ID. automatic charger parts

Discussion in 'The Projects Forum' started by Wheeler, Jun 18, 2009.

  1. Wheeler

    Thread Starter Member

    Apr 22, 2009
    Unable to find a diagram for a 1970s Allisson fully automatic tapered charge car battery charger.
    I did some repairs many years ago,now it quit again,and I wonder if I used the wright parts. The 2N6504 and the 2n3906 leave me wondering.
    Attached (I hope) is a drawing .
    I'm replacing old plate diodes with two stud rect. on a heat sink.
    I' ve somewhat less than basic knowledge of circuits. I'm sure someone out there can recognize this little circuit and reassure me. thks, Paul
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    No drawing got attached, but the stud rectifiers sound like a smart move.

    Why the angst over semiconductors? The part numbers should get you workable replacements.
  3. SgtWookie


    Jul 17, 2007
    The circuit didn't attach. The image may have been too large or may not be in an acceptable format.

    I prefer for people to attach images in .png format. They're compact and not "lossy" like .jpg images.
  4. Wheeler

    Thread Starter Member

    Apr 22, 2009
    Today ,I tapped , threaded the stud rectifiers right into the heatsink. I don't have heatsink compound. Is the heat resistant silicone you get with new brake shoes the same,its dielectric ,didn't seem right.
    And a .PNG format copy of the drawing. Now I,m missing that attachement paper clip,to send it . getting there.....
  5. Wheeler

    Thread Starter Member

    Apr 22, 2009
    Allissoninpxng.jpg the orange thingy is a cap.
    Last edited: Sep 23, 2009
  6. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    What help do you need with it? That's a nice picture, must have took a few hours work. :)

    It looks like a typical old SCR based battery charger. It full-wave rectifies the AC from the transformer, but does not filter it to a "proper" DC, instead it's like pulsating DC. This means they can use a SCR to switch the power on/off with reasonable energy efficiency.

    Some of the old timers I worked with back in industry preferred these types of battery chargers, they seemed to think the pulsating DC helped to "restore" the batteries a bit when charging and some guys would only use the old chargers there haha.

    Anyway congratulations on restoring a nice old piece of equipment, they just don't make them like that anymore.

    I looked in my old book for you in case you want to get a schematic going;

    2N6504 = SCR, 50v 25A IgIh <40mA pinout= KAG looking at front

    2N3906 = PNP 40v 0.2A 0.625w B>100 pinout = EBC looking at front

    MPS-A92 = PNP 300v 0.5A 0.625w pinout = EBC looking at front

    That all looks good comparing to your nice picture. There is a zener diode (and trimpot) controlling 2N3906, which turns on MPS-A92, which turns on the main SCR 2N6504.

    It looks like the whole "automatic" thing is just a voltage sensor that senses the battery voltage and when the battery voltage is high enough it stops turning the SCR on, or more likely changes to turning the SCR on later in the phase so it still puts a bit of current into the battery. It's probably quite a slow transition from full current to "float".

    I'm not quite sure about the diode (bottom left) but guessing it connects under the SCR heatsink to the Bat- wire? I *think* that this is done so the zener can only be biased ON when there is a battery connected, pulling the diode LOW and allowing the zener to provide the active low that turns on the 2N3906. If there is no battery connected the entire control circuit will float to 12v because of the lowish resistance of the filament of the orange light bulb. So that keeps the whole device turned OFF unless you connect a battery, pretty cool actually.

    It would be great to see this in a schematic, or even photos of the old beastie.

    PS. it's more likely 1960's technology than '70's although it would be common to be still selling the same product in the '70's.
  7. Wheeler

    Thread Starter Member

    Apr 22, 2009
    Re drawing: I started with a pencil sketch of the lead traces, made a copy and filled in the rest in bitmap. Its like eating salted peanuts, can't stop. Took about 6 hrs I guess.
    Re help: Needed encouragement more than anything (got it!) Now I know I put the right parts back then.
    Lots of good info, but it will take me a while to figure it all out, thats ok.
    Just one thing, my mistake, pardon me, the orange 'light' is a cap ( to smooth the pulses I guess?)(It was dark red ,like a real one on my screen)
    Last edited: Jun 23, 2009