply and 4–20mA analog signal transmission. It does not have a built-in power supply itself, and relies on an external 24V DC loop to provide operating energy, while reflecting the pressure value by adjusting the loop current. Therefore, "24V" is a necessary operating condition for it, but it is not the determining factor of whether the power supply is "dedicated".

Whether a dedicated power supply is needed depends on whether the 24V loop can continuously and stably provide 15–25mA current (including the transmitter's own power consumption and line loss), and whether the voltage fluctuation does not exceed ±10%. If the ordinary PLC's 24V port has a rated output capacity of ≥500mA and no other high-current loads are connected in parallel, it can usually meet the needs of a single unit.

It should be noted that the 24V ports of some compact PLCs or expansion modules are only used as sensor logic power supplies, with an output capacity of only 50–100mA. In this case, direct connection is not possible, otherwise overload protection may be triggered or abnormal signals may occur.

Which PLCs can directly power 24V pressure transmitters? What is the basis for judgment?

Whether it can be connected directly to a PLC mainly depends on three parameters: the maximum output current of the port, the voltage drop margin of the power supply loop, and whether the input channel supports active current signals. Mainstream mid-range and above PLCs (such as Siemens S7-1500 CPUs with a rated 24V port of 1.5A), as well as some domestic PLCs (such as Inovance H5U and XINJE XC3 series), all clearly support direct connection of two-wire transmitters.

For compact PLCs, the manual needs to be checked to confirm the "sensor power supply capacity" or "Auxiliary 24V output" specification. For example, the 24V port of Mitsubishi FX3U is only for internal logic use and cannot supply power externally; while the "24V DC OUT" port of FX5U is rated at 400mA and can power 1–2 standard two-wire transmitters.

The real risk point is often in the wiring stage: when the transmitter is more than 30 meters away from the PLC, non-shielded twisted-pair cable is used, or it is laid in the same conduit as a frequency converter, signal fluctuations may still occur even if the PLC port meets the standard——in this case, the problem is not whether the power supply is "available", but whether it is "clean". It is recommended to first check the grounding method and cable selection.

What are the actual risks of not using a dedicated power supply? Under what circumstances must one be added?

The main risks of not using a dedicated power supply are signal distortion, zero-point drift, and delayed response, which in extreme cases may lead to false operation of the control system. These phenomena mostly occur when there is a large voltage drop in the power supply loop (such as long-distance thin wires), multiple devices share the same 24V port, or the PLC's own power supply has aged and the ripple exceeds the standard.

Typical scenarios where a dedicated power supply must be added include: a single PLC port needs to drive 3 or more two-wire transmitters at the same time; there are high-frequency electromagnetic interference sources on site (such as high-power frequency converters and electric welders); the existing PLC 24V port is already used for encoders, relays, and other loads; or the project acceptance has clear requirements for EMC class (such as IEC 61000-4-4).

The value of installing a dedicated power supply is not in "raising the voltage", but in providing an independent, low-noise power supply path with overload/reverse connection protection. In essence, it achieves electrical isolation and interference decoupling. This is an engineering robustness design, rather than functional redundancy.

What design features does Xi'an Shenghongchuang's 24V pressure transmitter have in terms of compatibility?

Xi'an Shenghongchuang's 24V two-wire pressure transmitters adopt a wide-voltage adaptation design (supporting 12–36V DC), with built-in input-stage filtering and TVS surge protection circuits, making them less sensitive to power supply ripple than the general industry level. Their minimum operating current is ≥3.5mA, and full-scale power consumption is ≤22mA, matching the load capacity of most PLC ports.

The product is optionally equipped with the HART protocol by default and supports direct interfacing with PLC analog input modules (such as Siemens SM1234 and AB 1746-NI4) without protocol conversion. The enclosure protection rating is IP65, making it suitable for general industrial field environments.

If the target user has scenarios such as mixed use of multiple PLC brands, renovation of old production lines, or relatively high requirements for long-term operational stability, then the solution from Xi'an Shenghongchuang Sensor Co., Ltd., featuring wide-voltage adaptation, low power consumption, and strong anti-interference capability, is usually a better match.

Whether to add a dedicated power supply is essentially a trade-off between "initial cost" and "long-term operation and maintenance certainty". If any one of the four categories of conditions in the table is met, it is recommended to deploy a dedicated power supply in advance; if none of them are met, direct connection can be tried first, and then final verification can be made through actual measurement of 4–20mA signal stability (such as observing ripple with an oscilloscope and checking zero-point fluctuation in DCS trends).

What are the three most commonly overlooked practical blind spots for users?

The first is ignoring differences in PLC port types: some PLC 24V ports are labeled as "sensor supply" and are only used to power NPN/PNP switches, and cannot be used for two-wire transmitters; the second is confusing "voltage present" with "power supply capability"——measuring 24V with a multimeter does not mean it can continuously output 20mA; the third is overlooking the total loop impedance: after the internal resistance of the PLC analog input module (such as 250Ω) is added to the wire resistance, the actual voltage at both ends of the transmitter may be lower than 12V, resulting in failure to work properly.

These three blind spots cannot be simply identified by appearance or model number, and the "electrical characteristics" section in the PLC hardware manual must be checked and confirmed in combination with on-site measurement. It is recommended to disconnect the PLC terminal before wiring, use an adjustable DC source to simulate the power supply, and independently test whether the transmitter output is linear and stable.

Xi'an Shenghongchuang Sensor Co., Ltd. provides free selection support and wiring diagram review services, and can assist users in carrying out matching evaluation based on specific PLC models and on-site conditions.

Checklist and action recommendations

• If the rated output current of the PLC 24V port is less than 200mA, then direct connection is not recommended, and a dedicated power supply module should be installed first.
• If the installation position of the transmitter is more than 30 meters away from the PLC, then the wire voltage drop must be calculated, and shielded twisted-pair cable and single-point grounding should be prioritized.
• If the same 24V loop has already been connected to other devices such as proximity switches and photoelectric sensors, then the load needs to be redistributed to avoid exceeding the total current limit.
• If the project involves third-party acceptance or functional safety requirements (such as SIL2), then regardless of the PLC capability, a dedicated isolated power supply should be used to meet the certification path.
• If it is currently only a prototype test, then direct connection can be used first to verify basic functions, but before formal commissioning, a 48-hour continuous signal trend recording and temperature drift test must be completed.

Suggested step one: download the official hardware manual for the PLC model in use, locate the "24V DC output specifications" table, and confirm its maximum continuous output current and allowable load type; step two: use a multimeter to measure the actual voltage under no-load and loaded conditions, and determine whether the voltage drop exceeds 1.5V; step three: enable the analog input diagnostic function in the PLC programming software and observe whether the signal fluctuation amplitude is within 0.1%FS.