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555 timer problem

Discussion in 'Electronic Design' started by RKovach, Oct 20, 2004.

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  1. RKovach

    RKovach Guest

    I am trying to use two 555 timers to drive automotive ignition coils for
    testing and am having problems. I want to use 1 timer in monostable mode to
    run the second timer in astable mode to send the ground signal to fire the
    coil. I have tested the timers in a breadboard circuit and they each work as
    expected individually. The problem I am having is when I try to connect the
    output from the first timer to the second timer,(pin #3 on first to pin #4
    of the second) the first one will not have the correct pulse duration. It
    will only have the output if pin #2 remains grounded. I have also noticed
    that I have the same problem with the first timer when I have my scope probe
    connected to the output of the second timer, even if they are not connected
    to eachother except for the same power and ground. Also, if I do get these
    timers to work together, what would be the best transistor to use at the
    output of #2 timer to gound the ignition coil?
     
  2. Hello R Kovach,
    have a look at this article headed "the time machine"
    and the following article on the electric fence using an
    automotive ignition coil.
    http://www.homepower.com/files/electricfence.pdf

    This circuit, works the other way round, to the way you
    want, the astable then the monostable but it might still
    be worth a read through and you might get some ideas.

    Regards,
    John Crighton
    Sydney
     

  3. Assuming you have it configured correctly, it should work OK, as you
    see here:
    http://www.terrypin.dial.pipex.com/Images/555GatedMono.gif

    You've given no details of the coils' specs, so I've simulated that
    only simplistically.
     
  4. Fred Bloggs

    Fred Bloggs Guest

  5. On Wed, 20 Oct 2004 14:49:03 +0100, Terry Pinnell

    Hello Terry,
    on your simulator, just for fun,
    what happens to the yellow waveform if you
    hold the push button switch closed for a longer
    time period, say 0.1second or longer.

    I seem to remember seeing the monostable start
    when the push to close button was released. I was
    messing around a while ago with a similar circuit,
    not a computer simulator, just using components
    on a breadboard. I am just curious to know what
    your simulator does.

    Regards,
    John Crighton
    another hobbyist, Sydney
     
  6. John: The 555 mono trigger just needs to be shorter than the output.
    Here's the situation with the button held closed for 200 ms.

    http://www.terrypin.dial.pipex.com/Images/LongTrig.gif
     
  7. Hello Terry,
    I dug up the piece of scrap bread board that I was
    playing with a while ago and it was dodgy. Finger
    trouble on my part with wiring errors. Ooops!
    Thanks for adjusting your simulator for me. :)
    Regards,
    John Crighton
    Sydney
     
  8. RKovach

    RKovach Guest

    I did not have the 47uF cap (C3) in my circuit. I have added this and now it
    works fine.
    Thanks for the help.
    I have one more question. I have noticed that in all circuits that drive a
    relay or coil, diodes are used at the end to protect the circuit. I plan on
    using an IRF510 power MOSFET to ground the coil. The specs on this are 100V
    max. Do I need the diodes for protection on this circuit? If I do, will this
    affect the output of the coil? The purpose of this project is to view the
    coil waveform with a scope.
     
  9. Pleased to hear it.
    Yes. Most semiconductors could be destroyed/damaged by the high
    voltage spike generated when the magnetic field from the coil
    collapses when its current is removed. The diode provides a
    short-circuit path for it, harmlessly dissipating the energy as heat
    in the diode and coil.

    No. The reversed diode acts like an open circuit when the coil is
    being intentionally activated by current through the driver.

    Maybe you could post a picture of it when you've finished?

    My ECAD package, CircuitMaker, doesn't have an IRF10 in its library.
    But there's an IRF1010, which is rated at 55V and 75A. If you could
    provide some basic specs of your coil, including its inductance and DC
    resistance, then I'll see how it simulates. I will of course also need
    your mono output duration, and the on/off times of your astable,
    rather than the arbitrary guesses I used earlier.
     
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