Pressure sensitive resistance is a kind of voltage limiting protection device. By using the non-linear characteristic of the pressure sensitive resistance, when the over-voltage occurs in the voltage between the two electrodes, the voltage clamp can be clamped to a relatively fixed voltage value, so as to realize the protection of the circuit. The main parameters of the pressure sensitive resistance are: voltage, current capacity, junction capacitance, response time and so on.
The pressure sensitive resistance has its own sensitive voltage, when the voltage at both ends is equal to or beyond the sensitive voltage, the resistance will be from infinite, rapid decrease, similar to a short circuit, burn off the circuit before the fuse. Achieve the purpose of protecting the secondary circuit. For current and voltage high pulse, because it is short, so the high pulse is he a bypass, when, after the high voltage in the past, his resistance returned to infinity, so as to protect the circuit, not to be bad for the high voltage or high pulse.
Working principle of pressure sensitive resistance:
Varistor at work, each unit of ZnO varistors are assumed power surges, and the varistor element is roughly evenly distributed in the body of the resistance, is the resistor body are responsible for energy, and unlike zener diode voltage regulator tube that only district assumed power, this is ceramic varistor has of Zener diode voltage regulator tube had a lot of through flow and energy quota. The resistance value varies with the terminal voltage. Is the main features of the varistor voltage range (6 - 3000 volts, several files), voltage pulse fast response to several tens of nanoseconds), resistant to strong impulse current (up to 100 amperes 20 thousand amperes), low leakage current (less than a few to a few tens of microamperes), small temperature coefficient of resistance, of high quality and low price, small size is an ideal protection components. The circuit can be composed of an overvoltage protection circuit, a noise elimination circuit, a discharge circuit and an absorption circuit. Circuit symbols for the pressure - sensitive resistors, outline and internal structure see figure 1.
Figure 2 out of the pressure sensitive resistance in the circuit of the working waveform. The (a) said that in power supply network overlay over voltage pulse, is connected with a varistor, peak voltage waveform be flattened, limited within a certain range, (b) said, in the open or closed with a perceptual, capacitive load circuits, DC waveforms appear off sharp pulse, pressure-sensitive resistor in the circuit can absorb the counter electromotive force, so as to effectively protect circuit switch is not compromised. Was most or the automobile computer board power protection circuit used in automobile electric appliance, Toyota Car PC board with the varistor black discs, and most of varistors in American cars and Daewoo cars board computer is red, if the generator voltage regulator failure, build growing time voltage is too high or line of the battery and the battery pile loose, disengage and start the engine with high power supply startup, often leads to the damage of the computer board of the varistor.