Enhancing Circuit Protection with Varistors
Overview: This article reviews varistors for safeguarding electronic circuits against voltage spikes. It explores the construction, working principles, key parameters, and their crucial role in protecting electrical systems from transient overvoltages.
A surge is a transient occurrence that happens due to a high current or voltage. It damages various electronic components by abruptly and destructively raising the voltage.
Electronic components known as surge protection devices are used in communication and power supply systems to protect electric and electronic devices against voltage spikes.
Varistors are one type of device that discharges the surge current by functioning as an insulating circuit that changes into a short circuit.
What is a varistor?
A varistor, as shown in Fig. 1, is a non-linear, voltage-dependent semiconductor diode that quickly suppresses voltage to protect electrical circuits against spikes in voltage.
The varistor is also known as a voltage-dependent resistor (VDR) because its resistance changes in response to applied voltage. It shields the circuit's component parts from damage by absorbing potentially hazardous energy and dissipating it as heat.
Fig. 1: Varistor component used in electronic circuits. Source: Rakesh Kumar, Ph.D.
Construction of Varistor
A varistor is also referred to as a metal oxide varistor (MOV). As the name implies, it comprises a mass of zinc oxide and minor amounts of other metal oxides, like bismuth, cobalt, and manganese, sandwiched between two metal plates that act as electrodes, as shown in Fig. 2.
The substance has a crystalline structure made up of conductor-like metal oxide particles that are arranged at random angles. Resistive ceramic intergranular barriers separate these conductive particles.
The interface between each metal oxide grain and its boundaries forms a diode junction, enabling unidirectional current flow. This structure illustrates the characteristics of a P-N junction semiconductor.
Fig. 2: Illustration of the workings of a varistor. Source: Rakesh Kumar, Ph.D.
Working of the Varistor
When a moderate voltage is applied, the varistor usually acts as an open circuit with high resistance, preventing the current flow. A very slight current flow is detected because of the junction leakage.
When the voltage rises above the varistor's breakdown voltage, the diode junction breakdown causes a short circuit and lowers resistance, which permits current to flow. Conducting current via the varistor protects the other linked circuit components by clamping the voltage to safe levels.
Consequently, there is a non-linear relationship between the current-voltage characteristic of having low resistance at high voltage and high resistance at low voltage.
Applications
Varistors are bidirectional components used in AC and DC power electronic circuits to protect against transient overvoltage. It can reduce voltage spikes when used in conjunction with other voltage suppression components like diodes and zener diodes.
They are utilized in many different component assemblies, including motor controllers, high-power current converters, switch-mode power supply, bridge rectifiers, etc.
In industries, varistors are frequently used to control transient voltage spikes that result from:
- Source fluctuations in circuits
- Lightening
- Inductive switching of loads
- Electrostatic discharges
- Unexpected Overvoltage
They are affordable, very effective devices that protect high-capacitance circuits from transient voltage. They are effective in the removal of noise from electronic circuits.
Factors Affecting the Efficiency of the Varistor
When selecting a varistor for a specific application, it is important to examine several variables, such as rated voltage, clamping voltage, surge current, leakage current, and response time. These parameters are unique to various types of varistors and also depend upon the applications.
Rated Voltage
The VDR's rated voltage is the highest continuous AC or DC voltage it can tolerate when functioning as a high-impedance open circuit. It can also mean the highest voltage at which the equipment can function without breaking down. This value changes depending on the design and application and is unique for AC and DC signals.
Clamping Voltage
It is the highest voltage that can be used across surge-protecting equipment or varistor terminals. The varistor further clamps the input voltage and stops current flow to shield the linked component from excessive voltage exposure and avert damage.
Surge Current
The defined surge current of a VDR is the highest current it can withstand for a period of time before being damaged. The varistor acts as a short circuit during a transient voltage event, allowing the maximum current to pass through briefly until the transient voltage drops.
In cases of longer transient voltage durations, the other components are protected from damage, but the chances of varistor failure increase.
Leakage Current
Under normal conditions, a varistor experiences a minor current flow due to the reverse leakage of the diode junction, which is referred to as the leakage current. It should be as minimal as possible for effective varistors.
Response Time
It is defined as the amount of time it takes for the varistor to clamp the input voltage during a transient voltage event, at which point it closes the linked components to surge voltage. Response time should be as low as possible for the varistor to be effective.
A Varistor to Consider
Varistors are available in several packages, the most popular being a radial shape resembling a ceramic capacitor. These devices are available in a broad range of operating voltages, ranging from 10V to 1000V. One such varistor is described below:
VC120605D150DP, as shown in Fig. 3, is a TransGuard multilayer varistor. They are ceramic semiconductor devices that use zinc oxide and have voltage-current characteristics that are not linear.
They possess the additional benefit of enhanced capacity to handle higher electric current and energy levels. The temperature range for operation is -55°C to +125°C, and the working voltage is from 3.3 to 85 Vdc.
Fig. 3: VC120605D150DP Varistor, Transguard Series. Source: oemsecrets
It has a maximum operating peak current range of 20 to 2000 A. They are extremely dependable bi-directional EMI/RFI filtering components and have a very quick response time. They are utilized in many different contexts, such as smart grid applications, industrial controllers, microcontrollers, relays, keyboard protection, etc.
MOVs are a popular choice for energy dissipation devices in a variety of applications. Because MOVs are passive devices, repeatedly absorbing too much energy can nevertheless be "lifetime-threatening," as it can lead to nonlinear degeneration, overheating, and mechanical breakdown in the MOV structure. Therefore, it is crucial to understand the functioning of varistor specifications thoroughly.
Summarizing the Key Points
- Surge protection devices like varistors safeguard electronic circuits from voltage spikes, ensuring system reliability and longevity
- Industries frequently use varistors to reduce transient voltage spikes brought on by various factors like source fluctuations, lightning, inductive load switching, and overvoltage
- Proper selection and application of varistors can significantly enhance circuit protection, ensuring optimal performance and longevity of electronic systems
Reference
Chunmeng Xu et al., “Evaluation Tests of Metal Oxide Varistors for DC Circuit Breakers,” IEEE Open Access Journal of Power and Energy 9 (January 1, 2022): 254–64, https://doi.org/10.1109/oajpe.2022.3179453.
Ahran Min, Sin-Dong Kang, and Jae-Ho Kim, “Investigation on the Physical Properties of Varistors According to Temperature,” IEEE Access 10 (January 1, 2022): 115758–66, https://doi.org/10.1109/access.2022.3218047.
Erika Stracqualursi et al., “Analysis of Metal Oxide Varistor Arresters for Protection of Multiconductor Transmission Lines Using Unconditionally-Stable Crank–Nicolson FDTD,” Energies 13, no. 8 (April 24, 2020): 2112, https://doi.org/10.3390/en13082112.
Oemsecrets, “VC120605D150DP - Compare Distributor Prices & Inventory | oemsecrets.com,” oemsecrets.com, n.d., https://www.oemsecrets.com/compare/VC120605D150DP.