brusque@hotpop.com 19:05 2004-11-30:
>>Polyfuse will not be quick enough to protect the zeners
>
>    interesting. I'm just buying some polyfuses from Digikey to test its "speed". Hope to put my hands on them next week.

Polyfuses fuse by them heating up.  Considet he thermal mass ofa polyfise, compared with the zener chip...!  Then add that the zener have much higher voltage drop than the Polyfusem and you see which one take most power!

>>Tip: use glass encapsulated zeners: at failure they will more certainly
>>short cirquit, while plastic encapsulated ones blow open at high overload.
>
>    Thanks for the info, I didn't knew that plastic zeners blow open. 

At low overload the silicon chip P/N melt to short cirquit.  So do any zener i have tried.  But at high overload the plastic types crack mechanically apart by heat and maybe boiling silicon(?), while glass types hold together much better.  I played with different zeners and a large electrolyte cap to evaluate.  (wear eye protection!)

And I also agree with Dwayne to move it after the bridge so you need one zener only.

Even better is to let a small zener fire a SCR - the SCR wil have only one volt across it, so it wil take much higher current, enough to blow the fuse - even a polyfuse.  But to reset it you have to shut of the power of course.

/Morgan


--
Morgan Olsson, Kivik, Sweden

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