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Electronics Charging NiCad batteries..

Discussion in 'Modding' started by bigal, 5 Sep 2005.

  1. bigal

    bigal Fetch n Execute

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    Hi,
    i am working on a sort of advanced alarm for our workshop here, basically it is a IR beam across the door connected to a scantronic transmitter, the receiver is powered by 12V and has 2 relays to trigger a buzzer (as i am using it). anyway, what i am trying to do it put the receiver on a box and in the box have a battery, so the thing is wireless and can be moved around, but it can be plugged in and when plugged in it runs of the mains (transformed to 15VDC and run through a 7812 ans smoothed. i have this sorted), the problem comes with the charging the 10 cell 12v 800mah nicad. i have located this nice little circuit that should do the buisness but i just wanted to make sure that circuit is ok, i cannot see how it works... the text says to put a ammeter across the + and - of the battery but wouldent that short it out?
    cheers
    Alex
     
  2. cpemma

    cpemma Ecky thump

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    It's not made obvious, but you do that before connecting the battery pack to set the required current limit.

    They've used the 317T as a constant-current source, so the maximum current is limited to 1.25/(R1+VR) amps however high the load, even a dead short.

    There's around 3V minimum lost from the input across the 317T and resistors, so to charge 12V cells I'd start with around 18V DC (a 15V AC transformer will give that after rectifier & smoothing).
     
  3. bigal

    bigal Fetch n Execute

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    bah, cant get it to work, only ever gives out 11ma... bit weird..
    also it doesnt tell you what the resistor values should be (or rather how to calculate them)
     
  4. cpemma

    cpemma Ecky thump

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    If you wanted 200mA, (R1 + VR) resistance would be only 6.2R, so hard to find a suitable pot. And if there aren't enough supply volts for the load resistance, you'll get a lower figure that just depends on Ohms Law (you'll be running below the set limit).

    linear's got a clearer circuit and full explanation here.
     
  5. bigal

    bigal Fetch n Execute

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    lol, i was wiring the LM317 like i thought it should be, ie like a 78xx regulator with the GND as adjust, turns out it completely different...
    f00l of me
     

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