Power 5 pin scooter CDI unit off 120v ac?

Discussion in 'General Electronics Chat' started by rudyauction8, Jul 17, 2015.

  1. rudyauction8

    Thread Starter Member

    Jan 27, 2012
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    So I'm building a 3 wheeler and it uses a proprietary dc powered CDI unit. I am thinking about running one of the generic 5 pin cdi's off a 120v inverter to power them, since the stator coil only generates 2.5 volts while trying to start, which is not enough to fire the CDI. I'd wire the CDI's power in to the live wire on the inverter, and ground the inverter's ground pin and cdi's ground wire to the frame. I'm thinking its a good idea to put a resistor or low wattage light bulb between the inverter and the CDI. The CDI will still get its trigger pulse from the trigger coil near the stator.
     
  2. rudyauction8

    Thread Starter Member

    Jan 27, 2012
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    Forgot to mention, I'm not ordering a stock cdi for 2 reasons: they're very expensive ($200) and they're known to fail very often. The AC cdi is technically free because I already have a dozen laying around. This fix is temporary so I can get the engine tuned up before my generic DC cdi unit comes in the mail Monday.
     
  3. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    I'm probably missing something, but what powers the inverter input?
    Is the 'generic' DC CDI likely to be any more reliable than the stock CDI? If so, why?
     
  4. rudyauction8

    Thread Starter Member

    Jan 27, 2012
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    The inverter input is powered by the battery. The regular CDI requires 30+ volts (up to 200 at high rpm's) AC to function. The DC CDI has a dc-dc voltage booster built in.
     
  5. rudyauction8

    Thread Starter Member

    Jan 27, 2012
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    OK I gave it a shot, and it shorted the inverter every time. Then I wired a 60w light bulb in series and now I have a nice strong spark! Now I can focus on fixing the rest of the engine until the DC CDI comes in the mail. I'm also thinking about using 100w equivalent LED bulbs run from the inverter as headlights. Also I'll be replacing that 60w bulb with a small resistor bank.

    The generic DC CDI is watertight and known to be reliable if treated right, while the stock box is notorious for going bad all the time especially when exposed to water (it does not have any type of seal) according to several shops that have worked on these 3 wheelers, and I plan to do quite a bit of mudding so waterproof electronics are a must.

    As a side note I can now use these CDI's with standard ignition coils to generate high voltage whenever I need it.
     
  6. ian field

    Distinguished Member

    Oct 27, 2012
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    Most CDI schematics I've seen show the capacitor charged in series with the LT winding and the SCR briefly shorts the inverter.

    I've no idea how that doesn't damage the inverter, or how they commutate the SCR. Many 80s era motorcycles had a high voltage winding on the generator stator for the CDI. So no inverter.
     
  7. rudyauction8

    Thread Starter Member

    Jan 27, 2012
    252
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    This is a 1980's kawasaki 3 wheeler (klt200a), and they have an odd ignition system. The stator charges the battery (through a regulator/rectifier box) and the battery powers the CDI. They're known to lose spark while starting with a weak battery. The CDI I'm using is one of the chinese 5 pin black boxes, and it shorts the inverter even without being triggered, yet with series resistance it fires fine. The 60w bulb runs about half brightness and that doesn't change when I start firing the CDI. The CDI only fires when triggered by the trigger coil, I can have it powered up and no spark will come until I crank the engine.

    Remember the inverter is not part of the CDI, it simply gives the high voltage AC needed for it to work. In these CDI's is a diode-capactior voltage multiplier to provide the hundreds of volts fed to the coil. On mine the output to the ignition coil with the inverter is about 250 volts. On the DC version there's a DC-DC converter built in, that's why they are larger than the standard AC version.
     
  8. rudyauction8

    Thread Starter Member

    Jan 27, 2012
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    Also I think these CDI's charge the capacitor through a voltage multiplier and rectifier and the SCR shorts it through the ignition coil.
     
  9. rudyauction8

    Thread Starter Member

    Jan 27, 2012
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  10. debe

    Well-Known Member

    Sep 21, 2010
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    This is what I made as an inverter for a CDI unit. & it handles the shorting on the power feed when the SCR fires fine. I made it to use on an engine on a generator that the HV CDI winding was burnt out. CDI  3.JPG
     
  11. rudyauction8

    Thread Starter Member

    Jan 27, 2012
    252
    2
    Nice! The DC version has a similar circuit built in but remove the rectifier on the output and it'd work great for the AC version. If I have trouble with the DC version I'll give this a shot. I do have some small transformers from a couple 100 watt inverters laying around that I can use.
     
  12. rudyauction8

    Thread Starter Member

    Jan 27, 2012
    252
    2
    So I have a question for you guys: On a motorcycle regulator the regulator circuit supposedly shorts excess current to ground. This seems inefficient, and wouldn't it generate lots of heat in the regulator and stator? It seems that the regulator on my 3 wheeler gets hot when I used it to charge several batteries (less power being shorted) and stays cool when just running the ignition (more power being shorted). Is this what really happens or does it work in a more logical way of simply disconnecting the stator from the battery when it's full?
     
  13. ian field

    Distinguished Member

    Oct 27, 2012
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    Apparently the PM alternator is a current source - so shorting it doesn't do any harm, in any case, the regulator connects an SCR in parallel with half the rectifier diodes so it only shunts alternate half cycles.

    Better quality motorcycles have a field coil alternator that works the same way as the car type.

    The old British bikes with Lucas electrics used a dirty great 100W 15V zener diode.
     
  14. rudyauction8

    Thread Starter Member

    Jan 27, 2012
    252
    2

    I guess I was comparing the generator to a stalled DC motor. Now that I think of it you're right, it won't generate enough current to cause damage. But I think it will fight the motor a bit, think of spinning a small motor while shorted vs open circuit, it will be harder to spin when shorted.
     
  15. rudyauction8

    Thread Starter Member

    Jan 27, 2012
    252
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    `
    I did some testing and it looks like the regulator does short the coils. With it disconnected the coils go up near 100v and when connected go down to about 14v, even when the battery is full.

    The inverter powering the CDI would only let the engine rev to about 3,500rpm so I got a 120v-9v ac-ac transformer and wired it in backwards to feed ~100v to the CDI directly from the stator coils. I'm going to keep the inverter because I'm thinking about using an indoor LED 100w equiv. floodlight as a headlight.
     
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