How PLC Systems are Protected from Plant Voltages

Internally a PLC typically operates at 5 V dc. The external devices such as solenoids, limit switches, motor starters, etc. operate at voltages up to 110 V ac. The mixing of these voltages will cause irreparable damage to the PLC electronics. A less noticeable problem may occur from electrical noise introduced into the PLC from voltage spikes caused by interference on signal lines or from load currents flowing in ac neutral or dc return lines. Differences in earth potential between the PLC compartment and outside plant can also cause problems.

It is therefore imperative to separate the plant voltage supplies from the PLC voltage supplies with some form of barrier to ensure that the PLC cannot be adversely affected by anything happening in the plant. Such that a cable fault, for instance, 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 affected.

This isolation is achieved by optical isolators consisting of a linked light emitting diode and photoelectric transistor. When current is passed through the diode it emits light causing the transistor to switch on. Since there is no electrical connection between the diode and the transistor, very good electrical isolation is achieved.

A dc input can be provided as shown in Figure 1.0 below. 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 typically available for three voltage ranges: 5V (TTL), 12-24 V, 24-50 V.

A possible ac input circuit is shown in Figure 1.1 below. The bridge rectifier is used to convert the ac to full wave rectified dc. Resistor R2 and capacitor C1 act as a filter (typically 50 ms time constant) to give a clean signal to the PLC logic. A neon LPI acts as an input signal indicator for fault finding, and resistor R1 sets the voltage range.

The output connections also require some form of isolation barrier to limit damage from the inevitable plant faults and to stop electrical noise distorting the processor’s operations. Interference can be a bigger problem on outputs as higher currents are being controlled by the cards and the loads (solenoids and relay coils) are often inductive.

A transistor output circuit is illustrated in Figure 1.2 below:

Opto-isolation has been used to given the required 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 as illustrated in Figure 1.3.

The output can be observed on LED1. Figure 1.2 is a current sourcing output. If NPN transistors are used, a current sinking card can be made as shown in Figure 1.4.

AC output cards invariably use Triacs. A typical demonstration is shown in Figure 1.5. Triacs have the advantage that they can be made to turn ON at zero voltage and inherently turn OFF at zero current in the load. The zero current turn OFF eliminates the spike interference caused by breaking the current through an inductive load. 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, typically set by the printed circuit board tracks rather than the output devices. An individual output current will be set for each output (typically 2 A) and a total overall output (typically 6 A). Usually the total allowed for the card current is lower than the sum of the allowed individual outputs.

You may also read:

• What are PLC-Based Water Treatment Plants?
• The Basics of Ladder Diagrams for Programming PLCs
• Basic Features of Distributed Control Systems (DCS)
• Basic Features of Modern PID Controllers