Transils
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Transils (also know as transil) are electronic components similar to zener diodes that are designed especially to protect electronic circuits against overvoltage peaks. They have very short switch-on times (often in picoseconds), and much slower switch-off times (so they are unsuitable as rectifiers).
Transils act as clamping devices, suppressing all overvoltages above their breakdown voltage. They are similar to trisils, which act as crowbar devices, switching on when the voltage rises over their breakover voltage.
There are two kinds of transils:
- Uni-directional transils: that are expecting overvoltage of a well known polarity, behaving as a rectifier the other direction
- Bi-directional transils: that can handle overvoltage from both directions (they are made by internally connecting two uni-directional transils in piggy-back)
Usage
Typically the transils is connected between the electrical wire to protect and the ground. If the voltage on the protected wire becomes larger than the transils threshold, the transils conducts and creates a current towards the ground that prevents the voltage increase above the threshold.
Characterization
A Transils are characterised by
- Breakdown voltage: the voltage above which the transils begins to conduct
- Response time: this time must be the smallest possible because otherwise the circuits that need protection will be destroyed before the transils begins to act as a protection device
- Parasitic capacitance: The non conducting transils behave like a capacitor. Its capacitance can affect the behavior of the electronic circuit it protects
- Amount of energy it can dissipate: if this capacity is too small, the overvoltage will possibly destroy the transils and leave the circuit unprotected