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Registered Member #158
Joined: Sun Feb 12 2006, 09:53PM
Location: Central Ohio
Posts: 282
One of the last components I need for my new 'lytic powered cg projects are clamping diodes. I have found a few beefy ones for reasonable prices. My question is will a standard recovery work or does it need to be an ultra fast recovery type. The specs on the ones i have found cheap that are standard recovery are: DO5 stud mount 800V 80A avg 133A rms I(fsm) 1800A - I assume this is max single pulse I^2T 14000A - not sure what this is
Will these work? My caps are 250V and I may or may not series a pair or three so it is a good idea I get a high enough V rating that I can series some if I want.
Registered Member #158
Joined: Sun Feb 12 2006, 09:53PM
Location: Central Ohio
Posts: 282
Well I have never used diodes yet in my EM launchers so its kinda new to me. I was thinking that if the diode couldnt turn off fast enough then maybe when the pulse reversed it would kill the diode. Not sure how much V reversal there is after the first reversal that should be clamped by the diode. Is there a turn-on characterisitc? I mean if the diode cant go from off to on to clamp the pulse before it builds up to say 100V then your caps might be in danger? Not sure if this is somethign that can happen or not.
Registered Member #85
Joined: Thu Feb 09 2006, 01:22PM
Location:
Posts: 21
Diodes have negligible turn on times that are governed by the inductance of their terminals etc. So even slow diodes turn on extremely fast, and there is no chance of the voltage rising to any dangerous values. (and many people don't even care about those 100V in reverse as long its only for a small amount of time)
The turn off (reverse recovery time) of slow diodes is slow, but still in the microseconds. So you'd only get problems here if you would re-charge your cap bank within a few microseconds after the discharge. (the coil can't do this because it is clamped by the diode)
Registered Member #158
Joined: Sun Feb 12 2006, 09:53PM
Location: Central Ohio
Posts: 282
Ok sounds good. So a really high power standard recovery rectifier like the stud mount I described above would singly handle any coil so long as I dont approach the 800V limit right? And will 1800A be able to handle the inductive kick for any reasonable cg coil powered by up to a few kJ at 800V or less that I might build? Its hard enough to calculate the actual surge current let alone what the coil will surge once the magnetic field collapses... I dont know if this info is out there maybe I just havent found it yet.
I asked about the recovery because I beleive all the diodes mentioned from digi-key by someone in another post were all ultra-fast type I think.
Registered Member #85
Joined: Thu Feb 09 2006, 01:22PM
Location:
Posts: 21
The current the diode sees is slightly less than the peak current (dependant on the damping factor of you RLC curcuit). However this current can last much longer than the current that the thyristor sees(also dependant on the damping effect of the coil's resistance).
In my picture, i marked the place, where the diode takes over the current. Edit: Here 's the scope pic of my diode clamped coilgun:
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good diode for CG clamping?
