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Old 05-06-2010, 05:39 AM
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ozmatic ozmatic is offline
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Join Date: Jan 2009
Posts: 15
Hi Harvey,
Thank you for your detailed reply to my
I have to thank you for very gently pointing out to me that my coil is part of a capacitor discharge system - this is now glaringly obvious to me, and I have spent some time in the last two days trying to talk to the right engineer at Evinrude/Johnson to get the data on this component -with no result.
The motor it came from had a separate cdi module, so I automatically assumed that the cap was in the module, and that the coil was just a coil. Another of my litany of mistakes.There must be some cap inside it. I turned the diode around, and the on time of the spark gap increased substantially [more power ,as you predicted], but the ringing was very much less, and its the ringing thatI am interested in. The capacitative spike disappeared except for the shoot thru .The Tau is very evident in the decay of the ringing.
To get back on topic - the circuit you drew for me, was my effort to to find out what happens to the body diode in a mosfet heater circuit.
I use a circuit very similar to the one discussed here.I have a lot [20years] of practical experience with liquid lead acid batteries, and I reckon that the batttery is a fundamental part of the cicruit. You will not get the effect with a power supply. A liquid lead acid battery has low impedance and is capable of resonance - look at the ringing in the battery voltage in Glen`s fantastic scope shots. A liquid lead acid battery is not amenable to a strict mathematical analysis - a few crystals of PbSO4 fall off a plate during discharge, and suddenly you have increased voltage. To the knee in the discharge curve , you can use voltage as a guage of state of charge, - but I have yet to find it a straight line.I regularly discharge batteries from 12.8v to10.8v and the specific gravity of each cell will have dropped from 1.26 to 1.24 during the discharge, and if left unrecharged for 2 hours, will drop to 1.22 while the voltage will have risen to 11.8 at least. A lot of people cannot understand that a battery @11.8v is dead flat!
Also, pulse charging is definitely more efficient than brute force DC charging, where a lot of energy is wasted as heat.

I think that if you can match the resonant frequency[ or a sub division of it] of an inductive resistor, to the resonant frequency of a battery, - there can be effects worth looking at.
I do this with large batteries [100Ahr] @ low frequency. I keep a radio 2 feet away from the resistor, and when it starts squealing, I know I am getting near - and then the mosfet fries.- I have fried over 50 mosfets, and I blame the body diode. And yes, I know i am operating the mosfet way beyond its ratings, but I reckon the diode cannot handle the energy being bounced off it.
Glen, I would like to ask you if, once you hit your "sweet spot",do you have difficulty in maintaning it, as the resistor rises in temperature? It does not seem so from your description of start-up. You are operating at a much higher frequency than me , which may give greater stability.
All I want is a switch that can handle vast power, reversals of polarity, at any frequency - is that too much to ask!? Sooner or later, my large IGBT will be called to the frontline, but not before I think it has a fighting chance - it has a body diode too.
Another effect, before the mosfet fries, is that my mobile phone will switch off from over 20 feet away, and this worries me . I want shielding, but from what? -RF ? Microwave? X-Ray!? - of course ,if infra red is a large part , then we are in business!
Chris
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