Which side of the power supply for the external circuit at the DI/DO points is recommended?
Thread Content
This post was last edited by boligebo on 2020-5-20 at 18:01. With the leader’s departure, I am forced to take charge of both electrical and thermal control tasks; I seek guidance from experts regarding which side is more suitable for supplying the 24VDC power to the DI/DO points – whether it should come from the terminal board or from an external instrument power cabinet. Are there any differences in the recommended power supply methods for DCS systems and PLC systems? For the projects I’m working on, the power for the DO external circuits of the central DCS comes from the instrument power supply box, while the power for the DI circuits comes from the terminal block; obviously, according to the card’s specifications, both methods are acceptable.
Their uses are different, and the power supply requirements also vary. For example, in terms of DO points, there are dry contacts and wet contacts: dry contacts mean that the circuit does not require power from this point, while wet contacts mean that the circuit does need power from this point
Control systems are not very receptive to external DI signals with a source (so-called wet contacts). If you use a terminal block for DI, supply 24V to the terminal block as the circuit voltage for DI. DO is generally used to drive loads or go to the MCC, and relays are usually used for isolation. Given that the DO channel has a limited load-carrying capacity, the principle is as follows: 24V is supplied to the terminal block as power for the coil, while the power for on-site loads comes from the DC source in the power supply cabinet. The origin of both methods actually comes from a 24V DC power supply (usually configured in a redundant manner). It can be understood in this way: aside from the load voltage of the DO (commonly known as the user power supply), for which a separate 24V power supply is provided, all other power supplies fall under the responsibility of the system manufacturer. Choosing to power it from a terminal block facilitates maintenance, as the board is equipped with lights/fuses. For the Central Control 700 system, that’s exactly how I do it. Although PLCs offer the option of terminal blocks (refer to Siemens), they are not widely used in standard projects. Therefore, in the DI/DO circuits, the voltage often goes directly to the card, or a circuit is formed with the IO channels via terminal block jumpers.