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Delaying the saturation of a transistor

Discussion in 'Electronic Design' started by Mr. J D, Aug 19, 2006.

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  1. Mr. J D

    Mr. J D Guest

    Ok I am looking on how to delay the turn on of a transistor, or any
    other component. The precision of the timing circuit doesnt matter, it
    just needs to slow down the saturation of the transistor for safety
    purposes; to allow a digital component (a Tinylogic XOR Gate to be
    precise) to activate before the transistor does.
    If I were to use a resistor in series with a capacitor attached to
    ground, and the base of the transistor attached to the resistor, It
    will not only delay the turn on, but it the saturation of the
    transistor will constantly be going up and down. This is what I want to
    avoid, the transistor saturation going up and down. I just want it to
    delay and then hold at a specific saturation.
  2. Eeyore

    Eeyore Guest

    I don't think you means saturation. Why do you ?

  3. But what is the minimal delay required?
    If it is in miliseconds or parts of it then a 5V relay shorting the base
    to ground and activating on the power rail would create such delay.
    Reed relays may be too fast and mechanical relays may draw too much
    current, so give us some info as to what are the limits in which to move.

    Have fun

    Slack user from Ulladulla.
  4. Mr. J D may not be a native English speaker. He means 'collector
  5. Eeyore

    Eeyore Guest

    I suspect you're on the right track but if so why doesn't he want the collector
    current to go up and down ?

    Maybe he wants a delayed swich on with a 'snap' action ? In which case a
    comparator or dedicated reset type circuit would be appropriate.

    Perhaps he can advise.

  6. Tam/WB2TT

    Tam/WB2TT Guest

    You can prevent the transistor from going into deep saturation by connecting
    a diode with low voltage drop from base to collector; cathode to the
    collector. You can trim that even more by adding a low value resistor
    between directly in series with the base of the transistor. The ON voltage
    of the transistor will be slightly raised.

  7. Eeyore

    Eeyore Guest

    Baker Clamp !

  8. Fred Bloggs

    Fred Bloggs Guest

    At least you recognized that wrapping that delay into the single
    transistor will also slow the rise/fall times. The R+C method shunts the
    high frequency transistor drive and slows it down. The only way to
    prevent this is to Schmitt trigger the transistor input, with either a
    tiny logic Schmitt using an RC on its input, or another transistor. A
    tiny logic Schmitt is probably the cheapest, most compact, and most
    consistent performer of your choices. Of course, this answer is given in
    isolation and without knowledge of your specific circuit. If "safety" is
    critical, the *best* way is to use the XOR output as an input to
    whatever eventually drives the transistor. This may get complicated with
    an XOR and I won't waste time on conjecture without additional information.
  9. Mr. J D

    Mr. J D Guest

    Thanks for all the replies. I am a native english speaker. To clear up
    what I meant: I only need a few milliseconds of delay time, and then
    stay at the peak. Like a capacitor but with only one rise, and no fall.
    Someone mentioned using a diode to do this, can you explain more in
  10. Apologies. The use of the word 'saturation' instead of 'collector
    current' has echoes of French terminology.
  11. Jim Thompson

    Jim Thompson Guest

    You could do it logically. See...

    For your case just use the "TAU" output and drop the XOR portion.

    ...Jim Thompson
  12. Mr. J D

    Mr. J D Guest

    Thanks for the schematic. I don't want to do it logically, as it
    require an excess of parts, I have very limited space. I was almost out
    of space when I decided to add an XOR gate.
  13. Eeyore

    Eeyore Guest

    Can't you use a gate with schmitt inputs ?

  14. Fred Bloggs

    Fred Bloggs Guest

    The diode suggestion has to do with limiting the excess minority carrier
    charge in the transistor base by shunting base drive current around the
    base and directly into the collector at small BC forward bias voltage,
    which is why a Schottky diode is best and sometimes this combination is
    called a Schottky transistor. It is a speedy switch because the storage
    time, or roughly delay in time after removal of base drive until
    collector current begins to fall , is a function of the excess minority
    carrier base charge. This has little to do with your problem. You're not
    going to beat a tiny logic Schmitt trigger used to drive the transistor:
    View in a fixed-width font such as Courier.

    .. dual .---> OUT
    .. schmitt |
    .. .--|<|--. |
    .. | | |\ |\ |/
    .. IN >--+--[R]--+----| o--| o-[Rb]-|
    .. | |/ |/ |>
    .. === | | |
    .. | C | | |
    .. gnd1---------+-----+----' ---
    .. gnd2
    .. RxC~ 1.5ms
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