Back EMF controlled charger?

Discussion in 'The Projects Forum' started by psahd, May 18, 2014.

  1. psahd

    Thread Starter New Member

    May 2, 2014
    Hi All, I have a very unusual diagram that I need help on. It looks like it works by regulating the output of the transformer by creating a back EMF in TFR2 using the 2n3055. This is a 48V 30A charger.

    The circuit is voltage and current limiting, using 741s.

    If this circuit intrigues anyone and they can explain it, I would be very grateful. At present I have no control of either the voltage or current output.

  2. crutschow


    Mar 14, 2008
    That's an interesting circuit. It controls the output using an old form of power control, a magnetic amplifier. The control circuit regulates the amount of (pulsed) DC current through TRF2 (via Q1 and Q2) which is acting as an inductor in series with the TRF1 power transformer primary. This varies TRF2's effective inductance and thus the current through the primary circuit, which also controls the current/voltage from the secondary of TRF1. See this for an explanation and demo of how mag amps work.
    psahd and #12 like this.
  3. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    I havn't seen a mag amp since 1985.

    The sort version of the workings is TRF2 looks like a large inductance in series with the AC line, effectively shutting it off. The "seccondary" winding (which has a huge ammount of turns) of TRF2 is used to saturate that inductor so it appears as an open when Q2 is turned on. The large number of turns insures a small current can control a much larger current.
    psahd likes this.