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Off topic? Anyway CDI article.

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ozieagle Avatar
ozieagle Gold Member Herb Adler
Geelong Victoria, Australia   AUS
1958 Wolseley 1500 "Wooly"
1966 MG MGB "Bl**dy B"
Maybe this should go into the Off Topic forum, but since there has been a lot of discussion about points and Pertronix and electronic ign, I thought I'd post it here. It is quite pertinent to MGs as well as other cars.

Back in the 70s, I and my colleagues, were into CDI systems and built units according to the article attached. My unit worked for about 20 years, till I bought a new car with computer control. The CDI unit really made a difference to cold starting and running, being able to push the choke in at the end of the street, rather than miles down the road. Plugs started at 40 thou, and after a considerable time were checked and found to be at 80 thou. No performance degradation. Reset them and replaced for another extended time.

Current sensing tachs will not work, since the switched current is far too low. They can be modified, by winding 16 turns of finer wire around the current sensing transformer. either the external one or the internal one and connecting this wire in series with the points circuit. The power to the CDI must then be taken from a separate switched circuit. Ballasted systems should have the ballast resistor shorted out. Note that in this case it will not be viable to revert to standard ign, because the coil / points will be over loaded.

Herb



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Attachments:
Cdi complete.PDF    2.34 MB

Fred Winterburn Avatar
Ripley, Ontario, Canada   CAN
Interesting article. The circuit is similar to my father's, Lloyd Winterburn and my version, with some exceptions:
- The trigger circuit is the same for both positive and negative ground from my father's patent except for the complication of the extra transistor, which arguably is not necessary if the other component values are selected properly. The inverter runs at a very low frequency on this one so the transistors will be working harder and the zener diodes absolutely necessary to quell voltage spikes that could damage the transistors. The text says 50Hz, but I can't see it being that low, maybe add a zero and call it 500Hz. The old Hyland used a ferrite core and operated at 8kHz. The one I build will oscillate at no-load at 14 kHz (just outside my hearing range now), but easily heard by a teenager if they are close to the unit. I also use a small ferrite core.
- The voltage output at 400V is 100V higher than it should be for reliability.
- The current through the points with 50 ohms is marginal on a Lucas distributor. Should have a little more current to keep the points clean. I use a 39 ohm resistor for this.
- The graph showing relative spark energies is wrong. One can't equate the energy stored in the capacitor to spark energy. The total energy output for CDI is generally lower than Kettering, and this one will most definitely be lower. The spark will have a much higher power than Kettering, delivered in two or three current peaks with that design (alternating polarity like mine but fewer peaks).
- The unit I build would only need the ballast resistor bypassed if the spark rate of 533 sparks/second was needed. If the ballast resistor is bypassed, one should go to a 3 ohm coil so the points don't burn when the switch is selected to 'STD'. Keeping the ballast resistor in the circuit will not affect the ability of the unit to produce a spark at very low battery voltage. The unit amp draw is so low at cranking speed, that having the ballast in the circuit makes no difference. It will only make a difference at a very high demanded spark rate as the current draw increases with rising rpm (the opposite of Kettering). Fred

In reply to # 3571995 by ozieagle Maybe this should go into the Off Topic forum, but since there has been a lot of discussion about points and Pertronix and electronic ign, I thought I'd post it here. It is quite pertinent to MGs as well as other cars.

Back in the 70s, I and my colleagues, were into CDI systems and built units according to the article attached. My unit worked for about 20 years, till I bought a new car with computer control. The CDI unit really made a difference to cold starting and running, being able to push the choke in at the end of the street, rather than miles down the road. Plugs started at 40 thou, and after a considerable time were checked and found to be at 80 thou. No performance degradation. Reset them and replaced for another extended time.

Current sensing tachs will not work, since the switched current is far too low. They can be modified, by winding 16 turns of finer wire around the current sensing transformer. either the external one or the internal one and connecting this wire in series with the points circuit. The power to the CDI must then be taken from a separate switched circuit. Ballasted systems should have the ballast resistor shorted out. Note that in this case it will not be viable to revert to standard ign, because the coil / points will be over loaded.

Herb



'Anyone who likes liver, can't taste it'
'If you want to repair car electrical systems successfully, learn Ohm's Law'.

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