PLC Output connections
Output cards again require some form of isolation barrier to limit damage from the inevitable plant faults and also to stop electrical ‘noise’ corrupting the processor’s operations. Interference can be more of a problem on outputs because higher currents are being controlled by Computers and industrial
Computers and industrial control 25 the cards and the loads themselves are often inductive (e.g. solenoid and relay coils). There are two basic types of output card. Eight outputs are fed from a common supply, which originates local to the PLC cubicle (but separate from the supply to the PLC itself). This arrangement is the simplest and the cheapest to install. Each output has its own individual fuse protection on the card and a common circuit breaker. It is important to design the system so that a fault, say, on load 3 blows the fuse FS3 but does not trip the supply to the whole card, shutting down every output. This topic, called ‘discrimination’, is discussed further in Chapter 8. A PLC frequently has to drive outputs which have their own individual supplies.
A typical example is a motor control centre (MCC) where each starter has a separate internal 110-V supply derived from the 415-V bars. The card arrangement could not be used here without separate interposing relays (driven by the PLC with contacts into the MCC circuit). An isolated output card, has individual out-puts and protection and acts purely as a switch. This can be connected directly with any outside circuit. The disadvantage is that the card is more complicated (two connections per output) and safety becomes more involved. An eight-way isolated output card, for example, could have voltage on its terminals from eight different locations.
Relay outputs can be used (and do give the required isolation) but are not particularly common. A relay is an electromagnetic device with moving parts and hence a finite limited life. A purely electronic device will have greater reliability. Less obviously, though, a relay-driven inductive load can generate troublesome interference and lead to early contact failure. Optical isolation is again used to give the necessary separation between the plant and the PLC system. Diode D1 acts as a spike suppression diode to reduce the voltage spike encountered with inductive loads. The output state can be observed on LED1. If NPN transistors are used, a current sinking card can be made. AC output cards invariably use triacs, a typical circuit being. Triacs have the advantage that they turn off at zero current in the load, which eliminates the interference as an inductive load is turned off. If possible, all AC loads should be driven from triacs rather than relays
An output card will have a limit to the current it can supply, usually set by the printed circuit board tracks rather than the output devices. An individual output current will be set for each output and a total overall output . Usually the total allowed for the card current is lower than the sum of the allowed individual outputs.