Originally Posted by wwest
(Post 6766768)
You may note that the 2N3055HV is forward biased using the raw "unfiltered" alternator output voltage as the source.
The only source of reverse biasing, switching the 2N3055HV off, is the secondary winding of the step up transformer. Other than that reverse biasing ability occuring as the 2N3055HV collector current rises, there is basically no limit to the level the collector current can rise.
When the 3055 saturates the current through the inductor goes down and because L di/dt is negative the voltage on the base revereses and it shuts off. There was a disagreement there, some thought the core saturated.
I assume you will see, as I do, that with increased supply voltage, the harder it will become for the "transformer" reverse bias voltage source to switch off the 2N3055HV. So, the higher the supply voltage, the higher the 2N3055HV's collector current will rise for each "switch" cycle.
Let's assume that with a 12 volt supply source the collector current never rises above 6 amps. That would mean with a 16 volt source the collector current will rise to 8 amps. 8 amps into the primary of the step up transformer instead of only 6 amps....
Just so we have an understanding. The capacitor in parallel with the collector emitter of the 2N3055HV simply serves to limit, slow, the primary transformer inductive circuit risetime(***). The 82 volt 10W (24 ohm) Zener is there to protect the 2N3055HV. Once the ignition capacitor is fully charged there will be no load on the transformer secondary to be reflected to the primary so the peak rise of the collector voltage as the 2N3055HV is switched off must be limited.
Z1 does not breakdown and conduct unless you have a really high input voltage
*** The size of this capacitor, ~1MFD, seems extraordinary. Does this capacitor, along with the transformer inductance, form a resonant circuit at some specific, helpful, frequency?
C1 serves as a snubber to limit the voltage on the 3055. It also stores the energy not used in the output capacitor and returns it to the battery
So, now that we know a bit more about the operation of the Bosch CDI, why, how, did mine fail...?
There can be no question that the base causative reason was overcharging of the battery.
1. ) Obviously that might lead to overheating of the 2N3055HV.
2. ) Or it might lead to excessive voltage charge on the ignition capacitor resulting in self-firing of the SCR.
3. ) Lastly, the zener might be forced to clamp the transformer's inductive "kick" much more often.
But there remains one more clue to the failure of my CDI. Even once the source voltage problem was corrected my CDI proved to be to overly sensitive to heat, heating.
We could drive the car several miles before it would die. We could then let the CDI cool on it's own or we could "hit" the CDI case with a spurt or 2 of cool spray and then the car would again start right up. Repeatable.
I'm voting for the 2N3055HV to have been overheated to the level that an internal failure resulted in to much collector leakage current. Collector current flow even with no forwad bias at the base.
What say you, Loren. Do you have records? How did you fix my CDI..?
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