one, Overcurrent protection element for use in electronic equipment are:
1, the general fuse (current fuse),
2, resettable fuse (PPTC),
3, the fuse resistor (fuse resistance)
4, car fuse,
5, over-current protection (circuit breaker)

two, the thermal protection device used in electronic devices are:
1, thermal fuse (temperature fuse),
2, the temperature switch (bimetal thermal protection),
3, liquid expansion type thermostat (liquid Peng expansion type)
4, mechanical capillary temperature control switch (steam pressure thermostat)
They are generally connected in series in the circuit, In the circuit over-current or overheating and other abnormal phenomena, Will immediately cut off the circuit and play the role of protection, To prevent further expansion of the fault.

Third, the voltage protection element used in electronic devices are:
1, varistor,
2, over-voltage protection diodes, etc.,
They are generally paralleled in the circuit.

Various electronic protection element working principle:
1, the general fuse (current fuse):
      Fuse from the relatively large resistivity and low melting point of silver and copper alloy wire made of. Fuse are also known as fuses, IEC127 standard it is defined as "fuse (fuse-link)". It is a kind of installation in the circuit, To ensure the safe operation of electrical components circuit.    
When the circuit failure or abnormal, With the current rising, And the rise of the current may damage the circuit in some important devices or expensive devices, Also may burn the circuit or even cause a fire. Silver-copper alloy fuses will be abnormal in the current rise to a certain height and heat of the time, Its own fuse cut off current, Thus play the role of protecting the safe operation of the circuit. Current fuse can only work once, Fuse after the fuse can not work, So the circuit will stop working, The need to manually replace the fuse.

2, resettable fuse(PPTC):
resettable fuse (PTC) is an over-current electronic protection components. The use of polymer organic polymer in the high-pressure, high temperature, curing reaction conditions, Doped conductive particle material, after a special process from processing, resettable fuse with over-current, overheating protection, automatic recovery of dual protection. In the habit of PPTC (Polymer Positive Temperature Coefficient) also called resettable fuse.
At normal current and temperature, the polymeric resin tightly binds the conductive particles outside the crystalline structure, Constituting a chain-like conductive path. The self-recovery fuse at this time is a low resistance state (a). The heat generated by the current flowing through the resettable fuse (PPTC) on the line is small, Will not change the crystal structure, the product is in normal working condition.
When the line short-circuit or overload, The heat generated by the high current flowing through the resettable fuse (PPTC) melts the polymeric resin, The volume is rapidly increased to form a high-resistance state (b), The working current decreases rapidly, so as to limit and protect the circuit. When troubleshooting, the resettable fuse (PPTC) re-cooling crystallization, Volume contraction, conductive particles to re-form conductive pathways, The resettable fuse (PPTC) returns to a low-resistance state, Product back to normal working condition, No need to manually replace the fuse, resettable fuse (PPTC) are reusable over-current protection devices.

3, the fuse resistor (fuse resistance)
      As the name suggests, fuse resistor (fuse resistance),
Because the resistor and the fuse are similar in material and construction, So both the functions of both the fuse resistance, Usually when the resistor can be used, Once the current is abnormal to play the role of its fuse to protect the machinery and equipment. Because there are two with the protection function, the cost is relatively low.  Fuse resistance can be divided into: metal film fuse resistors, fuse-type wirewound resistors, fuse-type cement resistors. Power: 1 / 4W, 1 / 2W, 1W, 2W several, with the increase in power, Product Dimensions will continue to become larger.
Fuse resistance can only be a normal protection, fuse products must be replaced.

4, car fuse
In the structure, the general car fuse using the chip design, Fuse with engineering plastic shell, Wrapped with zinc or copper melt structure, Metal melt and pin connection. Automotive inserts fuse specifications are generally 2A to 200A, The ampere value is indicated at the top of the fuse. Car fuse can only be a normal protection, After the product is blown, the product must be replaced.

5, over-current protection
Is used for current overload protection device. When it enters the set state, The protection relay will automatically release, The protector exits the protection state, Effectively preventing the misoperation of the electric motor in the setting state. Protection of three-phase / single-phase motors and other electrical appliances:
● Overload protection
● Underload protection
● Stall protection
● Blocking protection
● Temperature protection
● Phase sequence protection
● Undervoltage protection
● Overvoltage protection
● Start time-out protection
● Phase failure protection
● Unbalanced protection
With automatic reset and manual reset function selection.

6, thermal fuse (temperature fuse):
       The main material of a thermal fuse is a fusible alloy which is meltable at a specified temperature. The resin mixture starts to liquefy at a specific temperature when the temperature rises, When the ambient temperature of the thermal fuse rises to its operating temperature, Its fusible alloy melt and under the action of surface tension and special resin help, Shrunk into a ball attached to the two pin ends. Thus, the circuit is permanently cut off.
    The temperature fuse can only be protected once, the product must be replaced after the fuse.

7, the temperature switch (thermostat):
       According to the temperature changes in the work environment, The physical deformation occurs inside the switch, Resulting in some special effects, A series of automatic control elements which generate an ON or OFF operation, Also known as temperature control switch, temperature protection, temperature controller, referred to as temperature controller. The temperature controller issues a switch command, Thereby controlling the operation of the device to achieve the desired temperature and energy saving effect. The basic composition of the structure: steam pressure thermostat, liquid expansion thermostat, gas adsorption thermostat, metal expansion thermostat.
    Temperature switch (thermostat) is a reusable overheating protection element, Sub-automatic reset and manual reset, The service life can reach 100000 times.

8, varistors and overvoltage protection diodes:
          Varistor is a limiting type of protection devices. Using the non-linear characteristic of varistor, When the over-voltage appears in the varistor between the poles, The varistor can clamp the voltage to a relatively fixed voltage value, In order to achieve the protection of the circuit after the level. The main parameters of varistor: varistor voltage, flow capacity, junction capacitance, response time.
          When the voltage applied to the varistor is lower than its threshold, The current flowing through the varistor is extremely small, It is equivalent to a resistance of infinite resistance. That is, when the voltage applied to the varistor is lower than its threshold value, It corresponds to a switch in the off state. When the voltage applied to the varistor exceeds its threshold, The current surge through the varistor, which is equivalent to an infinitely small resistance. That is, when the voltage applied to the varistor is higher than its threshold value, It is equivalent to a closed state switch
      The voltage limit of the varistor must not exceed the impulse withstand voltage (Urp) of the object to be protected, Excessive voltage will breakdown the varistor.