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Dec 19 2017

In-depth study of circuit protection

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With the development of science and technology, electric power / electronic products are becoming more and more diverse and complex. The circuit protection components used are not simple glass tube fuses. They usually have varistors, TVS and gas discharge tubes. Circuit protection has been developed into a wide range of emerging electronic components. Today let's go deep into the study of circuit protection.

Article Core

Circuit protection basics


Electronic circuit components

English name

Circuit Protection


Circuit Protection


Emerging electronic components

Protective device

TVS, gas discharge tube, etc.



I.What is circuit protection

2. Over-voltage

3.2 Functions for Circuit Protection Devices

1.1 Circuit protection concept

3. OT (Over-temperature)

3.3 How to choose Circuit protection device 

1.2 Circuit protection significance and importance

4. TFR

IV.Example of the method

1.3 How much amperage can a wire safely conduct

5. OCOV (over-current and over-voltage)

4.1 TVS tube in the protection circuit 

II.Common types

III.Circuit Protection Components and Device  

4.2 PTC Thermistor in Overcurrent Protection

1. Over current

3.1.Types of Circuit Protection Devices


I.What is circuit protection

1.1 Circuit protection concept

Circuit protection is mainly to protect components in electronic circuits from over-voltage, over-current, surge, electromagnetic interference and other circumstances without damage. With the development of science and technology, power and electronic products increasingly diverse and complex. Circuit protection components used are more than a simple glass tube fuse in the past, the usual protection devices are varistors, TVS, gas discharge tube. Circuit protection has evolved into a diverse field of emerging electronic components.


Circuit protection

1.2 Circuit protection significance and importance

  • (1)Since the integration of circuit boards is getting higher and higher, the price of the board is also increasing accordingly. Therefore, we must strengthen the protection.

  • (2) Semiconductor devices’ and IC’s operating voltage is much lower, while the purpose of circuit protection is to decrease energy loss, reduce the heat phenomenon and extend the service life.

  • (3) In-vehicle equipment, due to the conditions of the use of the environment worse than the average electronic products. The car driving conditions change frequently, and the car has a very large instantaneous peak voltage when starting and so on. Therefore, the power adapter for these electronic equipment products should generally use over-voltage protection components.

  • (4) Communication equipment and communication places have certain requirements for lightning surge prevention. Over-voltage protection and over-current protection components are important in these devices. They are the key to ensure the normal personal safety and communication.

  • (5) The failure of most electronic products is caused by overvoltage or circuit phenomenon in the electronic equipment circuit. As our requirements on the quality of electronic equipment are getting higher and higher, the electronic circuit protection becomes more and more necessary .

1.3 How much amperage can a wire safely conduct

The American Boat and Yacht Council (ABYC) publishes the following table showing how much amperage each size wire can carry:

how much amperage each size wire can carry

“Allowable Amperage” is not the only thing to be considered in sizing a wire. Voltage Drop must also be considered. Voltage Drop is the amount of voltage “consumed” as the voltage “pushes” the amperage through the resistance of the wire. Sometimes Allowable Amperage will be the determining factor in sizing a wire and in other cases Voltage Drop will dictate the wire size. The wire must be the larger of either the size required by the Allowable Amperage or the Voltage Drop.


II.Common types

1. Over current

In an electric power system, overcurrent or excess current is a situation where a larger than intended electric current exists through a conductor, leading to excessive generation of heat, and the risk of fire or damage to equipment. Possible causes for overcurrent include short circuits, excessive load, incorrect design, or a ground fault. Fuses, circuit breakers, temperature sensors and current limiters are commonly used protection mechanisms to control the risks of overcurrent.

Overcurrent protection is automatically cut off when the current is too large to prevent the electronic components of circuit from damage caused by exceeding the rated current.

See the video talking about Current limiting and short circuit protection

2. Over-voltage

When the voltage in a circuit or part of it is raised above its upper design limit, this is known as overvoltage. The conditions may be hazardous. Depending on its duration, the overvoltage event can be transient—a voltage spike—or permanent, leading to a power surge.

Electronic and electrical devices are designed to operate at a certain maximum supply voltage, and considerable damage can be caused by voltage that is higher than that for which the devices are rated.

Overvoltage protection is mainly to prevent electronic components damage from over-voltage or electrostatic discharge. It is widely used in telephones, fax machines and high-speed transmission interface (USB, IEEE1394, HDMI, SATA) and other electronic systems Products, especially electronic communication devices. This is particularly important to avoid damage to electronic equipment due to Over-Voltage or EOS (Electrical Over-Stress) or Electrostatic Discharge (ESD).

For example, an electric light bulb has a wire in it that at the given rated voltage will carry a current just large enough for the wire to get very hot (giving off light and heat), but not hot enough for it to melt. The amount of current in a circuit depends on the voltage supplied: if the voltage is too high, then the wire may melt and the light bulb would have "burned out real time". Similarly other electrical devices may stop working, or may even burst into flames if an overvoltage is delivered to the circuit.

3. OT (Over-temperature)

From the commercial to the present, temperature protection components has gone through a long time, the current over-temperature protection components are widely applied to the the occasions when specific temperature required. These protection components , in accordance with the principle of action, can be divided into chemical-activated and low temperature Alloy actuated type.  The main feature is that chemical-actuated products type can do low-temperature products (currently do 48 ℃), but more complicated and costly.  Another is low temperature Alloy actuated type, low-temperature alloy type action is mainly conducted through a diameter of more Large low-temperature fuse. We must ensure that the heat generated by the rated current will not melt the fuse. The low-temperature fuse is usually adjust the melting point by adjusting the tin (Sn), copper (Cu), silver (Ag), bismuth (Bi), indium (In) and other ingredients.

4. TFR

In recent years, with the upgrade of the application, simple temperature protection can no longer meet the needs of the ever-changing electrical appliances, motors and 3C products security protection, so components which can provide monitoring function and Timely protection due to temperature, current and voltage anomalies are re-developed. And the rise of lithium-ion battery and lithium polymer battery is the largest application.

5. OCOV (over-current and over-voltage)

With the complexity of modern electronic products, the requirements for the protection components are also increasing, such as comprehensive protection and limited reserved space. As these requirements are put forward, the protection component community has set off a wave of package encapsulation. Like mentioned above - over-temperature protection is also considered a combination package. But most of the ocov protection packaging products are still under research and development. There is no mature commercial products available.

III.Circuit Protection Components and Device  

Circuit protection devices automatically prevent dangerous or excessive temperatures, excessive currents or short circuits in electrical conductors. It limits the amount of energy released in the event of an electrical failure.

The video about Circuit Protection: Fuses, Breakers, Overloads

3.1.Types of Circuit Protection Devices

Classified as follows:

First, the components: Products which do not need energy devices can be called components if the factory does not change the molecular composition when processing. It includes: resistors, capacitors, inductors. (Also known as passive components )

  • (1) circuit devices: diodes, resistors and so on

  • (2) connected devices: connectors, sockets, connecting cables, printed circuit board (PCB).

Second, the device: the factory in the production and processing has changed the molecular structure of the device called the device. The company is located in:

Devices are divided into:

The active device’s main features are: 

  • (1)its own power consumption 

  • (2)need external power supply. 

  • Discrete device is divided into:

  • (1) bipolar transistor

  • (2) Field Effect Transistor

  • (3) SCR

  • (4) semiconductor resistance capacitance

3.2 Functions for Circuit Protection Devices

Listed below are some of descriptions of the components’ role. Electronic component is as following:



Resistance in the circuit is stated with "R". Resistors in the circuit's main functions are shunt, current limiting, partial pressure, bias, etc. Capacitance in the circuit generally is expressed with "C" . Capacitor is composed of two pieces of metal film with insulating material in the middle. The characteristics of the capacitor is mainly isolate DC and through AC.



The size of the capacitor capacity is the size of the electrical energy that can be stored. The blocking effect of the capacitor on the AC signal is called capacitive reactance, which is related to the frequency and capacitance of the AC signal.


Although the use of inductors in electronic production is not common, but they are equally important in the circuit. In our opinion, an inductor is also an energy storage element that converts electrical energy into magnetic energy, and stores energy in a magnetic field. The inductor symbol is “L”, its basic unit is Henry (H), commonly used millihenry (mH) as a unit. It often works with capacitors to form LC filters, LC oscillators and more. In addition, people also use the characteristics of the inductor to create a choke coil, transformers, relays and so on.

Now let's talk something about electronic devices:

Crystal diode

crystal diode

Crystal diode in the circuit commonly used "D" .The main characteristics of the diode is unidirectional conductivity. That is, under the effect of the forward voltage, the on-resistance is small; but the resistance becomes great or infinite uder the reverse voltage.

Because of these characteristics of diodes, they are often used in rectification, isolation, regulation, polarity protection, coding control, FM modulation and squelch circuits of cordless phones. The crystal diodes used in telephones can be classified into rectifier diodes (such as 1N4004), isolation diodes (such as 1N4148), Schottky diodes (such as BAT85), LEDs, and Zener diodes.

3.3 How to choose Circuit protection device 

  • Overvoltage protection device selection points

Overvoltage protection devices (OVP) are used to protect subsequent circuits from load dump or transient high voltage damage. Common overvoltage protection devices include varistors, transient suppressors, ESD suppressors and discharge tubes. Overvoltage protection device selection should note the following four points: 

  • 1)Turn off voltage Vrwm. General shutdown voltage is at least 10% higher than the maximum operating voltage of the line. 

  • 2)Selection of clamp voltage VC. VC refers to the voltage across the TVS during the ESD surge condition. It must be less than the maximum transient voltage that can be sustained by the protected circuit. 

  • 3)Selection of surge power Pppm. Different power, different protection time, such as 600w (10 / 1000us); 300W (8 / 20us) 

  • 4)the choice of the electrode capacitance. The higher the working frequency of the protected components, the smaller the capacitance of the TVS is required

  • Overcurrent protection device selection

In the process of breaking the circuit of the fuse, due to the existence of the circuit voltage, an arc will occur at the moment of the melt breaking. And one high-quality fuse should avoid such arcing as much as possible. After breaking the circuit, the fuse should be able to withstand Circuit voltage applied at both ends. Fuse damage by pulse will gradually reduce the ability to withstand the pulse. We should consider the necessary safety margin selection. The safety margin refers to the fuse of the total fuse (action) time, which includes pre-arcing time and arcing time both. We need to pay attention to its fusing characteristics and rated current of the basic conditions. In addition to consider the installation environment. The fuse will meltwhen it reaches the value of the melting heat. It will change the fusing time when colder, which is must be considered.

  • Fuse selection parameters required

  • 1)Rated current --- In

  • 2)Rated voltage --- Un

  • 3)Ambient temperature

  • 4)Voltage drop / cold resistance --- Ud / R

  • 5)Fusing characteristics

  • 6)Breaking capacity --- Ir

  • 7)Thermal energy value-I2t

  • 8)Durability / life expectancy

  • 9)Structural features and installation form

  • 10)Safety certification

  • Common temperature fuse model


Rated current(A)

Blown current(A)


Rated current(A)

Blown current(A)





























































  • Over-temperature protection device selection

NTC thermistor, also known as the negative temperature coefficient thermistor, which is characterized by a non-linear decrease in resistance as the temperature increases. NTC thermistor selection should consider the following points:

NTC thermistor

See the video talking about what is NTC thermistor

1)The maximum rated voltage and the filter capacitor value

The size of the filter capacitor determines the size of the NTC. For a certain size NTC thermistor, the size of the filter capacitor that is allowed to access is strictly required. This value is also related to the maximum rated voltage.

2)The maximum starting current value and the long-term load on the NTC thermistor working current that the product allows.

The maximum allowable starting current for electronics determines the resistance of the NTC thermistor. Current loaded on the NTC thermistor should not exceed the current specified when operating normally for Long-term in the specification.


IV.Example of the method

4.1 TVS tube in the protection circuit 

In the actual circuit, as shown in the figure, at the time of DC input, some transient pulses occur sometimes due to changes in the power supply environment. The best way to eliminate the instantaneous impulse damage to the device is to bring the instantaneous current to the ground from the sensitive device. In general, the TVS is connected in parallel with the protected circuit on the circuit board. Thus, when the instantaneous voltage exceeds the normal operating voltage of the circuit, the TVS will avalanche breakdown, providing an ultra-low impedance path to the instantaneous current. As a result, the instantaneous current is shorted to GND through the TVS to avoid the protected device. When the instantaneous pulse ends, the TVS diode automatically resumes to the high impedance state, and the entire circuit returns to the normal voltage state.

Typical circuit topology

The smart meter industry generally use RS-485 communication. The following figure shows the typical application of RS-485 communication interface. In the protection of the RS-485 interface form static or surge damage, SMBJ5.0CA usually used to do the interface protection, as shown in Figure 3. When the electrostatic or surge voltage is higher than 5V, SMBJ5.0CA will start to clamp the voltage At low levels. the final voltage is clamped to around 9V if the current exceeds the peak current IPP, thus protecting the latter stage from damage.

RS-485 interface protection circuit

Integrated operational amplifier to the external electrical stress is very sensitive. So, during the operation of the op amp, excessive voltage or current, especially surge and electrostatic pulses, often occur due to incorrect operation or abnormal working conditions, so as to easily damage the op amp or Failure. A 5V output of the DC power supply, with its regulator output with SMBJ5.0CA, you can protect the equipment using the power supply, but also can absorb the circuit in the transistor collector to emitter peak voltage to protect the transistor . Therefore, it is recommended to add an SMBJ5.0CA tube at the output of each regulated source, which can greatly improve the reliability of the whole machine.

Op amp output stage protection

DC power supply output stage protection

Surge protection devices may be categorized as voltage-limiting devices such as gas discharge arrestors, metal oxide varistors, suppressor diodes, triacs, diacs, and switches or current-limiting devices like fuses, circuit breakers, and thermal cutouts. Let’s take a look at voltage protection, as this is more pertinent to flare protection.

Wikipedia offers a good overview of the basic transient voltage suppressor devices:


Surge Capability (typical)

Lifetime - Number of Surges

Response Time

Shunt Capacitance

Leakage Current (approx.)


1A (small surface-mount device) to 15kA (large through-hole device)


≈ 1ps (limited by pin lengths)

< 1pF (small surface-mount device) to > 10nF (large through-hole device)


Metal-oxide varistor (MOV)

Up to 70kA

@ 100A, 8x20µs pulse shape: 1,000 surges

≈ 1ns

Typically 100–1,000pF +++


Avalanche diode, Zener diode


@ 50A, 8x20µs pulse shape: infinite

< 1μs



Gas discharge tube

> 20kA

@ 20kA, 8x20µs pulse width: > 20 surges

< 5μs

< 1pF

< 1nA

4.2 PTC Thermistor in Overcurrent Protection

Application principle

When the circuit is in normal state, the current through the PTC thermistor for overcurrent protection is less than the rated current. The PTC thermistor is in the normal state with small resistance. When the circuit fails, the current greatly exceeds the rated current, the PTC thermistor suddenly heat, shows a high impedance state, making the circuit relatively "off" to protect the circuit from being damaged. When troubleshooting, the PTC thermistor will automatically return to the low impedance state, the circuit back to normal operation.

There are generally three types of over-temperature and over-temperature protection:

1)the current overload (see the following picture): RL1 load curve is normal work state. When the load resistance decreases, such as transformer has a short circuit, the load curve turns from RL1 into RL2, beyond the B point, the PTC thermistor goes into the protection state.

Current overload protection diagram

2)the voltage overload (see the following picture): the supply voltage increases, such as 220V power line suddenly rose to 380V, the load curve turns from RL1 into RL2, which exceeds point B, the PTC thermistor enters into the protection state.

Voltage overload protection diagram

3)the temperature overheating (Figure 5): When the ambient temperature rises beyond a certain limit, PTC thermistor volt-ampere curve becomes from ABE into A-B1-F, load curve RL goes over point B1, PTC thermistor Enters the protection state.

 Over-current protection circuit


Book Recommendation

  • Protection of Electronic Circuits from Overvoltages (Dover Books on Electrical Engineering) 

Temporary failure and permanent damage of electronic systems are often caused by electrical overstresses such as lightning, electromagnetic pulses from nuclear weapons, and switching of reactive loads. Protecting industrial, military, and consumer systems from failure is critical; and until the publication of this volume, the related literature was scattered throughout journals, patents, conference proceedings, military reports, and elsewhere. This convenient text presents practical rules and strategies for circuits designed to protect electronic systems from damage by transient overvoltages. Because many circuits operate from AC supply mains, protection of equipment operating from the mains is also discussed. The five-part treatment covers symptoms and threats, fundamental remedies, types of protective devices, applications of protective devices, and validation of protective measures. Specific topics include damage and upset, environmental threats, standard test waveforms, and properties of nonlinear transient protection devices, plus protective applications related to signal circuits, DC power supplies, and low-voltage AC mains.

--Ronald B. Standler  (Author)

  • The Fundamentals of Circuit Breaker & Protection Maintenance (Switchgear Maintenance) (Volume 1) 

Low voltage (LV) and High Voltage (HV) electrical circuits have varying types of protection relays, circuit breakers and fuses for both safety and damage limitation purposes. All of which require maintenance to ensure continued safe and reliable service. Original Equipment Manufacturers (OEM) and numerous technical authorities have written textbooks, manuals and papers regarding switchgear. However, much of the information required for electrical fitters, engineers and maintenance technicians has to be extracted from different sources and gained through experience. The aim of this guidance document is to provide technicians, students and engineers with an overall appreciation of typical maintenance practices for both switchgear and protection.

--John Dunning  (Author)

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1 comment

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