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Registered Member #2099
Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716
Reverse voltage from an underdamped LCR discharge can kill most any charger that has just a diode between high voltage source and capacitor bank. Just look at the schematic, and figure what happens when charger output is connected to high voltage of the opposite polarity! Remember the boost converter MOSFET probably has an internal diode that wants to conduct when drain is pulled below ground.
One solution is a mechanical switch to disconnect charger before the impulse discharge.
Another is a series resistor between charger and main capacitor, to limit the current sucked from charger. With a boost converter like Uzzors's you would need a smaller capacitor "before" the resistor, to receive the current pulses from boost inductor. Are you going to be slowed down much if series R times main bank C is a second or two?
Registered Member #14
Joined: Thu Feb 02 2006, 01:04PM
Location: Prato/italy
Posts: 383
You need to clamp the coil with a R-D (resistor-diode) networt and a diode in antiparallel on the capacitors. This because the coil discharges the bank completely and then pulls the voltage negative (overdischarge or reverse charge) this causes the boost converter diode to conduct from ground to the coil (huge currents) and the mosfet as well. This happened to mee too, but since i was using an isolated transformer only the diodes blow (this happened because the R-D network has a failed resistor)
Registered Member #1525
Joined: Mon Jun 09 2008, 12:16AM
Location: America
Posts: 294
Since the SCR acts as a diode when triggered, how does a reverse charge manage to build up over the capacitor when current can only run from the cap to the coil?
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
The discharge isn't a simple singular event, with electrons going only one way following the polarity signs on the escalator. What you have is a train of damped waves, whose amplitude falls off according to the time constant.
So it could easily be possible that your second 360 degrees could easily put 67% of your initial PD inverted across your polarized capacitors.
(PS. It's very late at night, and my theory looks a bit rough there, but the substance of it still stands)
Registered Member #2099
Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716
Saz43 wrote ...
Since the SCR acts as a diode when triggered, how does a reverse charge manage to build up over the capacitor when current can only run from the cap to the coil?
Draw the voltage and current waveforms. Assuming your LCR circuit loop is underdamped, the capacitor is fully discharged at approximately the moment of maximum current. At that point there's no magic reversal of voltage or current at the SCR. The current continues to run from cap to coil, and the capacitor voltage continues its decline. The increasingly negative voltage on the capacitor now -aids- the circuit resistance to progressively reduce the forward current.
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reversed voltage killing charger?
