nicad charger help

Discussion in 'The Projects Forum' started by jody, Apr 24, 2014.

  1. jody

    Thread Starter Member

    Nov 12, 2012
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    0
    hi
    help im confused. my nicad charger went bang(duff nicad) and burnt out some components to the point i cant tell the original value. the transformer is a center tap 240v in and rared at 11.6v@18va. the relay coil appears to be 3v 25ohm0.12amp on the pull in coil. the nicad pack is 9.6v. on the attached schematic i have worked out(maybe) the zener diode should be 3v to match the relay coil and the resistor to drop the voltage to the relay coil should be 55ohms.
    im confused as to why the green led with a resistor value of 110ohm works out to be from a 6v source yet i think i have 9.6 returning from the battery once the relay has disconnected the rapid charge circuit.
    can i ask if i am correct in assuming that the charge current would be set at 1.75amp@9.6v as i believe the nicads are 1.75amp ones. or should the voltage be higher for the charge rate
     
  2. jody

    Thread Starter Member

    Nov 12, 2012
    39
    0
    forgot to attach ooooppps :rolleyes::rolleyes::rolleyes:
     
  3. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    5,435
    1,305
    Your schematic is very upside down and has lots of errors. :)

    The 1.1k resistor R1 is 1.1 ohm. That limits peak charging current and is the only current limiting.

    Normally in these 1 transistor relay switched chargers the transistor is controlled by the zener diode, so when the battery voltage gets too high the zener operates the PNP base, and the relay disconnects the battery.

    At that point the voltage will stay high (due to no battery load), so it will stay latched in that position.

    The zener voltage is normally much higher than 3v for a 9.6v charger.

    It will be a bit less than the battery charge voltage minus the transistor Vbe drop;
    An 8 cell NiCd has a charge voltage of about 1.4v*8 = 11.2v.

    11.2v - 0.6v = 10.6v
    the zener is usually a bit less than that as there should be a 2 resistor voltage divider, so I expect the zener voltage to be around 9 to 10v.

    We really need to see good clear photos of the PCB, so we can decode the correct schematic and see which parts are blown.

    The good news is that it is a very simple circuit and should be easy to fix.
     
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  4. jody

    Thread Starter Member

    Nov 12, 2012
    39
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    yes sorry my error it is 1.1 ohm. the items it blew were the zener and the resistor i labeled with 100000. it also fried the transistor but that ive replaced. i will try to upload some pics. the rest of the diagram is correct to the board im just not good at layout sorry.
     
  5. jody

    Thread Starter Member

    Nov 12, 2012
    39
    0
    photos. ignore the wire soldered on in the center of the board its a tep wire shouldnt be there
     
  6. jody

    Thread Starter Member

    Nov 12, 2012
    39
    0
    and yes the resistor in the picture is 1.5 ohm. now replaced with original 1.1 value.
     
  7. jody

    Thread Starter Member

    Nov 12, 2012
    39
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    i rechecked the circuit and drew just the cut off part. i dont know the value of the resistor 100000000 or the zener
     
  8. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    The zener looks fine?

    Thanks for the photos, that helps a lot, but your schematic still does not match the PCB.

    Where is the R1 you talk about? Your schematic shows R1 connecting to the cathode of the zener, but it is not on the PCB.

    Also, we need to know what those 4 coloured wires go to (red black blue green). The 3 wires at the bottom are AC from the transformer. But the other 4 wires need to be described. And why did you add the yellow wire?

    The other thing that will help is to lift one leg of each of those resistors clear of the PCB, then measure each resistor with an ohmmeter.
    :)
     
  9. jody

    Thread Starter Member

    Nov 12, 2012
    39
    0
    sorry i have introduced confusion, my fault. the green/red and the yellow are connected to a variable resistor and shouldnt be their (i was playing), instead that is where what i called r1 (labled r5 on the board) should be. the other wires are
    red battery poz
    black battery neg
    blue battery negative via a thermal cutout
    ive started again at the begining to see if i can understand each part of the circuit so ill ask one question at a time
    can i ask about the current limiting resistor. my understanding is that this is calculated to give max amperage at rated battery voltage ie 1.75a@9.6v 9.6/1.75 = 1.14 ohm which matches the original resistor.
     
  10. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    5,435
    1,305
    It's not quite that simple because the AC is rectified but not filtered. or not filtered very much (because this saves the cost of a large elector cap).

    So the battery is charged by pulsing DC. The easy way to find the current is to 'scope the waveform across that resistor when charging.
     
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