These devices can be thought of as high-current capable zener diodes. A TVS will be rated in terms of it’s reverse breakdown voltage which in practice is the maximum voltage you want to allow before a protection device, like a fuse, kicks-in. You place a TVS shunted across a DC voltage rail with the cathode on the positive voltage rail and the anode connected to the power supply common, or the reverse of this for negative voltage rails. The protection device will normally be in-line (in series) to remove power from the circuit being protected.
ON Semiconductor makes a series called the MiniMOSORB line that has impressive current handling capabilities. For example, the SA12A part is capable of limiting the voltage to about 14V and can withstand about 25A of current. So for example, a mobile device powered by 12-13.8 volts can be protected from overvoltage or reversing the supply leads (its still a diode) even though it may be protected by a 15A fuse; i.e. the TVS will present a short and ensure that the fuse is blown. You can find TVS diodes for sale at many supplier web sites and on eBay where sellers like myself have them for sale.
In effect, if your circuits cannot tolerate a higher than normal voltage spike that is transient or permanent, caused by a voltage regulation fault for example, then a TVS is a great way to protect the circuit from an overvoltage condition.
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