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Registered Member #75
Joined: Thu Feb 09 2006, 09:30AM
Location: Montana, USA
Posts: 711
Thanks Steve, the above shot was in fact a ferrite transformer, but I have also hooked up a Rogowski coil now. The ferrite transformer was most likely saturating, so I have replaced it by a powered iron core, which give me a rather more sinusoidal pulse.
Still the readings I get from the current transformer and the Rogowski coil (integrated with an RC network) are very different, so something must be wrong. And I even get a reading when I short out the work coil with the two measurement coils on it, so I suppose there is a lot of manetic field leaking around and inducing spurious voltages everywhere.
Registered Member #75
Joined: Thu Feb 09 2006, 09:30AM
Location: Montana, USA
Posts: 711
I am giving up on this project; not that I think the concept is without merit, quite the contrary actually. However my plumbing skills suck and soldering on bus-bars might not be the most clever prototyping solution.
Still I like closure and there is one thing that is bugging me: I needed to generate a 1ms delay so my scope had time to trigger before the actual discharge started. I did this by charging up an RC combo of 1k and 1uF, the charging voltage would also trigger my oscilloscope and once the capacitor was up to voltage it triggered a TC4422 gate driver which was used to drive the gate of the SCR. Another 10k resistor was connected across the cap to discharge it, so the TC4422 would not be high for more than a few ms. This resulted in the gate driver chips catching fire. I know it is a rather dirty trick to use a gate driver as a Schmitt trigger, but what exactly kills them in this application?
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Hi joe
I am giving up on this project
Hey.. no.. you can't really do that!
I wonder if it would at all be possible to use IGBTs, so all the stages of the gun could be powered by a single capacitor. This would save a lot of energy that would otherwise be wasted in a ringing discharge heating the coil. I don't really know how to determine if an IGBT is up to this other than by the "turn it up till it breaks and then back a bit" method. I think it boils down to the action, i.e. the integral of squared current. There was a thread about this on 4hv, but google wont turn it up.
I don't know why I never saw it, but why don't you use a diagonal halfbridge of SCR's? They are much cheaper than IGBT', although slower, but I don't think it would make big difference here since coil's resistance is much more massive dissipator.
With forcibly commutated SCR halfbridge you (may) need 2 more controls, but high side drive is easier.
Still I like closure and there is one thing that is bugging me: I needed to generate a 1ms delay so my scope had time to trigger before the actual discharge started. I did this by charging up an RC combo of 1k and 1uF, the charging voltage would also trigger my oscilloscope and once the capacitor was up to voltage it triggered a TC4422 gate driver which was used to drive the gate of the SCR. Another 10k resistor was connected across the cap to discharge it, so the TC4422 would not be high for more than a few ms. This resulted in the gate driver chips catching fire. I know it is a rather dirty trick to use a gate driver as a Schmitt trigger, but what exactly kills them in this application?
TC4422 will blow up because SCR gate acts as practical short circuit to anything above it's forward voltage, and if driver's output stays high for too long it will overheat.
You need to use a monostable and dial a pulse length (pulses of dozen tens of us are usually enough for triggering) and use an output resistor to limit peak current to safe value.
Lot of current isn't required neither, small 1A drivers should work fine.
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Design considerations for an induction coilgun
