Urgent: Need help in Power Electronics and PIR sensors

I am working on a project and nearing its deadline and still stuck in few problems ... can anyone help out?

I needed a transil diode for my circuit protection and I am having trouble acquiring it (surprising! isn't it) and for time been I am doing it with a Zener but I know it wont be enough when it get tougher i.e. when a high voltage transient gets in... so can anybody suggest me a solution or alternative. I actually needed SA5.0A unipolar transil diodes but can anyone suggest any other having same characteristics(like 600W 57Amps 5.8V clamp and 6.8V breakdown) or a zener perhaps would do?

can anyone tell me if I could use a VDR (voltage dependent resistor) instead of a MOV(metal oxide varistor)

one more thing.. does any one know a triac or like wise that requires low triggering current like 1 or 2 mA and PIR (pyroelectric infrared sensor) thats cheap and easily available

(1) You can get SA5.0A from the distributor Digikey

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in DO15 or axial package from Littelfuse, ON Semiconductor or Diodes Inc. The have stock now i just checked it. It takes a few days once you=B4ve made the order and the price is quite good even if you order a few samples.

(2) I have always heard VDR (voltage dependent resistors) to be another name for varistors (variable resistors).

(3) Whether you can use a standard zener or not instead of a TVS diode depends on the lines you are protecting and what you are protecting them against. If they are high-speed lines (i.e. communications) then you shouldn=B4t use a normal zener in general because of its higher capacitance as compared to that of a TVS. Also a TVS is much faster (typ 1ns). But if the only problem is the power capability then I can=B4t think of any big problem because when we say a 600W TVS it usually means 600W during 1ms or that order of magnitude.

(4) I don=B4t know of any triacs with gate currents lower than 10-20mA at all operation temperatures but there may exist. This current decreases with temperature so be careful to ensure proper operation.

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