Replacing dtdp relay with mosfets

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  1. #1
    Join Date
    Feb 2012

    Default Replacing dtdp relay with mosfets

    Hi Guys

    i would like to replace a DTDP relay with some descrete mosfets N and P pairs

    , but am having some issues with the cct i have made so far


    CCT A : works

    Q20 , Q18 pairs ensure polarity of the battery is correct as the battery charger modules dont have polarity protection

    Q19 isolates the bat charger MOD1 from the 7.2v when USB PWR is not on
    RLY1:B N/C connects the bat:B to bat:A when no USB PWR
    RLY1:B N/O connects bat:A to gnd so that MOD1 charger can charge it
    RLY1:C N/C uses the battery power when no USB Pwr
    RLY1:C N/O - connect USB to pwr cct , and charges battery

    Relay activated by USB pwr at 5v

    Note: USB power can be connected prior to batteries been installed in cct , as such incorrect battery polarity must be isolated , until corrected

    CCt B: does not work
    q100 = q20 IN CCT A
    q103 = q18 IN CCTA
    q101A works same as Q19 as described in cct A

    q102A: NCH replaces RLy1:B N/O in CCTA
    Q102B: Pch replaces Rly1:B N/C in CCtA
    Q101:B Pch replaces Rly1:C by isolating the 7.2v when USB PWr is on

    when q102A has USB_pwr = H , q102B remains on
    when q101:B remains on when USB_pwr is on

    suggestions ?
    power ccta.pdfpower cct b.pdf

  2. #2
    Join Date
    Jan 2005
    Montreal, Quebec, Canada

    Default Re: Replacing dtdp relay with mosfets

    I keep thinking about the "chicken or the egg".

    I'm trying to wrap my head around circuit B but can't find the exact problem.


  3. #3
    Join Date
    Feb 2012

    Default Re: Replacing dtdp relay with mosfets

    i think it has to do with the voltages on q102b , gate vers drain voltage

    gate has 5v when usb_pwr is on , Drain has 3.7v , so differance is - 1.3v and is still on as a result ???

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