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Capacitor discharge pulse circuit topology, series capacitor?

Discussion in 'General Electronics Discussion' started by Nanren888, Feb 10, 2018.

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  1. Nanren888


    Nov 8, 2015
    Just interested whether anyone knows a compelling reason for the series topology for capacitor discharge pulse circuits as illustrated below.
    I've just grabbed two circuits from a google search that look vaguely representative. I offer no endorsement of the circuits.
    The two main topologies I see are
    (1) as in the lower one, a capacitor is charged & then connected to the pulse output, eg transformer.
    (2) as in the upper one, the capacitor is charged, presumably slowly, through the output transformer, then discharged.effectively reversing the current in the coil.
    The only reasons I can think of involve net direction of current & such, which seem rather minor.
    The coil/transformer inductance presumably limits the initial charge current, but again this would seem minor for a pule transformer.
    Anyone know any compelling reason for the series version? CapacitorDischargeTopology.PNG
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    I would say there is little difference between them.

    The main distinction is that one charges the capacitor through the transformer and the other doesn't.

    If you're going to have a current limited charging circuit that charges at a rate which is lower than the holding current of the SCR, then the series charging circuit is simpler.

    The added series inductance could theoretically limit the initial charge rate, but given that the transformer will be typically designed to allow a rapid pulse of current when the capacitor discharges, and because this current may be many times higher than the charge current, this is unlikely to be a major factor.

    Where the supply used to charge the capacitor can provide sufficient current to hold an SCR on, then clearly the simple series approach can't be used.

    Another important factor, especially if charge current is high, is whether the induced voltage in the secondary caused by the charge current is a problem.

    In the case of something supplying a rectifier or requires an AC voltage with some particular peak value but little else, the answer is probably no. But where the discharges need to be timed, and where the initial charge current may be similar in magnitude to the discharge current, then the series charging may both limit the charge rate somewhat but also result in additional untimed pulses in the output as the charge cycle commences. The classic case of this would be a car ignition where you would not want additional sparks generated.

    I guess my understanding of the reason is simplicity, both in the circuit and in the requirements for the load.
  3. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    I should add that it's also entirely possible to have the switching element between the capacitor and the inductor, charging the capacitor directly before discharging it through the transformer.

    Again, this requires that the charge current is less than the holding current for the switching device, but the triggering could be slightly more complex.
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