Is a relay needed between the photoelectric switch and the PLC?
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This post was last edited by siena2008 on 2011-2-19 20:07. I’m not familiar with electrical automation; may I ask if a relay is needed between the photoelectric switch and the PLC?； Is the motor controlled by a PLC directly through a relay, or through a relay and then via a contactor? Thank you all so much! ! !
1. An optoelectronic switch, used as an input point for a PLC, should not require a relay; in fact, each DI point of a PLC functions like a small relay. A relay is needed when the load is relatively high. 2. A PLC can only directly control DC devices, while motors are generally AC-powered; therefore, a DC relay is required to control the AC contactor, which in turn controls the AC motor. Of course, there are also DC motors, but generally PLCs are unable to drive contactors (the load is too large); therefore, relays are needed
1\ Photoelectric sensors generally do not require a relay, provided that the PLC’s DI inputs can handle PNP signals. 2\ It depends on the type of motor used; if it is a small motor, a relay may not be necessary. For safety reasons, it is recommended to use a relay or contactor
If the PLC’s DI card is of the NPN type and the photoelectric switch is also NPN type, they can be connected directly; there are three-wire, four-wire, and two-wire connection types. The same applies to PNP. If they do not match, for example NPN to PNP type, a relay is needed for conversion. For example, a PNP-type photoelectric switch outputs a high level, and it is necessary to connect the relay between the signal line and the OV line of the photoelectric switch. Similarly, for NPN-type photoelectric switches, the relay coil needs to be connected in series between the signal line and the brown wire.
If the PLC’s DI card is of the NPN type and the photoelectric switch is also NPN type, they can be connected directly; there are three-wire, four-wire, and two-wire connection types. The same applies to PNP. If they do not match, for example NPN to PNP type, a relay is needed for conversion. For example, a PNP-type photoelectric switch outputs a high level, and it is necessary to connect the relay between the signal line and the OV line of the photoelectric switch. Similarly, for NPN-type photoelectric switches, the relay coil needs to be connected in series between the signal line and the brown wire.