HappyTime 2 Cycle Ignition Circuit Explained with Diagram

Discussion in 'Electrical' started by VTBikeman, Sep 12, 2008.

  1. VTBikeman

    VTBikeman Member

    As promised: A schematic of the HT ignition circuit and some musing.

    The magneto (generator, alternator) has 2 poles. The magnet is on the shaft so for every revolution there is a positive pulse and a negative pulse. The diode selects the pulse as the piston nears tdc and blocks the pulse that is made as the piston nears bottom dead center. The voltage I measured is about 120 volt peak and probably goes up with RPM. I used a drill to spin the motor with the plug out to measure this.

    The capacitor tunes the coil to resonate at a frequency that will give the coil's highest output.

    That actually leaves the negative half of the wave unused and if you use a half wave rectifier you should be able to tap some power off that low voltage winding to do something like charge a few batteries without affecting the spark. I didn't measure the output of that winding yet but the published spec is 6 volts or so that could yield a DC peak voltage of about 8.4 volts, not enough to charge the batteries (12v) on my bike directly.I also do not know which polarity you would get. You may wind up with a negative supply (pos ground) which is no big deal unless you've grounded your electric system as neg ground. You could probably still pull 3 watts off that winding without harm as you are using it 1/2 time. Feed a small dc:dc inverter and there's your battery charger.

    There is no scr because there is no separate trigger for it. The spark is generated solely by the magneto.-simple and elegant. Could a CDI be built for it with the oem alternator? Probably but you would need a separate detector to trigger the spark and one end of the magneto is grounded so the choice becomes a 1/2 wave rectifier or a voltage doubler to charge the big C. Once you have the cap charged up then the motor would start.

    There are some CDI schematics on line that use a microprocessor to advance the spark at higher rpm's but from the curve the advance doesn't kick in until 5K rpm's and now your up to the upper limit of the engine anyway. Has anyone studied spark advance for that motor? Maybe it would help at a lower rpm's.

    I Hope this helps.

    Attached Files:

  2. arceeguy

    arceeguy Active Member

    I am thinking that there has to be more to the ignition than that.
    Reason is that there needs to be something triggering the spark, as just connecting it to the magneto (even with diode and cap) will just produce a spark whenever the voltage potential reaches approx 2600 Volts to the plug. (using 75,000 volts for a spark to jump a 1 inch air gap) So as the engine RPM increases the timing will be greatly affected as magneto voltage output rises. If this is all that is needed for an ignition system, we would have seen breakerless ignitions in small engines and motorcycles in the 50's and 60's when silicon rectifiers became inexpensive and in widespread use.

    I am thinking that there is some sort of solid state device (transistor/SCR) that is triggering on the falling edge of the waveform. If you look at a magneto on a weed whacker or lawn mower engine, there is no separate trigger coil unit. Just a coil and associated electronics all in one, with the spark triggering done by simple wave form processing.
  3. VTBikeman

    VTBikeman Member

    Well, there are no more wires than what I showed and I took some measurements to confirm the presence of a diode and someone else reported the presence of a cap so I drew what I thought was in the box.

    I get resistance measurements that confirm uninterrupted circuits in the magneto and the box. Maybe there's an SCR that triggers at some point but if that were so I wouldn't get a constant diode drop (.512v) when measuring the input to the device. The cap would charge to a volt or so and the current would stop.

    The spark starts when the magnetic field collapses in the spark coil, not when it is fed. If that cap charges up to the peak voltage then the collapse starts as soon as the input goes below the charge voltage as the diode blocks any reverse current. The cap discharges into the coil and then the field collapse starts.

    All the lawn mowers I grew up with had a magneto with the magnet on the flywheel with the spark wire going right to the plug and a switch attached to the throttle to short some turns to kill the motor. No timing issues as it was made to run at a constant speed.

    The only difference here is that the spark coil is outboard in an epoxy box, like a HV supply in a tv. Probably cheaper and safer and more reliable this way. We do have modern insulators available now.

    Remember- this is an engine that costs very little money. It only has to work, not work great. I'm amazed it works as well as it does.
    Last edited: Sep 12, 2008
  4. arceeguy

    arceeguy Active Member

    Right, but there was a set of points to precisely time the collapse of the magnetic field and produce a spark.

    Producing a spark the way you describe is easy, but timing it precisely enough for an engine to run well over a wide RPM range (1500-5000+RPM) is a different story.

    There has to be more in that box IMO.
  5. VTBikeman

    VTBikeman Member

    I think the diode gives it the precision as it shuts off the magneto current when the magneto voltage drops below the capacitor voltage. Then the cap has to discharge through the coil. The cap will charge before the coil conducts as the coil resists a change in current, the main characteristic of inductance.

    Now that I think of it lawn mower engines did have points but as a 4 cycle engine with 1 magnet flying around the magneto it had to fire on ALTERNATE revolutions and it ran off the camshaft which turns at 1/2 speed the speed of the crank. I don't think it did critical spark timing functions. I think it just killed the spark on the exhaust-intake cycle.

    There has to be a small engine repairman out there who can set this straight.

    If you have an extra box take it apart. I'd love to see it. I went on what I could measure and what others had to say about what's in the box. There is certainly a diode voltage drop and it is continuous indicating the circuit I provided is consistent with the measurements.
  6. arceeguy

    arceeguy Active Member

    A small B&S engine with points/condenser magneto ignition fires every revolution. The points are opened and closed by an eccentric on the crankshaft, not the camshaft. The points are under the flywheel. It actually fires during the exhaust stroke too, but nothing happens. A lot of cars do the same, it's called "waste firing". (for example, a six cylinder engine with 3 "direct" ignition coils, two plugs per coil - one cylinder fires on compression stroke, the "sister" cylinder fires simultaneously on the exhaust stroke.) The breaker points perform the essential function of interrupting the current through the primary of the ignition coil, causing the magnetic field to suddenly collapse and generate the high voltage pulse in the secondary winding. For the ignition coil to function, the power must be "pulsed" in this manner whether it be from mechanical breaker points, a transistor, or an SCR dumping the charge of a capacitor. (CDI) The "tuned circuit" you theorized would not work for an ignition application.

    The "diode voltage drop" you are seeing can be caused by any number of solid state components. A pocket radio will give you about the same reading, and there is much more to a radio than a diode, capacitor and transformer.

    In another thread, someone hacked apart a HT ignition module and posted pictures. I think the results were inconclusive because everything was still basically covered in epoxy.
  7. HI,

    Just curious, but with everybody looking for better ignition strength and more power to run lights etc....How hard would it be to modify the HT engines to make 12V power at some acceptable wattage?...Is it as simple as messing with the windings (as in adding more?).

  8. arceeguy

    arceeguy Active Member

    I'm working with a local generator/starter shop to rewind the coil to produce 14V,and run a motorcycle CDI unit and 12V lighting. The tech at the shop says I should get about 30 watts output. I plan on using a trail bike rectifier/regulator and a small lead acid battery. The CDI is less than 20 bucks, but it will require me to rig an external trigger coil and sensor magnet to the HT engine.
  9. VTBikeman

    VTBikeman Member

    I hooked the box up to an isolation transformer and a variac and took wave forms for input voltage and input current at an input voltage that creates a spark, about 80 volts ac at 60 sparks/sec. Gives about 1/8-1/4" spark.

    The spark timing is independent from the input voltage and starts at the end of the charging cycle that uses the negative half of the power wave from the alternator. The cap is charged during that cycle and it is fired, probably via an scr, after a fixed delay from the end of the charging cycle. The timing may be accomplished with a cheap timer chip or a microprocessor or some other method. This syncs the spark to the output of the alternator independently of the voltage.

    I'll drive the isolation transformer with an audio amp and see what happens to the timing vs frequency.

    This is indeed more sophisticated than I thought. I am going to retract and redo the circuit. What's in the box has to be diagrammed in block diagrams since it is potted in epoxy. There are different ways to do the timing in either the analogue or digital domain.

    It is indeed a CDI and I have the wave forms to show it. I just don't have the circuit in the black box.

    You are correct about waste firing on a 4 cycle engine.
  10. arceeguy

    arceeguy Active Member

    Wow - great work VTBikeman! I like your idea of driving it with an audio amp and signal generator, this way, you can test if it has some sort of rev limiter without putting your engine at risk!

    As far as what is inside the box, I doubt there is any embedded processor considering how inexpensive they are. I just think there is a diode, capacitor, and SCR and a few other components (maybe a R/C network and a transistor) to trigger the SCR on the falling edge of the AC wave form. (after the cap is charged) If there is any timing advance, it is probably done with the SCR trigger circuit and not with an "intelligent" processor.
  11. VTBikeman

    VTBikeman Member

    I don't think there is an embedded processor as it wouldn't like the Hv environment. It triggers on the rising side of the wave form. When I drive it with an amp I'll know if there is any advance at all. The spark occurs about 10% of the way into the rising waveform and it looks like there is plenty of room for the speed to double or triple before it runs out of room. That is faster than the motor can probably go. 60 hz= 3600 rpm.
    The circuit charges on the negative half of the waveform so technically I guess that is the start of the cycle which puts the spark at about 60 to 70% of the time in.

    When I post the photos you can take over figuring it out. My head hurts.

    Again, timing is independent of voltage in.
  12. arceeguy

    arceeguy Active Member

    That makes sense.
    So I guess you have a dual trace scope and took photos of the input and output of the CDI?

    I have a variac, and an isolation transformer so I could duplicate your test. I also have a signal generator and an amplifier that could feed a 12V transformer as a "step up" to drive the CDI. The big question is if the CDI likes rock, country or rap music. (or perhaps even disco?)
  13. VTBikeman

    VTBikeman Member

    I took photos of both the input voltage and input current across a 10 ohm resistor in the ground leg of the transformer, not the box. I left the box on the bike, ground connected as usual, put a screw in the spark lead and held it close to the motor to see the spark. The spark trigger is obvious in the scope trace with sharp spike and some HF ringing on the input voltage wave form. I used a 12 watt 1:1 transformer which limits current. The motor alt winding is 320 ohms or so so it also limits current.

    I think the diode may short out the negative input pulse due to the heavy loading on the voltage wave form and the current waveform shows current draw. This is consistent with a steady diode drop of about .595 volts. The heavy loading on the negative side shows up as severe clipping with some tilt so some cap is getting charged somewhere. It may a lower voltage CDI running on 120 volt peak, not rms.

    Timing is not voltage dependent so it isn't a zener or other voltage dependent device tripping it.

    I also don't see any hash that might indicate an inverter.

    Maybe a 555 timer chip with a film cap to determine delay timing in the cycle??? This is all relatively slow so the delay may not require super accuracy. Engines run even if the spark isn't exactly where it needs to be.

    Will post photos later.
  14. VTBikeman

    VTBikeman Member

    These are shots at 60 hz (=3600 rpm).

    I ran it as high as 500 Hz (=30,000 rpm) and it still fired but things looked worse. Of course that is faster than any of these engines will ever go.

    The timing from A to B seems to be constant even when I drive it from an amp into a step up xfmer and vary the frequency.

    The - input voltage peaks under load at about -20V and the negative current is fairly sinusoidal, with less distortion than my off - the - line ac wave. There is some kind of load that behaves resistively after the diode. If it was a capacitor it should show a tilt. It could be an inverter but it is awfully quiet.

    I ran neg DC into the box from a current limited supply. I could dump 275 ma at 15 volts into it continuously. That doesn't indicate to me an inverter whose input current should go down as a cap charges. It looks like a fairly low ohm resistor.

    At higher frequencies the negative current was greater, off the scale.

    At B you have the spark evidenced by ringing and overshoot.

    Note the small positive current starting at A and note that it doesn't start until the input voltage in the reference frame hits 50 volts. It also peaks and goes to 0 current at spark time. That indicates to me that there is a cap or a circuit that is 50 volts above ground.

    There is no neg current drawn until the input wave falls to 50 Volts. Note the heavy negative current and note the assymetrical wave.

    It is your turn to figure this out. I have to give the designer credit, it is quite elegant.

    This is like doing the NYT Sunday xword puzzle. Quite a brain teaser.

    I'd like to kill my first post with the photo as it is obviously wrong and the post on the other list which is obviously wrong. It was a good guess but no cigar.

    Attached Files:

  15. VTBikeman

    VTBikeman Member

    Photo at 500 hz, (30,000) Rpm.

    0 point of the lower trace is 1 division higher and shows -600 ma peaks.
    As you can see it is running out of time on the positive peak to fire.

    Varying the frequency without changing the sweep rate showed a constant time from A to B as in the other photos.

    I attribute some of the differences to the driving amplifier/transformer characteristics. The amp didn't like being driven too hard at 60Hz but was OK at 500.

    Attached Files:

  16. arceeguy

    arceeguy Active Member

    Great information, thanks!

    We may not know exactly what is inside one of these two wire ignition boxes, but thanks to your super sleuthing, we know that it is a "true" CDI and it does not have any sort of rev limiter.

    I wonder why these units are subject to SIDS. "Sudden Ignition Death Syndrome" Could it be transient spikes from the magneto instantly destroying the circuitry? If so, maybe a couple of back to back Zener diodes or even a small capacitor across the magneto could prevent them from dying? Hmmmmmmmmm.......
  17. VTBikeman

    VTBikeman Member

    SIDS? Not so far. May be just that it isn't made to mil spec or component selection isn't high quality. I doubt that a zener or a cap will save any given all the odd waveforms. It may even delay the spark a degree or 2 if the cap is too big or it may load the alt.

    I'd bet a bad ground could kill it. The ground and hot connectors are cheesy and could easily open from vibration.

    Who TF knows? Anyone with an xray machine out there?

    I've spent way too much time on this nonsense. I get on the bike and it goes forward which is all I care about.

    I have some other ideas to think about.
    Last edited: Sep 13, 2008
  18. VTBikeman

    VTBikeman Member

    Sorry, I didn't mean to sound so mean about it. It was great fun but at the expense some other things that I SHOULD have been doing.
    You know . . . Would-a . . . Should-a . . . Could-a . . .
  19. arceeguy

    arceeguy Active Member

    Maybe the CDI is more reliable than I thought. I guess we only hear the bad things about the engines, while the vast majority are chugging along just fine. I dunked my CDI in a glass of water while the engine was running and it didn't miss a beat submerged for 10 minutes - but then again, I haven't started it today. :shock:
  20. emdude

    emdude Member

    for yor considereation...

    ...regarding the ignition system:

    Based on the thread, the scope shots and speculation as to how the CDI box works I have the following comments. It is in fact a CDI unit, that is a "capacitor discharge ignition" system. I looked at the magneto coil in my engine (haven't even installed it in a bike yet) in order to get some idea of how the thing must work and what if any timing advance there may be. Noting that the engine runs clockwise when viewed from the left side and noting TDC of the piston, it becomes clear that the magnet rotor passes the (approximate) center of the coil a few degrees before TDC. Ignition ALWAYS occurs when the positive going voltage begins to decline. Or so I surmise based on the research done and explained in the thread. There will be a very slightly advanced spark at higher revs because the "pulse" as it were is of shorter duration and the declining voltage does so a bit sooner. However I expect this to be nothing more than a side effect of the design and not any intended advance.

    As to how it works:

    It's simple...and yet maybe not. The high voltage from the magneto coil to the CDI unit...or what ever you really want call it clearly indicates a capacitor discharge system. The diode and cap shown in earlier drawings are correct. The cap is charged on the rising voltage. The "primary" of the ignition coil is most likely in series with an SCR to ground. The SCR's gate is held low until the voltage begins to decline. A few volts or so after peak, it is triggered by some circuit which of course fires the coil by discharging the cap through it. Up to that point it's simple. A circuit to do this could be made from what most have in their junk boxes.

    A problem may be that since once the cap is discharged the process could start all over even on the now only declining voltage. So there may be something in the unit that only allows the above triggering to occur once for every positive going pulse. But this could be worked out as well.....actually on second thought, that may not be that difficult either.

    I ponder all this because I wanted to know if the timing on these engines is static or has some type of dynamic advance or retard. It seems two strokes do better with more advance at low rpm's and less advance at rpm's. If this engine is set up for optimum advance at lower to midrange rpm's, gains could be had by creating a new CDI unit that functions as the above but has electronic delay for higher rpms. i.e. retarded timing.
    Last edited by a moderator: Dec 15, 2015