PLC Input cards
Internally a computer usually operates at 5 V DC. The external devices (solenoids, motor starters, limit switches, etc.) operate at voltages up to 110 V AC. The mixing of these two voltages will cause severe and possibly irreparable damage to the PLC electronics. Less obvious problems can occur from electrical noise introduced into the PLC from voltage spikes on signal lines, or from load currents flowing in AC neutral or DC return lines. Differences in earth potential between the
PLC cubicle and outside plant can also cause problems.
The question of noise is discussed, but there are obviously very good reasons for separating the plant supplies from the
PLC supplies with some form of electrical barrier. This ensures that the PLC cannot be adversely affected by anything happening on the plant. Even a cable fault putting 415 V AC onto a DC input would only damage the input card; the PLC itself (and the other cards in the system) would not suffer.
This is achieved by optical isolators, a light-emitting diode and photo-electric transistor linked together.
Protection of the PLC from outside faults. The PLC supply L1/N1 is separate from the plant supply L2/N2
switch on. Because there are no electrical connections between the diode
and the transistor, very good electrical isolation (typically 14kV) is achieved.
A DC input can be provided. When the push-button is pressed, current will flow through D1, causing TR1 to turn on,
passing the signal to the PLC internal logic. Diode D2 is a light-emitting diode used as a fault-finding aid to show when the input signal is present. Such indicators are present on almost all PLC input and output cards.
The resistor R sets the voltage range of the input. DC input cards are usually available for three voltage ranges: 5V (TTL), 1224V, 2450V.
A possible AC input circuit. The bridge rectifier is used to convert the AC to full wave rectified DC. Resistor R2 and capacitor C1 act as a filter (of about 50ms time constant) to give a clean signal to the PLC logic. As before, a neon LP1 acts as an input signal indicator for fault finding, and resistor R1 sets the voltage range. The isolation barrier and monitoring LEDs can be clearly seen. This card handles eight inputs and could be connected to the outside world.
