Connect with us

Another simple transistor question.

Discussion in 'Electronic Basics' started by AJ, Sep 5, 2006.

Scroll to continue with content
  1. AJ

    AJ Guest

    Hi all

    I know that a PNP transistor will not work if the collector is higher than
    the emitter but will it be damaged?


  2. Eeyore

    Eeyore Guest

    May I ask why you want to know ?

  3. If by higher, you mean more positive, then this just reverses the
    roles of emitter and collector. As long as this doesn't break down
    the base to emitter junction in the reverse biased direction, nor
    exceed any current rating, no damage is done. Ordinarily, the reverse
    beta (collector and emitter roles reversed) is only a fraction of the
    forward (normal reverse biased collector to base junction and forward
    biased base to emitter junction) current gain.
  4. Of course you may ask. No one can stop you from asking. Ask. ;-)
  5. AJ

    AJ Guest

    I have a 3.6V Lithium battery and a 5V supply that power a 3.3V regulator.
    I have a diode on the input to the 3.3V regulator and I wanted the battery
    to power this regulator when the 5V disappears. I don't want to have to put
    a diode in between the battery and the 3.3V regulator because of the voltage
    drop but I need something there to stop the battery receiving a charge from
    the 5V rail (via the diode) instead of its proper charger circuit. This is
    where I was thinking of using PNP transistor with tis base permanently tied
    low. If the battery is dead flat, the collector could be higher than the
    emitter, thus my original question. I have since thought that I could use a
    zener/transistor to detect when the input to the 5V supply drops and switch
    a MOSFET accordingly but my first idea would be better if it would work ok?

    I will give it all a test in the next couple of days but its 2am here in
    Australia and my eye's are the size of pea's, not sure I can cope right now.
    Anyway, its always nice to hear what other people think first, and I always
    get good answers, Thanks John.


  6. jasen

    jasen Guest

    something like this maybe,

    | | |
    +-/\/\----. | |
    | | | |
    | ----- | |
    | 7| \ | |
    | / \ | |
    Vbat------|-----' `------' |
    | |
    | |
    / |
    \ |
    / |
    \ |
    | |

  7. ehsjr

    ehsjr Guest

    You could use a relay:

    + 3.6 -------------+
    N/C V --------
    -------|3.3V reg|----
    +5 ----+-----+-----^ --------
    | | N/0
    [D1] [Ry1]
    | |

    Normally closed point connects the 3.6 volt
    battery to the reg until +5 is present - then
    the relay transfers disconnecting the normally
    closed point and connecting the normally open

  8. Chris

    Chris Guest

    Hi, AJ. I'm assuming you'd also want seamless transfer to avoid
    glitching on your regulator.

    Doing this well really isn't that simple, but it's obviously a common
    problem with consumer electronics. So sometimes the best thing to do
    is just cast around and see what the semi manufacturers who play with
    these things have come up with.

    I'd recommend taking a look at the Linear LTC4411 for your problem.
    (Maxim makes some good product too, but their great ideas tend to
    disappear after a couple of years). It's only available in a 5-pin
    SOT-23 package, but it's otherwise just about perfect for what you're
    doing. It's got an internal 140 milliohm Rds(on) P-channel MOSFET that
    turns off if the output voltage is more than the input voltage, like
    this (view in fixed font or M$ Notepad):

    | Shottky .-----------.
    | + Diode | 3.3V | +
    | o-------->|-------------------o--| LDO |---o
    | | | |
    | | '-----------'
    | .-------------. | |
    | | LTC4411 | | |
    | | | | |
    | .-----oVin OUTo--' |
    | | | | |
    | 5V 3.6V| | | | 3.3V
    | +| | | |
    | --- .--oCTL Statuso |
    | - | | | |
    | | | | | |
    | | o--oGND | |
    | | | | | |
    | | | '-------------' |
    | - | | | -
    | o------o--o----------------------------o---------o
    (created by AACircuit v1.28.6 beta 04/19/05,C1,C1003,C1142,C1079,P2430
    (This is the product page -- you can download the data sheet from

    If you don't mind a "store-bought" one IC solution, and you can live
    with an SOT-23 package, it'll git r done.

    Good luck
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day