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effect of reverse b/e current over time

Discussion in 'Electronic Basics' started by kell, Nov 10, 2005.

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  1. kell

    kell Guest

    I'm looking at using an astable multivibrator, like the one in the
    diagram at this link
    http://www.play-hookey.com/digital/experiments/rtl_astable.html
    running probably at a few tens of kiloherz
    to drive a mosfet in a forward converter.
    I'll probably run the multivibrator somewhere between 12 and 18 volts.
    That will put a reverse bias on the base-emitter junctions of the
    transistors
    in the multivibrator as the base voltage gets pulled below ground
    during oscillations.
    I've heard that reverse biasing the base emitter junction affects gain,
    but that the effect
    is of limited severity. I can live with the loss of a little bit of
    gain. But what about the long term effects -- will it damage the
    transistors, or will they operate reliably for something like their
    normal lifetime if they are not abused otherwise?
     
  2. I have seen such circuits fail, when the gain fell so low that
    oscillation stopped. It is a reliable circuit with a 5 volt supply,
    but one that damages the transistors when run from a 12 to 18 volt supply.
     
  3. Tim Williams

    Tim Williams Guest

    I would imagine a resistor (comparable to the collector resistors) in series
    with the base and a diode across E-B to clamp the negative voltage to 0.6V.

    I don't know if a series resistor would stop it from oscillating.

    Tim
     
  4. kell

    kell Guest

    I'll put a zener in series with each transistor's base.
     
  5. PeteS

    PeteS Guest

    You could get around the issue of breakdown by using FETs instead - a
    VN2222LL (TO92 package for through hole) or 2n7002 (SOT-23 SMT) should
    work fine. They are both rated at Vgs of +/- 20V continuous, 40V
    non-repetitive pulse.

    Cheers

    PeteS
     
  6. John Larkin

    John Larkin Guest


    Beta degradation as a result of b-e zenering is a legend, but I've
    never seen a quantitative estimate of the effect. Somebody could do an
    experiment or two...

    John
     
  7. Fred Bartoli

    Fred Bartoli Guest

    Not an urban legend, at least.

    I just have made the experiment with a brand new BC547C.

    New, @VCE=2V, IC=1mA, (beta ~ 370).

    Carefully zener it (no cap discharge) for 15 seconds at 10mA (BE broke down
    at 10.1V)

    Then under the same conditions, IC dropped to 965uA, a 3.5% beta drop.
    That's not insignificant at all.

    What's more noticable is that as I'm writing this I can see the beta still
    sligthly dropping to 957uA (more than 4%). Perhaps the starting 965uA
    benefited from the 100mW heating from BE zenering.

    I now have to get my little daugther to a painting exhibition.

    More zenering this evening...
     
  8. Fred Bartoli

    Fred Bartoli Guest

    "Fred Bartoli"
    So, more zenering...

    30s @ 10mA, IC dropped to 950uA -> 5% beta drop;
    2min @ 10mA, IC dropped to 938uA -> over 6%.

    Time to be a bit more nasty.
    1 min @ 30mA, IC dropped to 922uA -> 8% beta drop.

    2 hours @ 30mA, IC dropped to 843uA -> almost 16% beta drop.

    One noticable thing is that the normal working VBE constantly dropped with
    the cumulative zenering, from 615mV down to 604mV.

    It could also be interesting to see what happens with cap discharge like
    with a multivibrator but I've no more time now.

    Maybe someone else?
     
  9. John Larkin

    John Larkin Guest


    Nice, first numbers I've seen on this. Looks like the beta degradation
    is falling off as the current-time product increases.

    I'll have to try this myself one of these days.

    I wonder if LEDs are degraded by reverse current.

    Thanks.

    John
     
  10. Fred Bartoli

    Fred Bartoli Guest

    I've become amuzed with this now and I'm emulating the 20V supply astable
    case with a 50ohm/20V generator @ 10kHz, and a 22nF cap. Ouch, about 160mA
    peak zenering current. Probably about what it'd be in such a circuit.

    10 min at this regime and beta dropped further down to -18%
    30 min later again brings us down to -20% and 603mV VBE.

    I don't know which region of the BE junction is zenering first and what the
    degradation mechanism is.
    Maybe Jim has some knowledge about this.

    When I've time I'll open a TO18 or a TO5 (if I can still find one) and try
    to have a look.


    You'll have to be pretty persuasive with LEDs. The ones I've tried happily
    withstood 60V with under 10pA leakage IIRC.
     
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