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LM217's in parallel is OK?

Discussion in 'General Electronics Discussion' started by eem2am, May 25, 2013.

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  1. eem2am

    eem2am

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    Aug 3, 2009
    Hello,

    Is the following 400mA NiCd battery charger, involving five LM217 current regulators in parallel OK?

    Each LM217 is supposed to pass 80mA, but i wondered if parallel connection will make only one LM217 pass the 80mA, and the others will not be able to?
     

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  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    Placing current sources in parallel should not be a problem.
     
  3. eem2am

    eem2am

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    Aug 3, 2009
    Are you sure, i mean, if each LM217 passed 80mA as required then that would mean exactly the same voltage across each LM217...under those conditions i would not expect each to pass the same current?

    ...this is just a charger for use in the lab, and we dont want to bother with a heatsinked 1.5A device and hassle with heatsink paste etc......its easier to just solder 5 TO220's in parallel.........the Vin is 15V, and the battery could be down as low as 0V, so five in parallel each carrying 80mA means theres definetely no heatsink needed for any of the five LM217's.

    We need this charger for the lab because the sales and apps guys are constantly coming to us for fully charged batteries for the demo kits, and we cannot find any off the shelf NiCd chargers for five D cells in series.
    We will get about 10 of these built , and we can leave the batterys on them overnight and they will be charged fully in the morning...(its a C/10 charger).
     
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    Even if each 217 passes a slightly different current, in parallel they will have the voltages across them the same. That's fine.

    If you placed them in series you would expect unusual things to happen since the current would be determined by the one that pases the least current. The voltage would rise across that one and fall to as close to zero as is possible across the others.

    So the restriction is, you couldn't place several in series to use them with a higher voltage supply, but you can go to town placing them in parallel.

    A similar (except exactly opposite) thing is the rule for voltage sources. You can't put them in parallel because the one with the highest output voltage will try to power the entire load. But you can place them in series...

    If you wanted to place voltage sources in parallel, you'd have to place a resistance in series with each of them to equalise the currents. If you wanted to place current sources in series, you'd have to place resistors in parallel to equalize the currents.

    edit: a switchmode voltage regulator with a current limit would seem to be a better solution.
     
  5. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Nov 28, 2011
    Good advice from Steve.

    Why are you using NiCd cells? I thought they were banned under RoHS (Cadmium in the waste stream is a no-no). What country are you in?
     
  6. eem2am

    eem2am

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    Aug 3, 2009
    UK,

    Cadmium is still legal in emergency lighting, because NIcad is the most robust tech for constantly being trickle charged.....nicad is THE battery for emergency lighting...giving cheapness and constant full charge due to the constant trickle that it can take....put a normal nimh on trickle, and it will die..........noone wants to spend big bucks on batts for emergency lights...as they hardly ever get used and most people think they can get out of a building within a minute or so anyway, so they dont need the regulatory 1 hour evacuation time. (or they dont think they do, and thats what counts for marketing)

    Nicad is THE emergency lighting engineers choice, thats why its banned in al apps except nicad.....nicad is about 50% cheaper than nimh..and the nicad chargers are also 50% cheaper than nimh or li chargers.

    cadmium is heavy metal...but them so is the mercury in fluorescent tubes, and theyve never banned that.

    nimh contains "gremlin" chemicals too
     
    Last edited: May 26, 2013
  7. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Nov 28, 2011
    Interesting. Thanks for the info.
     
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