Some field devices such as an inductive proximity switch may be designed as three wire devices that require a 24VDC source of power. In such cases it is possible to tap into the 24 volt bulk power supply in the rack room and run twisted pair to the device. However, to protect against a short circuit in the field it is necessary to also provide some type of protection such as a fuse. The special wiring, installation instructions and documentation may make such an installation quite costly. An alternate approach is to use a 24VDC High Side Discrete output to provide the 24VDC. In most DCS the discrete output, such as the DeltaV CHARM Discrete output, includes built in protection against a short circuit in the field. Also, taking this approach has the advantage that it is possible to turn on/off the power to the field device by activating or deactivating the discrete output. This capability to selectively turn the device power on/off can come in handy when initially installing and working with the device.
A few years ago we put together a discrete demonstration that included two inductive proximity switches that required a 24VDC power source – shown below.
In this case we used one 24VDC High Side Discrete output to provide power to these proximity switches as is illustrated below.