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Registered Member #75
Joined: Thu Feb 09 2006, 09:30AM
Location: Montana, USA
Posts: 711
I have been wondering whether it would make sense to replace the freewheeling diode in a coilgun with a MOV (varistor). This would have a number of advantages: -Virtually indestructible, since there is no single PN-junction that could overheat -Clamps at rated voltage, so it would dissipate huge amounts of power and quickly quench the current. - Cheap, can probably be paralleled.
I wonder if there is anything wrong with this idea? The voltage rating of the MOV would have to be greater than the capacitor voltage, so it does not start conducting during the forward cycle. Then, the freewheeling current would build up enough voltage to break down the MOV and all energy would be dumped there. Obviously the main switch (SCR) voltage would have to be above the MOV breakdown so it does not get reverse overvolted.
Registered Member #75
Joined: Thu Feb 09 2006, 09:30AM
Location: Montana, USA
Posts: 711
Yes, you are right, so I corrected the original post. I think TVS is a more general term for any kind of transient suppressor, wheres MOV is more specific to those little blue disks I was actually talking about.
I have actually tried it on my coilgun now and it seems to work rather well. I am running a nearly critically damped condition though, so maybe things look different if there is more energy to be dissipated.
Registered Member #191
Joined: Fri Feb 17 2006, 02:01AM
Location: Esbjerg Denmark
Posts: 720
If I understand your post correctly, you replaced the freewheeling diode with an MOV. So when you fire, when capacitor voltage drops to zero, what's stopping it from charging the capacitor backward. Current is still flowing so SCR would still be on, and if the MOV can stand up to the forward voltage, it will just sit there and watch the capacitor being charged from behind.
Registered Member #75
Joined: Thu Feb 09 2006, 09:30AM
Location: Montana, USA
Posts: 711
Jimmy is right, the idea it totally flawed. I am embarrassed to bring the thread to the top again, but I really don't want anyone to try this. Luckily I just caused the rectifier diodes charging the cap bank to fail short, but at more than a couple of 100J something might have failed explosively.
I have to disagree with Dago though, apparently MOVs are extremely fast with a switching time on the order of a nanosecond. Also they should last indefinitely if the maximum energy is not exceeded. (from )
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MOV instead of freewheeling diode?
