How to use robust circuit protection to ensure PLC reliability and efficiency

Figure 1:Robot circuit protection ensures PLC reliability. Source: LittelfuseFigure 1:Robot circuit protection ensures PLC reliability. Source: Littelfuse

Meeting the challenge of improving speed and product yield in manufacturing operations resides in control system intelligence. Advanced software technologies and algorithm innovations provide the capability to realize Industry 4.0 efficiencies. However, the system hardware must operate reliably. Otherwise, interruptions will slow manufacturing and control processes or negatively impact yield, resulting in increased costs. The intelligence of the Industry 4.0 system, the programmable logic controller (PLC), must be highly reliable to achieve the full potential of Industry 4.0. Thus, PLCs require protection from current overloads, voltage transients and electrostatic discharge (ESD).

An Industry 4.0 system’s heart is the PLC, providing monitoring, control and data collection; managing the system based on factory floor inputs; and transmitting critical data on manufacturing processes and tests, machine status, and quality control. The industrial IoT gateway sends data, receives commands from a central command center, and stores all data in the data center and cloud.

Figure 2: Industry 4.0 manufacturing system. Source: LittelfuseFigure 2: Industry 4.0 manufacturing system. Source: Littelfuse

A PLC is a special-purpose computer optimized for monitoring and control operations. Because PLCs operate in manufacturing environments with large, motorized equipment, warm temperatures and manufacturing residue in the air, they must be more robust than general-purpose computers.

Figure 3: PLC block diagram. Source: LittelfuseFigure 3: PLC block diagram. Source: Littelfuse

PLC block diagram

The PLC block diagram identifies components that protect the PLC from hazards, including overcurrent conditions, high-voltage transients and ESD as well as components that help reduce the PLC’s power consumption.

Figure 4: Recommended protection and power control components. Source: LittelfuseFigure 4: Recommended protection and power control components. Source: Littelfuse

Protecting the PLC

Here are several recommendations to protect the PLC circuits from overcurrent and transient voltages.

The power supply converts the AC line voltage to 24 V DC, which operates the circuitry. The power supply requires protection from current overloads and voltage transients on the AC line, surges on the AC line and voltage transients resulting from large motors turning on and off.

A time-delay fuse provides AC line current overload protection and avoids nuisance shutdowns due to current transients. Find a voltage rating of at least 250 V AC for use on a 230 V AC power line. Also, ensure that the fuse has a current interrupting rating of at least 50 A. Select a fuse that complies with UL/CSA 248.

Absorb the high-voltage transients induced on the AC line using a metal oxide varistor (MOV) or a protection thyristor-MOV combination to extend MOV lifetime. Select one that withstands over 400 J of energy and absorbs a surge current of up to 10,000 A. Some MOVs include an added feature: a thermal element protects the MOV by opening the connection to the leads if the MOV begins to overheat from a large overvoltage.

Consider adding a protection thyristor in series with the MOV. This combination allows selection of a lower-rated-voltage MOV, which enables clamping transients at lower voltages. The protection thyristor responds to transient overvoltage by turning on within nanoseconds. The protection combination has a lower leakage current than MOVs alone and increases end-product reliability.

To protect sensitive semiconductors, consider replacing the thyristor-MOV combination with a transient voltage suppressor (TVS) diode. Any portion of an overvoltage that passes through the MOV or the thyristor-MOV combination will be further absorbed and clamped at a low voltage.

Use a polymeric positive temperature coefficient (PPTC) resettable fuse for overcurrent protection of the power supply’s power-factor correction (PFC) circuit. These devices have low internal resistance (under 10 mΩ) for many current ratings. PPTCs have fast trip times, typically under five seconds, and consume a minimum of PCB space with surface-mount packages as small as the 0402-form factor.

A time-lag fuse, a series protection thyristor-MOV, a TVS diode and a PPTC fuse will ensure a thoroughly protected power supply circuit.

The DC/DC converter changes the 24 V DC to the voltages required for the control and input-output circuits (typically 5.0 V and 3.3 V). If 24 V DC directly powers the PLC, the converter needs protection from overcurrent and transient conditions propagating on the unprotected 24 V DC line. For overcurrent protection, use a time-lag fuse compliant with IEC 60127-2. Consider a surface-mount TVS diode to absorb and clamp transient voltages. Surface-mount versions withstand peak pulse power up to 3,000 W and peak surge currents of 300 A.

The input module converts digital and analog input signals into levels and digitized values that the application processor can read. Because the input signals arrive from the external environment, they can carry hazardous transients. Use a bidirectional or unidirectional TVS diode to protect the input module from transient overvoltage. Models that can safely absorb 600 W of pulse power and 100 A of surge current provide suitable protection.

User interface, wireless and wired communications circuits experience exposure to hazardous ESD via operator contact or exposure to ESD strikes from the outside environment via the antenna.

To protect the user interface touchscreens, choose a TVS diode array consisting of two diodes, connected anode-to-anode, to withstand ESD strikes from direct contact of ±12 kV. Diode arrays also protect wireless circuits from through-the-air ESD strikes up to ±18 kV and have a low capacitance of under 0.1 pF to avoid transmission and reception performance degradation.

Figure 5: Anode-to-anode TVS diode array. Source: LittelfuseFigure 5: Anode-to-anode TVS diode array. Source: Littelfuse

For wired communications, the methodology depends on the communications protocol. For information on protecting communication ports, reference “General port protection.”

How to optimize control, maximize efficiency and reduce power consumption

In switch-mode power supplies, investigate Schottky barrier rectifiers. The dual-diode package enables fast switching, more efficient high-frequency switch-mode circuits and saves PCB space.

The power supply’s PFC circuit reduces total PLC current drawn by keeping the voltage and current in phase while reducing current harmonics. Using an active PFC, a transistor acts as a switch to control current to an L-C network. To enable low-power, fast switching and further maximize the PFC circuit’s efficiency, consider using a MOSFET.

Figure 6: Common cathode, half-bridge Schottky pair. Source: LittelfuseFigure 6: Common cathode, half-bridge Schottky pair. Source: Littelfuse

For greater efficiency, use a silicon carbide Schottky diode. They have low forward voltage, and high pass currents at high operating temperatures, plus low leakage current and low recovery times for fast switching.

In output modules, use solid-state relays to drive motors and other devices. For reduced power consumption, consider optically isolated relays with turn-on currents under 10 mA. Be sure to ensure noise immunity between control and load circuits.


Protecting a PLC, improving its efficiency and enhancing its reliability does not require many components. Littelfuse is available to assist engineers with selecting protection and control components for their PLC designs. Littelfuse application engineers:

  • Help select the most cost-effective protection and control components
  • Provide compliance guidance on applicable safety and performance standards
  • Perform pre-compliance testing that helps identify problems before standards compliance testing.

By partnering with component manufacturers, designers can save development time and compliance testing costs. Protected from overcurrent and transient voltages, the resulting PLCs are robust, reliable and more efficient, enabling the realization of all Industry 4.0 benefits.

Download Stay Connected and In Control of Your Future Factories, Circuit Protection Products Selection Guide and Power Semiconductor Product Catalog, courtesy of Littelfuse, Inc.