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Loop Power vs External PSU: Powering and Wiring Notes

When to use loop power, when an external PSU is required, and how resistor, diode, and isolation options affect wiring continuity.

Applies to

  • Loop devices

Summary

This article explains whether the device requires a separate power supply or can be powered from the loop, and lists practical notes for powering and wiring.

Can the device be powered from the loop?

  • The device cannot rely solely on loop power for all functions. An external power supply is required to obtain the full functionality described for these devices.
  • When powered only from the loop, some advanced behaviors or features will not be available.

Powering recommendations and wiring notes

  • Use an external power supply (PSU) to power the device when full functionality is needed.
  • Devices may include a resistor (and in some configurations a diode plus resistor) in the line to provide resistive monitoring or line continuity behavior.
    • The resistor option allows you to configure line continuity so that, in some setups, removing a downstream device can still leave the loop functional; default behavior without the resistive option may result in loss of the loop when a device is removed.
  • Some device variants are supplied with short-circuit isolation built in; select the appropriate variant if short-circuit isolation is required for your installation.

Practical steps

  1. Fit and connect an appropriate external power supply when the installation requires full device functionality beyond simple loop powering.
  2. If resistive monitoring or line continuity is required, ensure the resistor (or resistor + diode variant) is installed in the line as provided or specified.
  3. If your installation requires short-circuit isolation, choose the device variant that includes short-circuit isolation.

Troubleshooting tips

  • When resetting or power-cycling, be aware that the line may be shut down temporarily; use the external PSU to restore the loop if needed.
  • If you see unexpected loss of loop continuity when a device is removed, verify whether the resistive option or continuity wiring has been installed.

Conclusion

An external power supply is required to enable the device’s full set of features; relying solely on loop power will not provide all capabilities. Use the supplied resistor/diode options and select short-circuit-isolated variants where necessary to achieve the desired wiring and monitoring behavior.

 

Last reviewed: 2026-06-16 — Reviewed by: Takashi Ishikawa