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Registered Member #1576
Joined: Sun Jul 06 2008, 12:48AM
Location:
Posts: 13
I'm using the 555 circuit posted by Dr. Kilovolt a while ago to drive my flyback transformer, and for some reason I can only get a 1/2 cm spark. Also, the mosfet gets hot enough to melt the solder. Anybody have any idea whats wrong?
Registered Member #540
Joined: Mon Feb 19 2007, 07:49PM
Location: MIT
Posts: 969
What MOSFET are you using? Is there a heatsink on your MOSFET? What's the input voltage? What freq is going to the flyback (if you don't know, what are the values of the components?) Is the output of the 555 enough to turn the MOSFET all the way on?
Maybe you could make a totem pole trasistor configuration. Donno if that would help... Pics can be usefull too.
Registered Member #540
Joined: Mon Feb 19 2007, 07:49PM
Location: MIT
Posts: 969
Try adding a diode across the MOSFET so that the anode is going to the neg. This will help suppress the spikes. Also a high voltage cap that is about .01uF across the MOSFET might help too.
Registered Member #834
Joined: Tue Jun 12 2007, 10:57PM
Location: Brazil
Posts: 644
When the transistor turns off, the current trough the primary coil can't be interrupted immediately, and produces a very high voltage pulse over the transistor. With a capacitor in parallel with the transistor (or the coil), it absorbs the current and limits the pulse to a voltage that doesn't force the transistor to conduct again with high power dissipation. The pulse must exist, because its voltage is multiplied by the transformer turns ratio to produce the high-voltage output. Most flyback transformers will not produce more than 1 cm of spark between smooth surfaces. Maybe 2-3 cm between points, somewhat more if the arc is stretched after it forms. If the transistor is too hot to be touched, it is too hot and there is something wrong. Verify if the 555 is really oscillating, at least at a few kHz.
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flyback driver problem
