In electronic circuits, it's crucial to manage inrush current that occurs when turning devices on or off. To prevent this, a thermistor is often placed in series within the power circuit.
Power-type thermistors are specifically designed to suppress surge currents during switching operations. When connected in series with an AC circuit, they act as a form of "current protection." Meanwhile, varistors are typically used in parallel with the AC line to "limit high voltage" and protect sensitive components from voltage spikes.
So, how does a varistor actually work? Let’s take an example: a varistor rated at 300 volts. Under normal household electricity conditions, the voltage is around 220 volts. However, if the voltage rises to 310 volts, which exceeds the 300-volt rating, the varistor will activate.
Once triggered, the varistor conducts a large amount of current, which then causes the fuse to blow. This protects the rest of the circuit by cutting off the power supply. The varistor reacts very quickly, and once the fault is cleared, it returns to its original state.
However, both varistors and thermistors can degrade over time due to repeated switching under normal operating conditions. As they heat up, their resistance decreases, reducing their ability to suppress surges effectively. This means their protective capabilities are not infinite and can be limited after prolonged use.
Therefore, while these components are essential for protecting circuits, regular maintenance and replacement may be necessary to ensure long-term reliability. Understanding their limitations helps in designing more robust and safe power systems.
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