In electronic circuits, it's crucial to manage the inrush current that occurs when turning power on or off. To prevent this, a thermistor is often connected in series within the power circuit. This component helps limit the initial surge of current, protecting sensitive components from damage.
Power-type thermistors are especially effective at suppressing surge currents during switching. When connected in series with an AC power line, they act as a form of "current protection." Meanwhile, varistors are typically placed in parallel with the AC circuit to control high voltage spikes, serving as a "voltage limiting" device.
So, how does a varistor actually work? Let's take an example: a varistor rated for 300 volts. Under normal household conditions, the electricity supply is around 220 volts. However, if a voltage spike causes the voltage to rise to 310 volts, which exceeds the varistor’s rating, it will trigger a breakdown.
Once this happens, the varistor conducts a large amount of current, which can cause the fuse in the circuit to blow. This protects the rest of the circuit from damage. The varistor reacts very quickly, and once the fault is cleared, it returns to its normal state, ready for the next event.
However, both varistors and thermistors have limitations. After prolonged use under normal operating conditions, repeated switching on and off can lead to overheating. As their temperature rises, their resistance decreases, reducing their ability to suppress surges effectively. This means that their protective performance becomes less reliable over time, making regular maintenance and replacement important for long-term safety.
In summary, while these components play a vital role in protecting electrical systems, they are not infallible and require careful consideration in design and application. Always ensure proper selection and installation to maintain optimal performance and safety.
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