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Oscilloscope question

Discussion in 'Electronic Basics' started by Dan Beck, Sep 13, 2008.

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  1. Dan Beck

    Dan Beck Guest

    Hello all,

    years ago I was given a 10 MHz Heathkit oscilloscope. Both channels work
    fine, and I use it to probe circuit boards powered up on the workbench. I
    now own an old hobby car with the old fashioned plugs/points/capacitor
    ignition. Is there any way I could use the 'scope to observe the ignition
    signals, and would there be any value to it, besides academic information?

    Thank you in advance for any and all thoughts!

  2. That sort of thing was done in the old days, I've never had a car so
    I don't know how useful it was, or even if it matters now (I gather
    a lot of cars are now beyond the ability of the average person to
    do much repair work).

    It would have been detailed in countless magazine articles, and
    likely in all those "101 Things To Do With Your Oscilliscope".
    That sort of book can be quite useful when starting out with
    an oscilliscope, you might want to dig up a used copy.

  3. neon


    Oct 21, 2006
    YES you can . the max car frequency is not 10mhz more like 400hz. at the points there is 12 volts being switched on-off. looks like you could put a scope there right NO! when the secondary releseases the energy there is the same ratio 1:xxxx reflection so there is probably 3-400v spikes can you see that yes but i sugest you putting a resistor attenuator of 1000:1 so the cheap scope doesn't break down the input. same applies to the secondary at spark gap output. there is usualy about 100mw of power there so don't kill the circuit with low resistors. have fun.
  4. whit3rd

    whit3rd Guest

    Two ways. First, you can look at the primary winding on
    the spark coil; the opening of the points will make a
    few hundred volts spike (an attenuator probe is wise
    here), and the sparkplug firing will determine the trailing
    edge of that transient. Imbalance of cylinders in
    breakover voltage is clearly visible, can be caused by
    obvious things (dirty or misgapped plugs) or unobvious
    ones (gas mixture in the cylinder changes the breakdown

    Second, you can use a clamp-on transformer to monitor the
    current through the HV wires (either the wire to
    the coil, OR the distributor wires to one plug at a time).
    Be careful about grounding here, of course. A burden
    (shunt) resistor on the secondary of your transformer is
    VERY IMPORTANT; ten to a hundred ohms should
    be sufficient. The transformer can be a pair of ferrite
    core halves and a single turn of wire for the secondary.
  5. ian field

    ian field Guest

    The LT back emf could spike as high as 600V (maybe even higher) if the scope
    is all tube you should be OK, but if it uses transistors in the Y amplifiers
    some caution might be wise. Study the spec for the x10 probe and if it isn't
    rated for that voltage then make a voltage divider yourself, here again you
    need to check the voltage rating of the resistors - if necessary make the
    dropper section of the divider out of a number of series resistors.

  6. It might be prudent to add a neon lamp or a TVS bidirectional zener type
    suppressor across the scope inputs. I'm not sure how fast a neon lamp
    triggers, but the type of suppressors used in telephone equipment should be
    pretty good, and rated about 60 volts.

    Here are links:

  7. Guest

    You could take this oscilliscope, and check the rate of transfer
    during disconnect of the 12V btty.
    This will show you when your wires are degrading. And, might even
    indicate moisture due to acidic influence, a common ailment.
  8. Jasen Betts

    Jasen Betts Guest

    at 60 they'd fire on the ring voltage.

  9. ian field

    ian field Guest

    Might not be so bad if the OP builds a scope lead breakout box to add the
    neon so it can be shunt connected between a x10 probe and the scope input.
  10. Jamie

    Jamie Guest

    If you're interested in the HV current Pulse, You can wrap a couple of
    turns around a plug wire and load the output to a low value resistor.
    This will act as a current xformer and provide you a safe a low voltage
    reference signal for your scope to monitor the current pulse at the
    electrode on the plug.
    They make engine scope probes to clamp on plug wires and in line taps
    At work, we actually use an automotive HV inline coupling to monitor
    a signal via a scope on irradiation units.

    You could also consider using a HV probe for scopes. The one you
    would need could be expensive."
  11. Jamie

    Jamie Guest

    And hope the condensor(capacitor) in the ignition system is good as you
    connect your sensitive scope to this potential arching point."
  12. Jamie

    Jamie Guest

    Invest in some TVS diodes. They'll work great for your application.
    Just select the ones that hits the limits of your scope input or just
    under it.
    Get the bi-directional types and lay one of them across the scope's
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