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  1. #1
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    Default Gayrad battery cherger

    Hi.. This is the charger I would like to get running on 12V again.. for some reason it only runs on 6v, and was expecting to find something burnt inside, but all looks OK. A check of the switch reveals it is working in both directions.
    Explaining the wires, 240 enters the coil on one side, on the other the two cloth covered wires go to the rectifier and are marked AC... the red wire from the coil runs to the 6V side of the switch. on the rectifier the first and last plates are connected by a bridge and are marked N (yellow wire) the red wire is marked P.
    What do I check ?
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  2. #2
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    Not enough detail of the important parts in the circuit but my best guess is its a centre tapped transformer and crudely switching between half and full wave rectification.

    If the above information is not clear enough for you to diagnose it then I would take it to an electrician for assistance.
    It is a very simple device requiring basic electrical skills, but is also vary capable of killing an unqualified person.

  3. #3
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    Quote Originally Posted by kenny_10 View Post
    A check of the switch reveals it is working in both directions.
    Do you mean it moves in both directions, or it's actually making contact in both positions of the switch? I.e. have you checked it with a multimeter?
    Chris

  4. #4
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    Quote Originally Posted by jack620 View Post
    Do you mean it moves in both directions, or it's actually making contact in both positions of the switch? I.e. have you checked it with a multimeter?
    Checked with a meter.. continuity in both positions

  5. #5
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    It looks like you have a selenium rectifier. Apparently they break down with age. If you are up to it, you can replace it with a silicon bridge rectifier like this:

    https://www.jaycar.com.au/400v-35a-b...ifier/p/ZR1324

    If not, you should probably toss the charger out.
    Chris

  6. #6
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    Hi Kenny, Guys,

    If you have a multimeter the voltages you should see assuming that the transformer is OK. The voltages on the picture are nominal ones, you will probably measure higher than those. The transformer normally will output on those black wires about 15 or 16 volts AC. You cannot simply replace the rectifier with a bridge, you will have to replace it with discrete diodes if you want to keep the six volt output.

    HTH.

    100_3857.jpg
    Best Regards:
    Baron J.

  7. #7
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    Quote Originally Posted by BaronJ View Post
    Hi Kenny, Guys,

    If you have a multimeter the voltages you should see assuming that the transformer is OK. The voltages on the picture are nominal ones, you will probably measure higher than those. The transformer normally will output on those black wires about 15 or 16 volts AC. You cannot simply replace the rectifier with a bridge, you will have to replace it with discrete diodes if you want to keep the six volt output.

    HTH.

    100_3857.jpg
    I think you missed the centre tap on the transformer and the fact the bridging bar is the negative output of the bridge plate rectifier, there is no reason a solid state rectifier could not replace it.
    I believe the 6/12 volt is crudely derived by using half wave rectification for 6 volt and full wave for 12 volt, that was the normal for similar type equipment of that age.

    If the switch measures ok then the full wave rectification side of the system is non functional, either one side of the transformer windings or one side of the bridge is open circuit.

  8. #8
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    This is what a circuit diagram for a similar device would look like.
    A suitably qualified technician will be able to confirm if it matches your device.

    circuit.jpg
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  9. #9
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    [QUOTE If not, you should probably toss the charger out.[/QUOTE] AAAAAAAAAAAGH !!

  10. #10
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    Hi Droog,

    Quote Originally Posted by droog View Post
    I think you missed the centre tap on the transformer and the fact the bridging bar is the negative output of the bridge plate rectifier, there is no reason a solid state rectifier could not replace it.
    I believe the 6/12 volt is crudely derived by using half wave rectification for 6 volt and full wave for 12 volt, that was the normal for similar type equipment of that age.

    If the switch measures ok then the full wave rectification side of the system is non functional, either one side of the transformer windings or one side of the bridge is open circuit.
    Yes your right, a brain fart there !
    The drawing you posted is correct as well.

    I was thinking about, if you replaced the rectifier with a modern silicon bridge the output voltage would be much too high, so a couple of extra diodes in there would reduce the output voltage to something more reasonable.
    Best Regards:
    Baron J.

  11. #11
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    Quote Originally Posted by BaronJ View Post
    Hi Droog,



    Yes your right, a brain fart there !
    The drawing you posted is correct as well.

    I was thinking about, if you replaced the rectifier with a modern silicon bridge the output voltage would be much too high, so a couple of extra diodes in there would reduce the output voltage to something more reasonable.

    Might also want to note that with 16v Ac (RMS) your going to end up with around 25v Dc once rectified, which will probably cook most 12v lead acid batteries pretty quick.

    We used to use elevated voltage to try and recover failing SLA batteries with high resistance cells with forced charging, the results were not very successful.

    And yes the voltage drop is lower on solid state rectifiers but so minimal that it will have next to no effect, these are crude devices and you cannot leave them connected if you want a useable battery at the end.

  12. #12
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    I think the OP said in another thread he uses it for electrolysis, which would be fine. I wouldn't put it anywhere near a battery.
    Chris

  13. #13
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    Quote Originally Posted by jack620 View Post
    I think the OP said in another thread he uses it for electrolysis, which would be fine. I wouldn't put it anywhere near a battery.
    I'm not sure about "fine". Yes it will work but 25V DC is going to generate lots of gas, erode the anode and contaminate teh electrolyte with sludge pretty quickly - Somewhere between 3 and 6V is all that is needed for electrolysis.

  14. #14
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    Quote Originally Posted by BobL View Post
    Yes it will work but 25V DC is going to generate lots of gas, erode the anode and contaminate teh electrolyte with sludge pretty quickly - Somewhere between 3 and 6V is all that is needed for electrolysis.
    Fair enough. However, the 16VAC/25VDC is an assumption. It seems unlikely a 12 battery charger would be designed to output 25V. But it would still be a lot more than the 3-6V you recommend.

    Perhaps this thing really should go to the tip.

    BTW- 16V RMS gives about 21VDC after allowing for the drop across the bridge.
    Chris

  15. #15
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    Hi Guys,

    Since the transformer output would be, say 16 volts AC for argument, the peak DC would be 1.414 times that, around 22.5 volts. But this is not a pure DC output, there is no smoothing capacitance to knock off the peaks.

    At 6v setting it is only half wave rectified, so the 16 volts AC would become roughly half that times 1.414 at the rectifier output, about 14 volts, much nearer to the voltage required to charge a 12 volt lead acid battery rather than a 6 volt one.

    Also it should be possible to use a light dimmer to adjust the voltage input to the transformer to reduce the transformer output voltage, turning the charger into a crude variable voltage supply more suitable for electrolysis.

    JMTPW.
    Best Regards:
    Baron J.

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