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Dec 27 2017

Principle and Applicaions of OverCurrent Protection Circuit

Warm hints: The word in this article is about 2800 and reading time is about 15 minutes.


Many electronic devices have a rated current. Once the device exceed the rated current, it will burn out the device. Therefore, these devices do a current protection module, when the current exceeds the set current, the device automatically power off to protect the device, which is over-current protection. Such as the USB interface on the computer motherboard, USB overcurrent protection generally have to protect the motherboard is not burned. This article will show you what is overcurrent protection;OverCurrent Protection types; its Principle and Applicaions.



I.What is OverCurrent Protection

IV.Application Examples of OverCurrent Protection Circuit

II.OverCurrent Protection Principle - Introduction

V.Over-Current Protection Circuit Switching Power Supply Design

III.OverCurrent Protection types



I.What is OverCurrent Protection

OverCurrent Protection (Over Current Protection) is the current protection device action when the current exceeds a predetermined maximum. When the current flowing through the protected original exceeds a preset value, the protective device is activated and the timing is used to ensure the selectivity of the action, tripping the circuit breaker or giving an alarm signal.

overcurrent protection

Many electronic devices have a rated current. Once the device exceed the rated current, it will burn out the device. Therefore, these devices do a current protection module, when the current exceeds the set current, the device automatically power off to protect the device, which is over-current protection. Such as the USB interface on the computer motherboard, USB overcurrent protection generally have to protect the motherboard is not burned.

Below we use a chart to show how the overcurrent protector works, as shown in following picture. 

how the overcurrent protector works

Overcurrent protection include short circuit protection and overload protection. Short-circuit protection is characterized by larger setting current and instantaneous action. Electromagnetic current release (or relay), fuses are often used as short-circuit protection components. Overload protection is characterized by smaller setting current, inverse time action. Thermal relay, delay-type electromagnetic current relays commonly used as overload protection components.

Fuses are also commonly used as overload protection components without much impact current.

In the TN system, When use fuses for short circuit protection, the melt rated current should be less than 1/4-phase short-circuit current. with the circuit breaker protection, the circuit breaker instantaneous action or short delay action overcurrent release’s setting current should be less than 2/3 of the single-phase short-circuit current

learn the knowledge about overcurrent protection more intuitively: 


II.Overcurrent protection principle - introduction

The overcurrent protection circuit described here is suitable for DC overcurrent protection, such as various battery-powered applications. If the load current exceeds the preset value, the electronic insurance will disconnect the DC load. To reset the circuit, simply turn the power off and then on again. The circuit has two connection points (A, B mark), can be connected to either side of the load.

Electronic Insurance (Overcurrent Protection) Load current flows through transistor T4, resistors R10 and R11. A, B voltage and load current is proportional to the majority of the voltage distribution in the resistance. When the power is just turned on, all the power supply voltage on the insurance. Transistor T2 by the R4 current conduction, the collector current value is determined by the following formula: VD4 = VR7 +0.6. Because the voltage at D4 (VD4) and the voltage at R7 (VR7) are constant, the collector current at T2 is also constant. The transistor provides a stable base current to T3, thus turning it on, which in turn provides a stable base current to T4. Conductive, load current flows. When the power is turned on, the capacitor C1 provides a delay, so as to prevent T1 from turning on and keeping T2 off.

DC overcurrent protection

The voltage on the fuse (VAB) is usually less than 2V depending on the load current. When the load current increases, the voltage rises and reaches the shunt portion T2 base current when the diode D4 is on and the collector current of T2 is therefore limited. As a result, the voltage on the fuse further increases until approximately 4.5V, the Zener diode D1 breaks down, turns on T1 and T2 turns off, which causes T3 and T4 to also close, at which point the voltage on the fuse increases, and Generate positive feedback to keep these transistors off. The purpose of C1 is to give a short delay so that the fuse can control short-term overloads such as the switching current of an incandescent lamp or the starting current of a DC motor. Therefore, changing the value of C1 can change the length of the delay. The circuit voltage range is 10 ~ 36V DC, the delay time of about 0.1 seconds. For component values given in the circuit, the load current is limited to 1A. By changing the component value, the load current can reach 10mA ~ 40A. Select the appropriate rating of the components, the circuit voltage can reach 6 ~ 500V. By using a rectifier bridge (such as the following power circuit), the insurance can also be used for AC circuits. Capacitor C2 provides instantaneous voltage fuse protection. Diode D2 Avoid discharging the load through C1 when the voltage on the fuse is low. The above is some simple analysis of the principle of over-current protection circuit.

A protective device is provided at the origin of the circuit concerned (see the following picture)

Circuit protection by circuit breakerCircuit protection by fuses

Acting to cut-off the current in a time shorter than that given by the I2t characteristic of the circuit cabling

But allowing the maximum load current IB to flow indefinitely

The characteristics of insulated conductors when carrying short-circuit currents can, for periods up to 5 seconds following short-circuit initiation, be determined approximately by the formula:

I2t = k2 S2

which shows that the allowable heat generated is proportional to the squared cross-sectional-area of the condutor.


t = Duration of short-circuit current (seconds)

S = Cross sectional area of insulated conductor (mm2)

I = Short-circuit current (A r.m.s.)

k = Insulated conductor constant (values of k are given in Figure 5)

For a given insulated conductor, the maximum permissible current varies according to the environment. For instance, for a high ambient temperature (θa1 > θa2), Iz1 is less than Iz2 (see Fig. 5). θ means “temperature”.

I2t characteristic of an insulated conductor at two different ambient temperatures


ISC = 3-phase short-circuit current

ISCB = rated 3-ph. short-circuit breaking current of the circuit-breaker

Ir (or Irth)[1] = regulated “nominal” current level; e.g. a 50 A nominal circuit-breaker can be regulated to have a protective range, i.e. a conventional overcurrent tripping level similar to that of a 30 A circuit-breaker.

III.Overcurrent protection types

  • 1.1 Complex type: a variety of protection in line.

  • 1.2 Limited power type: limited output of the total power

  • 1.3 Rewound type: the initial current is constant, the voltage drops to a certain value current began to decrease.

  • 1.4 Playing type: over-current, the current voltage dropped to 0, and then began to rise again and again.

  • 1.5 Constant current: constant current, voltage drop

  • 1.6 Comparison of several over-current protection methods

Several overcurrent protection methods are listed in Table 1.

Circuit mode

Components used

Debugging difficulty

Protection performance

Power consumption

Impact on efficiency

Resistor primary current limit circuit






Base drive current limit circuit






No power current limit circuit






555 timer current limit circuit






IV.Application examples of overcurrent protection circuit

Overcurrent protection is when the fault current in the circuit reaches the action value of the current relay, the current of the current relay is set according to the requirements of the line selectivity. PTC thermistors for overcurrent protection reduce the residual current by limiting the consumption of the entire line by a sudden change in their resistance. They Can replace the traditional fuse, widely used in motors, transformers, switching power supplies, electronic circuits, over-current thermal protection, the traditional fuse can not be restored after the line is blown, and over-current protection with PTC thermistor after the fault is removed Can be restored to the pre-protection state, when the fault occurs again can achieve over-current protection function.

The primary voltage of the voltage transformer is 220V, the secondary voltage is 16V, the secondary current is 1.5A, the primary current of the secondary abnormality is about 350mA, the protection state should be entered in 10 minutes, the working environment temperature of the transformer is -10-40 ℃, 15 ~ 20 ℃, PTC thermistor installed near the transformer, please select a PTC thermistor for primary protection.

  • 1.1 Determine the maximum operating voltage

Given transformer voltage is 220V, considering the power fluctuations, the maximum operating voltage should reach 220V × (1 + 20%) = 264V

PTC thermistor maximum operating voltage selection 265V.

  • 1.2 Determine no action current

After calculation and actual measurement, the primary current of the transformer is 125mA when it works normally. Considering that the ambient temperature of the PTC thermistor is up to 60 ℃, it can be determined that the non-operating current should be 130 ~ 140mA at 60 ℃.

  • 1.3 Determine the operating current

Taking into account the PTC thermistor mounting position of the ambient temperature can reach -10 ℃ or 25 ℃, the operating current can be determined at -10 ℃ or 25 ℃ should be 340 ~ 350mA, the operating time of about 5 minutes.

  • 1.4 Determine the nominal zero power resistor R25

PTC thermistor in series in the primary, the resulting voltage drop should be as small as possible, PTC thermistor itself, the heating power should be as small as possible, the general PTC thermistor voltage drop should be less than 1% of the total power, R25 Calculated:

220V × 1% ÷ 0.125A = 17.6 Ω

  • 1.5 Determine the maximum current

The actual measurement, the transformer secondary short-circuit, the primary current up to 500mA, taking into account the primary coil short-circuit when there is a greater part of the current through, PTC thermistor to determine the maximum current above 1A.

  • 1.6 Determine Curie temperature and dimensions

Taking into account the PTC thermistor ambient temperature  of the mounting location can go up to 60 ℃, selected Curie temperature should be on the basis of 100 ℃. But taking into account the low cost and the PTC thermistor is not installed in the transformer line package, the higher surface temperature will not produce adverse effects to transformer. So the temperature can be selected Curie temperature 120 ℃, so that the PTC thermistor can reduce the diameter, and the cost can be reduced.

  • 1.7 Determine the PTC thermistor model

According to the above requirements, consult specification sheet, selected standard as below: 

Namely: the maximum working voltage is 265V, the rated zero power resistance is 15Ω ± 25%, the operating current is 140 mA, the operating current is 350 mA, the maximum current is 1.2A, the Curie temperature is 120 ℃ and the maximum size is 11.0mm.


V.Over-current protection circuit switching power supply design

A switching power supply commonly used over-current protection circuit.

  • 1.1 current sensor for current detection

Overcurrent detection sensor works as shown in Figure 1. Through the converter secondary current obtained by the converter I / V converts to voltage. After the voltage becomes DC form, it is compared with the set value by the voltage comparator . If the DC voltage is greater than the set value, the identification signal is issued . However, this detection sensor is generally used to monitor the load current induction power supply. So we should take the following measures. Since the start-up current is several times as the rated current at the start-up of the inductive power supply and is much larger than the current at the end of start-up. in the case of simply monitoring the current battery, the necessary output signal should be obtained at the start of the inductive power supply. We must use a timer to set prohibit time, so that induction power supply does not get an unnecessary signal output before the end of start-up. After the end of the timer, the power supply will turn into the scheduled monitoring state.

  • 1.2 start inrush current limit circuit

Switching power supply will generate high inrush current when power is on. Therefore, soft start device to prevent inrush current must be installed at the input end of the power supply to effectively reduce the inrush current to the allowable range. Inrush current is mainly caused by the filter capacitor charging, the capacitor on the exchange showed a lower impedance at the beginning of the switch is turned on. In the absence of any protective measures, the inrush current can approach hundreds of A.

Rectifier filter circuit

Switching power supply input generally use capacitor filtering circuit shown in Figure 6, the filter capacitor C can use low-frequency or high-frequency capacitors, low-frequency capacitor need to be parallel with the capacity of high-frequency capacitors to bear the charge and discharge current. In the figure, the current limiting resistor Rsc, which is inserted between the rectification and the filtering, is to prevent the impact of the inrush current. Closing Rsc limits the charging current of capacitor C. And after a period of time, the voltage on C reaches the preset value or the voltage on capacitor C1 reaches the operating voltage of relay T, and Rsc is shorted. At the same time, SCR can also be used to circuit Rsc. When closing, due to the SCR cut-off, the capacitor C is charged through Rsc. After a period of time, the SCR is turned on, short-circuiting the current-limiting resistor Rsc.

  • 1.3 using base drive circuit current limiting circuit

Under normal circumstances, we can use base drive circuit power supply control circuit and the switching transistor isolated. Control circuit and the output circuit in common, the current limiting circuit can be connected directly with the output circuit, the working principle is shown in Figure 7, when the output overload or short circuit occurs, V1 is conducted, R3 voltage increases. Compare the voltage with the reference voltage of inverting the comparator to control PWM signal on and off.

Current limiting circuit using base drive circuit

  • 1.4 through the detection of IGBT Vce

When the power output has an overload or short circuit, IGBT Vce value becomes larger. According to this principle, we can take protective measures on the circuit. This usually uses a dedicated drive EXB841, whose internal circuit can be done well down the gate and soft-shutdown, and has an internal delay function. You can eliminate the interference caused by malfunction. Its working principle is shown in Figure 8, Vce with IGBT overcurrent information is not sent directly to the collector voltage monitoring pin 6 of the EXB841, but is rapidly recovered by the diode VD1. Then connected to the pin 6 of the EXB841 through the comparator IC1 output Elimination of the forward voltage drop varies with the current situation, the use of threshold comparator to improve the accuracy of current detection. In the event of an overcurrent, the driver: The EXB841's low-speed shutdown circuit will turn off the IGBT slowly to prevent collector current spikes from damaging the IGBT devices.

Using IGBT overcurrent principle circuit



In recent years, the switching power supply is widely used, and its reliability also has higher requirements. Once the electronic product fails, if the input end of the electronic product is short-circuited or the output end is open, the power supply must turn off its output voltage to protect the power MOSFET and the output end device from being damaged. Otherwise, the electronic product may be further damaged or even cause Electric shock and fire of operators. Therefore, the switching power supply over-current protection must be improved.

Book Recommendation

  • Electric Power System Protection and Coordination: A Design Handbook for Overcurrent Protection

A guide to the implementation of electric power protection in both new and existing systems in individual and commercial facilities. Focusing on systems in the low to medium volt range, the book helps in the solution of protection and co-ordination problems by use of microcomputers as well as more traditional methods. The text provides step-by-step procedures for quick problem solving. It shows how to engineer intelligent switchgear and includes critical information on setting up a protection and co-ordination workstation. The text should help meet the requirements of the National Electrical Code and the National Standards Institute.

--Michael A. Anthony (Author)

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