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What Turns Off The Transistor In A SSG Circuit

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  • What Turns Off The Transistor In A SSG Circuit

    Following on from a discussion with John Koorn in another thread, here is what defines the duration of the input pulse in SSG circuits and the mechanism that turns the transistor off :

    The current induced by the rotor magnets in the trigger windings only initiates the input pulse. What turns the transistor fully on and off is independent of the rotor magnets. Transistor have a current limiting property. For example if we have a transistor with a gain of 10 and we allowed 10ma to flow through the base, then a maximum of 100ma of current will be allowed to flow through the Collector/Emitter junction, regardless of the voltage across the C/E junction or series load.

    Here is the series of events on the trigger circuit :

    Step One : The rotor magnet passes TDC and begins conducting a tiny amount of current in the trigger winding that flows through the B/E junction.

    Step Two : This allows a relatively small amount of current to begin flowing through the C/E junction and the primary coil.

    Step Three : As current begins to increase through the primary coil, it generates a growing magnetic field.

    Step Four : Through transformer action the expanding magnetic field induces a much larger current (EDIT : meaning larger than the original current induced by the magnets) through the trigger winding and through the B/E junction. From here the current induced by the rotor magnet has no influence and the trigger current is almost entirely from the transformer effect.

    Step Five : The larger current flowing into the base now puts the transistor into saturation. As an example, let's say the transistor has a gain of 40 and 50ma is being allowed to flow into the base (the amount of current flowing into the base is dependent on the base resistance). This means that as long as the current flowing through the C/E junction is less than 2amps (40 x 50ma) the transistor will be in saturation and have very low impedance. From here the main impedance is the induction of the primary coil and so current will continue to increase through the coil until the current reaches about 2 amps.

    Step 6 : At this point the transistor starts entering its Active Region and so very quickly starts restricting current through the C/E junction and so the rate the magnetic field is expanding slows down and very soon stops expanding altogether. Since the current flowing into the base is due to the transformer effect, and that is dependent on the rate of change of the magnetic field, once the magnetic field's rate of expansion slows down, the current in the base drops off, which plumates the active region lower and ultimately, and very sharply, turns off the transistor altogether. Pretty much an avalanche effect.

    Step 7 : Once the magnetic field starts collapsing, the current in the trigger winding reverses and flows through the 1N4001 diode which reverse biases the B/E junction by about 1.5v and so clamps it shut.

    Step 8 : If the magnet hasn't reached a great enough distance from the stator by the time the magnetic field has finished collapsing, then the whole process starts again and gives us multiple pulses per magnet pass until the rotor magnet is either out of range, or the stator is directly between two rotor magnets.

    That's pretty much how it works. Worth noting that the trigger current during step 4 and 5 is suprisingly level and doesn't change significantly until it reaches the active region.
    Last edited by sephiroth; 11-23-2012, 05:50 AM.

  • #2
    Hi sephiroth,

    Nice explanation on the transistor.

    Do not procrastinate! Make something happen...even if it is wrong. Once begun half done!