Transil
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A Transil is an electronic component similar to a Zener diode that is designed especially to protect electronic circuits against overvoltage peaks. It has a very short switch-on time (often in picoseconds), and much slower switch-off time (so it is unsuitable as a rectifier). Transils are avalanche diodes manufactured in the same way as Zener diodes, only are more thoroughly tested to make sure they are capable of withstanding high impulse loads.
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 Transil:
- Uni-directional Transils: that expect an overvoltage of a well known polarity, behaving as a rectifier in the other direction
- Bi-directional Transils: that can handle overvoltage from both directions (they are made by internally connecting two uni-directional Transils back-to-back.)
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[edit] Usage
Typically the Transil is connected between the electrical wire to protect and the ground. If the voltage on the protected wire becomes larger than the Transil's threshold, the Transil conducts current to the ground that prevents the voltage increase above the threshold.
[edit] Characterization
Transils are characterised by
- Breakdown voltage: the voltage above which the Transil begins to conduct
- Response time: this time must be the smallest possible because otherwise the circuits that need protection will be destroyed before the Transil begins to act as a protection device
- Parasitic capacitance: The nonconducting Transil behaves 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 Transil and leave the circuit unprotected
