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Why do you Need a Separate Power Supply for Your PLC?

PLCs are general-purpose, microprocessor-based controllers which provide logic, counting, analog, and timing control enhanced with a variety of network communications capabilities. It is mainly used in industrial computing systems to control manufacturing processes, assembly lines, and other industrial automation equipment that require a highly reliable control and fault diagnosis method.

A PLC system consists of a Central Processing Unit (CPU) module, control module, communication module, a power supply, Digital, and Analog Input/Output modules. The required quantities of the aforementioned modules are mounted on a common electrical interconnection and physical support structure referred to as a rack. This article focuses on the PLC power supply unit.

Power Supply Module

A PLC system without a power supply is more of a laptop without a battery or a car without fuel. It is the powerhouse that fuels/ energizes the PLC system to perform its designated functions. Hence, the more powerful a power supply module is, the more powerful the PLC will be. The PLC power supply converts an electrical distribution/ line voltage, commonly 240Volts AC (Alternating Current) or 120V AC, into useable signal level voltage to power the PLC processor and its other modules which is mainly 24V DC (Direct Current)

To change the 240V AC into a useable 24V DC, the PLC power supply uses a Regulated Power Supply (RPS) Circuit, illustrated in the block diagram below. The Alternating current (AC) utility power enters the PLC power supply through a primary input filter whose main function is to block transmission of unwanted high-frequency electrical noise to the input AC power line. In the first stage, a step-down transformer is used to step down the 240V AC line voltage into 24V AC. In the second stage, a rectifier is used to convert the incoming 24V AC to 24V DC. The rectifier used may be full-wave, half-wave, or bridge. In the smoothing stage, large electrolytic capacitors are used to heavily filter AC ripples and pass the voltage to the regulator. The capacitors also provide a reservoir for power storage, enabling the PLC power supply to ride through power dropouts of the utility power.

In the last stage, a linear voltage regulator with an active (MOSFET or BJT) pass device (shunt or series) and controlled by a high-gain differential amplifier, is used. This regulator compares the output voltage of the power supply with a precise reference voltage, it then adjusts the integrated pass device to maintain a constant and acceptable level of output voltage. The regulator can maintain a DC voltage of either 12V or 24V or 5V, depending on your PLC voltage rating and the power requirements of the PLC components. Also, the voltage regulator reduces the ripples left by the capacitor filters, by eliminating any voltage drops or surges that could damage the components of the PLC system plugged into the power supply.

All the components shown in the block diagram above, are all packed into the small looking PLC power supply. Note, PLC power supplies are available in different power ratings and sizes depending on the specifications of the PLC system, just as PLCs are supplied in different sizes and with different features for varying applications. Though, as previously mentioned, the common output voltage for most PLC power supplies is 24 Volts DC.   

Any PLC power supply has a label showing its reference current, rated in amperes or amps. The common current ratings for power supplies in smaller PLC systems range between 2A(amps) to 10 amps, while those in larger and more power PLC controllers are rated up to 50A(amps).  The current is an important parameter of consideration during the design or modification of a PLC system, as it directly affects the performance of the controller. 

How is the Power Supply connected to the PLC? 

As previously mentioned, the power source for a PLC power supply is usually a single-phase and 240V AC. Hence, if the PLC system is installed in an enclosure, the two AC power leads (L1 hot and L2 common) normally enter the PLC enclosure through the top part of the cabinet. This is to minimize electrical interference with other control electrical lines.

For most PLC systems with modular-style racks, the power supply is plugged into the rack or backplane. The rack is sort of the PLC base, in which all the other PLC modules plug into so that they can function as one single unit. In some PLC systems, a single power supply is used to provide power to all the connected modules via a bus system in the backplane. While in other PLC systems, these modules are wired individually to different power supplies throughout the rack.

Why a Separate PLC Power Supply? 

A) To Minimize Line Voltage Transients 

Typically, a PLC is powered from a standard AC utility power, while in some cases a 24V DC power source is used. From these two sources, the PLC power supply creates tightly regulated and constant DC voltages that are vital to the proper functioning of the PLC’s volatile memory, internal CPU, and the rest of PLC’s internal electronics. The power supply can regulate the DC output voltages over a wide utility regulation range. However, the regulation voltage range cannot be infinite. This means that whenever the input voltage gets out of the normal operational range of the PLC power supply, the critical DC output voltages go out of regulation. Such an occurrence causes the PLC system to malfunction thereby disrupting the processes or equipment being controlled, and in the worst-case scenario, the PLC components could get damaged leading to loss of profits or human life. This mainly occurs during ‘spiky’ input line voltages or transients.

A transient voltage is an unwanted voltage in an electrical circuit that lasts for a short duration. It can vary from a few volts to high magnitude voltage peaks comprising of fast-rising edges, mainly referred to as surges. Often, the transient voltage spikes can get up to 6000V, lasting from a few microseconds up to no more than a millisecond. Some voltage transients can happen only once, while some may be repetitive. Transient voltages result from a sudden release of stored electrical energy caused by various incidents like contact bounces, unfiltered electrical equipment, lightning strikes, arcing, generator, or capacitor bank being switched ON and OFF. In an industrial setup, the most common sources of line voltage transients are faulty contactors, relays, and motors within the process or system being controlled. These transient voltages bring about an excessive voltage spike in the input line voltage to the PLC power supply.

PLCs are highly susceptible to line transient surges, as they are used in factory floors that have motors and other types of inductive loads. A short-duration line voltage transient, that can severely damage the electrical circuits within a PLC system if proper protection is not employed.

The design of the PLC power supply, along with an additional filter protection scheme and surge suppressor, enhances the survivability of the PLC system in the occurrence of voltage spikes or surges. However, it is highly recommended that you mitigate the transient voltage before it reaches the PLC system. This strategy is considered to dramatically increase the odds of your PLC system components surviving damages related to line voltage transients. To mitigate transients on the AC power line connected to the PLC power supply, you can use a Constant Voltage Transformer (CVT) that acts as an isolation transformer.

Depending on its rating, the constant voltage transformer is capable of suppressing lightning-induced voltage transients as well as capacitor/motor switching transients of up to 3.5kV line-to-ground and 4kV line-to-neutral. Although the surge occurrence can lead to a flashover between ground and line conductors at the 3.5KV level, the output of the isolation transformer will always remain steady. A CVT is especially desirable when heavy equipment or machines on the factory floor are likely to introduce transients into the AC line going to the PLC power supply. Hence, to integrate the isolation CVT transformer with the input AC line, a separate PLC power supply would be required, which would not be possible if the PLC system is connected directly to a power source.

A clamping or protection circuit can also be used to protect the PLC power supply from voltage spikes. A protection circuit usually consists of a diode that sets the clamp voltage, as well as a Pass Field-Effect Transistor (FET) that allows protected current to flow to the power supply. Unfortunately, clamping circuits require additional components, they take a lot of space and can only be integrated with a separate PLC power supply. Non-isolated synchronous buck converters with FETs can also be integrated with the PLC power supply to protect downstream PLC components’ circuits, though the available such-like converters have a rating of up to 100V. A separate PLC power supply can allow the use of a common AC source to the PLC system and I/O modules.

A common Alternating Current (AC) Source, minimizes line voltage transients and prevents faulty input signals that stem from an unstable AC source to the I/O modules, but a stable AC source to the PLC power supply. Also, keeping the PLC power supply and the I/O modules on a common AC source, allows the user to fully utilize the monitoring feature within the power supply line. Such that, whenever the power line conditions exceed the recommended maximum operating level, the PLC power supply will detect the abnormal changes in operating conditions and signal the CPU. The CPU module will then stop reading data from input devices and turn off all outputs, a safety precaution aimed at protecting the PLC components. 

B) To Improve Electrical Noise Immunity

Electrical noise occurs whenever unwanted signals (less or more random electrical signals) get coupled into circuits with desirable signals. These unwanted signals usually disrupt information-carrying signals. In any given circuit, electrical noise can originate from power lines, ignition systems, nearby conductors, RF transmitters, or motors that are switched on or off thus drawing sudden large currents. In a PLC system, other external sources of noise include Electromagnetic Interference (EMI) caused by current flowing in nearby cables or conductors. As well as Radio Frequency Interference caused by radiating signals emanating from wireless systems. Note, electrical noise can occur on both signal and power circuits, but it becomes a major problem whenever it occurs on signal circuits.

PLCs use data-transmission systems like RS-485 and a variety of other communication protocols like Ethernet, Profinet, or Profibus. If the PLC power source is remotely located, it can be susceptible to large ground-potential differences due to multiple and non-standardized grounding techniques, which result in multiple ground current paths and loops. Ground current loops are usually very high, as they connect several ground potentials using low-impedance wiring. These extremely high current lines are a major source of electrical noise that interferes with the PLC’s data transmission.

Therefore, a separate PLC power supply enhanced with galvanic isolation is required to break the ground loops and prevent loop currents. The use of galvanic isolation with power supplies is the most reliable method in solving high ground-potential differences. A PLC power supply with galvanic isolation allows the ground-referenced input (on the input side) to be independent of the ground-referenced output (on the output side). This significantly improves the noise performance of the PLC system and enhances common-mode rejection. You can further customize the PLC power supply to counter electrical noise, by setting a section of its circuit board that is “isolated’’ from potential noisy grounds. The most popular technique used to achieve this isolation is through the implementation of a 5V input to a 5V output via an isolation barrier. A push-pull transformer can be used to create the isolation barrier.

C) To Separate Field Power and PLC Power

All input and output modules connected to a PLC system are isolated (can be through optical isolation) from the internal signals and power in the PLC. The isolation is used to assist in preventing electrical noise issues. This is made possible by using a separate DC power supply for the PLC and another one for Field Power. To provide maximum noise immunity to the PLC system, it is recommended that you use a common AC source for the PLC power supply and the Field power supply. But this does not mean that you use the same DC power supply for both the Field devices and the PLC system, as this will bypass the isolation in the PLC allowing electrical noise to reach the CPU module. The possible damage by the electrical noise can be time-consuming and very difficult to diagnose and resolve.


Most PLC-controlled processes are so critical in nature, that any PLC failure or malfunction can result in huge financial losses or even loss of human life. Such processes require PLCs with an extremely high level of reliability. Through this article, it is clear that a separate power supply is vital to the overall reliability of the PLC system. As the power supply creates tightly regulated DC voltages without excessive AC ripples that can damage the PLC components. Also, a separate PLC power supply circuit can be integrated with isolation transformers (Constant Voltage Transformer and a Push-Pull Transformer) or clamping circuits, to help prevent line voltage transients and electrical noise from interfering with the proper operation of the PLC’s volatile memory chips, its CPU module and all other in-built PLC electronics.

Imagine, a PLC powered by a 24V DC source gets damaged or malfunctions due to motor-induced line voltage transients. This could cause glitches on the PLC controlled system, and if it is a safety-critical process, the hitch can even lead to loss of human life valued at approximately $6million USD. Yet, all that can be prevented by simply using a PLC power supply integrated with a CVT transformer circuit. This scenario shows just why you need a separate power supply for your PLC system. For more information or to discuss which equipment might be best for your application, please visit our website here, or contact us at sales@pdfsupply.com or 1-919-535-3180. 

This entry was posted on October 11th, 2021 and is filed under Allen-Bradley, Automation, Education, Electrical, PLC, Uncategorized. Both comments and pings are currently closed.

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