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princeton
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Join Date: Sep 2010
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Quote:
 Originally Posted by MPaulHolmes Forget the capacitors for a second. Where does the future 1200 amps go that will be flowing through the mosfets, once they shut off? It's going to go through your resistor and diode. It WILL be the same current for a short period of time, because current can't change instantaneously. So, if you are suddenly forcing the current to go through a resistor and the freewheel diode, you are still stuck with Ohms law. V = I*R. I = 1200amp (let's say). Was the resistor like 20 Ohms or something? I forgot. Let's say 20 Ohms. V = 1200*20 = 2400v. You should see 2400v drop from Drain to Battery + for an instant. So, the voltage that the mosfet will see from Drain to Source would be 2400v + Pack voltage. The mosfets are rated for 300v. Am I missing something? Let's see if we can come up with an analogy for this... Oprah, a very very large land mass, is moving at 100 mph. Suddenly, she is forced to go into a bog instead, since the path that her wheelchair is on is closed off. Then she starts singing the theme song to the Phantom of the Oprah... I hope that clears it up.
What does this have to do with the capacitors? My assertion is that the capacitors are useful for adding longevity to the batteries but are almost useless at preventing inductive feedback.

In regards to the above analogy, I agree that a field is generated around the inductor (motor) which contains energy. This energy must be dissipated somehow, or face consequences of mosfet failure. We seem to disagree about the role of the capacitors in the protection of the mosfets. I do agree that the flyback diodes do offer some protection for the mosfets. Unfortunately, the flyback diodes drain the current from the inductor too quickly and this high current produces a magnetic field around the inductor of opposite polarity. Without a resistance to dissipate this energy, this produces an oscillation of voltages. We have created an inductor-capacitor oscillator. (The mosfet drain capacitance is the capacitor).