The conventional surge protection technique applicable to electric products can be understood by referring to the Taiwan patent publication No. 518808 “Power Socket Module with a Surge Protection Circuit”. It is characterized in that a surge protection circuit is installed in a power cord socket and electrically connected between two alternating current (AC) electrodes of the power cord socket in parallel. The surge protection circuit is used for eliminating the electric pulse between the two AC electrodes. The surge protection circuit is substantially described in the patent description and claims 4 and 5. A varistor and a fuse are connected between the two AC electrodes in serial. Wherein, when the voltage of the electric pulse exceeds a predetermined value, the varistor will conduct to eliminate the electric pulse. If the passing current of the varistor exceeds a predetermined value, the current fuse will fuse, or a light emitting diode will be off to indicate the damage of the surge protection circuit.
Another conventional surge protection device is described in the Taiwan patent publication No. M297037 “Lightening Strike Protector with a Protection Device”. It is characterized in the lightening strike protection element and the point discharging module mentioned in the patent description and claim 1. When the pulse voltage is higher than the preset voltage of the point discharging module, the lightening protection element is activated.
Therefore, the principle of the above-mentioned two conventional surge protection techniques is substantially based on the existence of a core element: the varistor (or the lightening strike protection element). The varistor is capable of changing its resistance according to the voltage bridging over two ends of the element. According to such a principle, the resistance of the varistor is very high in a normal voltage. However, when the voltage increases to a certain value, the resistance will drop down rapidly in an extremely short time. In a general case, the response time is about several nano-second (ns). Therefore, when the varistor is applied to the surge protection, it will be connected to the front end of the electric product in parallel. Consequently, when the normal operation voltage is supplied to the electric product, the varistor will be in an open-circuit state. However, when the voltage rises abnormally, the varirstor will be in a short-circuit state to abuse the energy in front of the electric product, so that the electric pulse cannot influence the electric product.