Mechanism of Operation
The bridgewire is usually made of gold, but platinum or gold/platinum alloys can also be used. The most common commercial bridgewire diameter and length is 1.5 and 40 mils (0.038 mm and 1 mm), but lengths ranging from 10 mils to 100 mils (0.25 mm to 2.5 mm) can be encountered. From the available explosives, only PETN at low densities can be initiated by sufficiently low shock to make its use practical in commercial systems as a part of the EBW initiator. It can be chained with another explosive booster, often a pellet of tetryl, RDX or some PBX (e.g. PBX 9407). Detonators without such booster are called "initial pressing detonators" (IP detonators).
During initiation, the wire heats with the passing current until melting point is reached. The heating rate is high enough that the liquid metal has no time to flow away, and heats further until it vaporizes. During this phase the electrical resistance of the bridgewire assembly rises. Then an electric arc forms in the metal vapor, leading to drop of electrical resistance and sharp growth of the current, quick further heating of the ionized metal vapor, and formation of a shock wave. To achieve the melting and subsequent vaporizing of the wire in time sufficiently short to create a shock wave, current rise rate of at least 100 amperes per microsecond is required.
If a current is supplied in lower rate, the bridge may burn, maybe with deflagrating the PETN pellet, but will not cause detonation. PETN-containing EBWs are also relatively insensitive to a static electricity discharge. Their use is limited by the thermal stability range of PETN. (Cf. slapper detonators, which can use high density hexanitrostilbene, allowing their use in temperatures up to almost 300 °C and at both vacuum and at high pressures.)
Read more about this topic: Exploding-bridgewire Detonator
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