A varistor is an electronic component with an electrical resistance that varies with the applied voltage. Also known as a voltage-dependent resistor (VDR), it has a nonlinear, non-ohmic current-voltage characteristic that is similar to that of a diode. In contrast to a diode, however, it has the same character for both directions of traversing current.
Traditionally, the varistor is composed of two rectifiers, such as the copper oxide rectifier or the germanium oxide rectifier, in an antiparallel configuration. At low voltage, the varistor has a high electrical resistance, which decreases as the voltage is raised. Modern varistors are primarily based on sintered ceramic metal-oxide materials that exhibit directional behavior only on a microscopic scale. This type is commonly known as the metal-oxide varistor (MOV).
Performance and characteristics:
a. Absorb sparks caused by carbon brush and commutator rectification
b. Reduce electrical noise
c. Extend motor life
d. Press E1/E10/ to select varistor voltage
e. Good welding resistance, the small change rate of E10 after welding
Note: E10 value is the voltage value when 10mA current is applied in between the two poles of the varistor.
The role of varistor in the circuit
The voltage protection function of the varistor in the circuit generally can be combined with the fuse or other overvoltage protection. Usually used for lightning protection; when an overvoltage occurs, the varistor will be broken down, showing a short-circuit state, thereby clamping the voltage at both ends of it at a lower level, and the overcurrent caused by the short-circuit will burn the front fuse or force the air The switch tripped, thereby forcibly cutting off the power supply.
Generally speaking, it has little effect on other components after damage. Only check the circuit components connected to it. If it is breakdown damage, it will blow this fuse. The varistor plays the role of “operating overvoltage” protection in the circuit.
2. Lightning resistance requirements
The varistor is resistant to lightning strikes; when the varistor is broken down and short-circuited by a large current, the fuse is immediately burned to protect the circuit.
3. Need for safety test
Varistor safety testing is required. With the development of technology and the improvement of people’s living standards, a large number of electronic products have entered our lives, including our commonly used home appliances, so there are higher requirements for the safety performance of home appliances . Varistor as an electronic component is also listed as a safety certification requirement.
In order to protect the safety of consumers and safeguard the interests of consumers, the country has successively formulated a series of laws to regulate these conditions in the market to rectify defective products. If electronic products that have not undergone safety monitoring cannot be sold on the market, this is based on the safety of customers and must be implemented. If non-compliance is found, legal penalties or even criminal liabilities will be imposed.
The application of varistor in the micromotor is mainly to absorb the reverse electromotive force of the motor at the moment of commutation. Prevent commutator pole in the moment of commutator short circuit sparks, commutator and brush surface by high-temperature burns, affect the service life of the motor, and generate electromagnetic waves, interfere with the use of other electronic products.
Varistors have many advantages and can be used in many different types of applications for the suppression of mains borne transients from domestic appliances and lighting to industrial equipment on both AC or DC power lines. Varistors can be connected directly across mains supplies and semiconductor switches for the protection of transistors, MOSFETs, and thyristor bridges.