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Discussion in 'Misc Electronics' started by Ken O, Jun 17, 2006.

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  1. Ken O

    Ken O Guest

    I have been working on a project here, I thought it was simple but now I
    come to a dead end. I used a 555 timer with output at 2Khz 7v peak
    impulses. I hooked the output of the 555 to the base of a 2n3055. the
    emitter to the battery ground and the collector to a coil in wich is
    connected to the positive of the battery.
    I thought the coil would follow the discharge of the 555 timer, but I get a
    dc voltage at the collector.
    any idea???

    thanks

    ken
     
  2. Ken O

    Ken O Guest

    HI

    I did you a voltage divider. , I have about 4 volt to the base.
    The battery supplying the coil is a 12 v battery and I am barely getting 5
    volt impulses from the coil.
    i am not doing any Em kick back as you mentionned.

    ken
     
  3. Jamie

    Jamie Guest

    i am not sure if i am following you here.
    the last project i did like that consisted
    of keeping the coil at saturation and then
    generate a narrow pulse to allow the kick
    back discharge to take place!, this means
    inverting your signal from the 555 or tailoring
    the components to generate a narrow off pulse for
    the EM Kick back.
    P.S.
    you should think about a small cap on the Collector
    to ground so to not allow high voltage to appear at the
    transistor.
    also, the 555 does not go to the - rail completely if
    memory serves, this means you need a voltage divider at the
    base.
     
  4. default

    default Guest

    Are you measuring the voltage with a scope?

    Duty cycle of the 555?

    DC resistance of the coil and inductance? or what kind of coil.

    If you are using a scope to measure voltage . . . the inductor may not
    be saturating fully before you take the pulse away. It takes time.
    With the inductance and resistance of the coil you can calculate the
    time constant and have some idea if your pulse width is great enough.

    If you are pulling lots of current through the 2N3055 it may not
    saturate without a lot of base drive current - the 3055 doesn't have a
    lot of gain

    From my old Texas Instruments "The Power Semiconductor Data Book"

    F hfe 20 KHZ minimum

    Static forward current transfer ratio between 5 at 10 amps collector
    current and 20 at 4 amps (minimums)

    Collector Emitter Saturation Voltage
    Ib = 400 milliamps drive, Ic 4 amps collector current = 1.1 volt
    Ib = 3.3 amps , Ic 10 amps = 8.0
    volts!

    So, if you're sinking lots of current don't expect a 3055 to pull to
    ground and even if you're not, the 555 can only supply so much base
    drive current to the 2N3055
     
  5. Ken O

    Ken O Guest

    hi

    what I have tried doing is putting some voltage (from 6 to 40 v) to the
    base. with the collector connected to the a simple coil ndn the coild to
    another 12 v supply, still, there is nothing going through the emitter. i
    used a 2n3055, I have a few of those, so I do not think they are deffective.
    I just want the coil to discharge.
    It does discharge if i connect it diretly to the battery, then I get an
    eletromagnet, this is what i want to do woth the transistor.

    ken
     
  6. Pooh Bear

    Pooh Bear Guest

    Directly with no resistor ? That may be a problem for startes.

    Graham
     
  7. default

    default Guest

    You doubtless have a resistor between the output of the 555 and base
    of the transistor, you've calculated how low you can go and still have
    the 555 survive the experience or how high and still expect the 3055
    to saturate. Right?

    Another trick that may come in handy - bypassing the current limiting
    base drive resistor with a capacitor to increase the rise time and
    dump a little more "get moving" energy into the transistor's base.
    Size would be best determined by some experimenting.
     
  8. kell

    kell Guest

    The 555 doesn't have enough power. Use the 555 to drive a smaller
    transistor that will then drive the 2N3055.
    Do a gain calculation to find out how much current you need into the
    base of the 2n3055. How much collector current will you need the
    2n3055 to conduct -- in other words, how much current do you need to
    flow through the coil? Look up the gain of the 2n3055 at that current
    level.
    If minimum gain at that collector current is say 5, then divide coil
    current by 5; that is the current you need to drive the base of the
    2n3055 with. Bung in a transistor between the 555 and the 2n3055, and
    use a resistor to control current into the base of the 2n3055.
     
  9. Pooh Bear

    Pooh Bear Guest

    How do you know ( you mean current anyway ) ? The OP hasn't specified his load.
    The 555 has a reasonably healthy output current.
    This is called a darlington arrangement.
    Or use a mosfet.

    Graham
     
  10. Ken O

    Ken O Guest

    Lots of options here.
    i was not sure to try an intermediate transistor. i try that and the
    capacitor too.
    Also anotehr thing.
    if hook up the coil to the battery I get an electo magnet. if I put a 50 ohm
    resistance in series or even an higher resistance i get nothing. measured
    the resistance of the coil. its 50 ohms. the resistance get very hot, but no
    electro magnet.. why

    ken
     
  11. Pooh Bear

    Pooh Bear Guest

    The capacitor 'thing' is just for 'high speed circuits' it's irrelevant here.
    Yes. That's normal.
    Your measurements sound like they are in error then. You should still get a
    weaker magnetic field with the sereies resistor. The resistor gets hot because
    it's dissipating power.

    Graham
     
  12. kell

    kell Guest

    For the sake of argument I'll assume you have a twelve volt battery
    driving the coil.
    So you need 12/50 or 0.24 amps collector current (coil current).
    .05 amps base current to the 2N3055 ought to get you by, which your
    555 can probably deliver. I seem to remember seeing 0.2 amps source
    current in datasheets for typical NE555.
    Here's my guess what's happening:
    The base-emitter diode of the 2N3055 is forming a shunt to ground,
    overloading the output of the 555. You should put a base resistor in
    there and see if that eliminates the problem. Try a 100 ohm, one watt
    resistor. That will give you .07 amps to the 2N3055 base if your 555
    output is 7 volts as you stated. Ohm's law says power=v^2/r=49/100 or
    about half a watt. I suggested using a one watt resistor to stay
    within the power rating.
    Also if you're driving the coil with less than 12 volts, this resistor
    value will still work.
     
  13. Ken O

    Ken O Guest

    The capacitor 'thing' is just for 'high speed circuits' it's irrelevant
    yes , its about 90Hz

    there is a minuscule difference with even the smallest resistance. The point
    I was getting at as soon as i add resistance to this network the electro
    magnet is not considered operational if we consider operational the ability
    to repulse a weak ceramic magnet.

    I added a TIP41c NPN transistor between the timer and the 3055 but no
    difference.
    thats why earlier I put a 6 v battery directly to the base of the 3055 and
    no effect. I was hoping to activate a dc current in the electro magnet .

    ken
     
  14. Ken O

    Ken O Guest


    I tried a 100Ohm resistor in betwe the output of the timer and he base and
    nothing changed. swapped the 3055 for another.. no change.
    This I thought to be a very simple design is giving me a headache

    K
     
  15. Ken O

    Ken O Guest

    i tried using just the TIP41C instead of the 3055. to my surprise it do not
    heat up, but I get a very week voltage in the coil now, far from connecting
    it directly to the 12v battery.
    This time the setup was; battery -coil- transistor- ground and timer to the
    base

    ken
     
  16. Try connecting the coil in the emmiter circuit of you 3055. Not from the
    collector to 12 supply. Use the resistor in the base circuit of 3055 to
    limit the base drive, and connect the collector of 3055 to +12 volt -
    emmiter to coil and coil to ground. Also add a diode collector to emmiter of
    3055 reversed bias. This may help drive the coil a little better. JTT.
     
  17. Ken O

    Ken O Guest

    I added the diode across thebase and emitter, I got good results,
    switching the coil around to the emitter did not help.
    I am getting some power on the electro magnet but just around6 v, not the
    11.5v I was anticipating

    ken
     
  18. default

    default Guest



    somewhere you state that the resistance of the coil is 50 ohms - well
    if you put a 50 ohm in series with a 50 ohm coil half the voltage will
    drop across the coil and half across the resistor. Your total current
    with 100 ohms total resistance should be 120 milliamps and close to a
    watt of power in the resistor - plenty to cause heating.

    Your current with no resistor should be .24 with a power dissipation
    of close to three watts in the coil causing it to heat (and depending
    on its physical size and construction it may take awhile to heat
    noticeably)

    As for the electromagnet effect: with the resistor you cut the
    current in half and you cut the voltage across the coil in half. You
    drop the effective "ampere turns" by a factor of four so the magnet
    should be 1/4 as strong as it was with no resistor.

    It doesn't sound all that hard to do even with a 555 and a 3055

    So something is obviously wrong . . .

    What became of the 2 KHZ you were initially driving it with? Is/was
    that a square wave? Is your coil very large?

    I had a little 555 circuit driving a car ignition coil. The coil
    resistance was ~3 ohms. I used a TIP 120 to drive the coil and it
    worked reasonably well (made sparks). I forget the duty cycle, but
    the frequency was adjustable and duty cycle fixed (I think - this was
    several years ago)

    This shouldn't be a big problem unless the inductance of the coil is
    very high - and then it may be hard to pump any real energy into it at
    2 KHZ with only 12 volts.

    And when dealing with MOSFET's and inductors, like someone suggested,
    it takes some savvy to protect the mosfet from the inductive kick to
    keep from destroying them. You need a relatively high voltage part
    and some high speed diodes or zeners to protect them - it would be a
    good idea to read up on their characteristics first.
     
  19. Pooh Bear

    Pooh Bear Guest

    Actually, the diode is there to repvent *damage* not to improve results, which
    it won't.
    So, there's 6V across the 2N3055 too ?
    There is clearly something fundamentally wrong here. it wil be something to do
    with how you've connected things.

    I suggest you post a schematic in a suitable place ( not here since this isn't a
    binaries group ).

    Graham
     
  20. Ken O

    Ken O Guest


    I tried the buz21 mosfet , it work good, but heats up real quickly.
    thats a reason why I wanted to use the 3055. . The buz21 only work when I
    apply at least 5v to the base. Itried a new 3055 with only 1v at the base as
    Jamies suggested, but it did not work.
    I really want to get it going with the 3055 transistor

    k
     
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