Mensaje originado el: 12/08/10 a las: 23:03 z



From: G8MNY@GB7CIP.#32.GBR.EU
To  : TECH@WW

This is a simple SCR controlled charger. I made it to give 27.5V at 15A for
charging a train carrage's 500 AH 24V battery lighting system. It is not
suitable for use with electronic equipment that can't operate on hummy
supplies!

Using a dual winding transformer or a single one with 2 other diodes so the
SCRs form part of a bridge... Or 1 SCR arm following a bridge rectifier.

                       SCR1
AC1   __________________!\!____________________
       !         _____  !/!\                  !
       !____!\!__!1K  !_____!__!\!__          !
            !/!  !____!        !/!  !         !
                                    !         !
                       SCR2         !         !
AC2   __________________!\!___________________!
       !         _____  !/!\                  !
       !____!\!__!1K  !_____!__!\!__!         !
            !/!  !____!        !/!  !       __!__
                                  __!__      ===
                                   / \ '      !BATT
                                   ---        !
AC RETURNS                   ZENER  !         !
    ________________________________!_________!
    __!__
    /////

The Zener can be a virtual one for voltage adjustment with current limiting
changing the voltage as well.
                                     !
                        ----------------------------
 VIRTUAL ZENER          !         !          !     !
 & CURRENT LIMIT        ! +      c \!       POT   c \!
                       ---          !------>10K      !---------
               100uF   ---       e /!        !    e /!  !      !
                        !         !          !     !    !+     !
                        !        ---\        !     !   ---    1K
                        !        / \         !     !   ---     !
                        !        ---         !     !    !10uF  !
                        !         !          !     !    !      !
         -VE ret   ---------------------------------------\/\/---- Batt return
                                                       SHUNT

The Zener can be 5-10V (12v system). Both transistors are NPN. Set the voltage
pot so that the Zener action is 1V higher than the required Battery voltage.
Make the shunt resistor (thick wire link eg a few inches of coat hanger) so at
the maximum current the 2nd NPN conducts to limit the trigger voltage & hence
the current.
The transformer must deliver an RMS voltage slightly greater than the battery
voltage. If too low Triggering can be difficult, but a 2nd 2v winding feeding
the trigger circuit can keep the unit working well on low mains (train PSU's
problem <200V AC at times).

This circuit is very efficient, only the transformer heats up with the high
pulse current, & SCR losses, no half power lost in dropper regulator! So a
thermal trip mounted on the transformer in case it overheats in heavy loading
conditions is recomended.

For recharging 12V batterys use 14.5V DC setting, for float (infinate charge
rate) use 13.8V, with sealed types use 13.5V MAX.

73 De John, G8MNY @ GB7CIP