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Registered Member #1451
Joined: Wed Apr 23 2008, 03:48AM
Location: Boulder, Co
Posts: 661
Those would work for the design that I'm working on, but this one will probably be more along the lines of a normal amateur coilgun. Data sheet says it can handle a current pulse of 300A. I assume, similar to SCRs, that it can handle a larger pulse if it has a shorter duration. Still not a stud package but the die may very well be much larger than the surface mount parts.
That is exactly what we need, good job, Turkey! The size of the die can be rougly estimated by a "parasitic" capacitance of a junction - the bigger it is, the larger the crystal. In our case, that capacitance will not have any impact on performance, as well as a reverse leakage current; voltage stabilization precision does not need to be high also. Generally saying, all we need is just a cheapest zener with highest pulsed reverse current rate. The one above is suitable. Maybe, it doesn't even need to be paralleled. Howether, looking deeper into a datasheet it is mentionable, that 300A is a rating not for 200V ones. Advertising even in an electronics, yeah... Well, taking into account much shorter pulse time, I would be dare to say it will not burn, but you can not be sure by 100%. BTW, there are bidirectional zeners available - with those damper diode is not necessary - just bidirectional zener in parallel with the coil and that is it.
Depends on the voltage - see the table. The higher the voltage, the smaller the pulse current. So basicly, before buying those diodes one should find out the magnitude of the current when coil is shutting off. Say, it is 300A. Assume you decided to use those 1500W diodes. Than divide 1500W by 300A and get 5V - that is the zener's rating you looking for if you choosed this type of diodes. Pretty small, huh? Don't worry, for single pulse 50V rated will be suitable. So buy dozen of those to get 600V spike connecting them in series. Or the same number of 600V ones in parallel (I would recommend series connection). But it is better to search for bigger zeners (more W), so to use just one. But the best solution is to return energy back into cap instead of burn it on zener.
Check this one: Even though it says 5kW, peak current rating is 219A for 274V clamped. And price is good.
Registered Member #2939
Joined: Fri Jun 25 2010, 04:25AM
Location:
Posts: 615
Yandersen wrote ...
But the best solution is to return energy back into cap instead of burn it on zener.
Absolutely. Especially for multi-stage systems where you'd like to have that unused energy available for successive stages. The drawback is complexity. Zener clamping is a reasonably simple step along the road to perfection.
If obtaining monster zeners is problematic, why not shift some of the load to a nice big transistor?
LOL! Transistor can not handle as much as zener can. Well, if you have some to spare... Anyway, if you have transistor that can handle such a current, why not to build a halfbridge?
Registered Member #2939
Joined: Fri Jun 25 2010, 04:25AM
Location:
Posts: 615
You can LOL all you like, but there is a much wider range of big transistors (FETs and IGBTs would also work) available. Impulse handling comes down to die size. As for why not a halfbridge? I'll say it once again: simplicity.
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Zener vs Fast-Act Diodes
