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Preventing the inrush current saturation in a toroidal/EI transformer
Preventing the inrush current saturation in a toroidal/EI transformer
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Old 14th September 2012, 04:30 PM   #111
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by nigel pearson View Post
................... Like the old contacts of a motorcycle points system ( HT coil LT side ) . On a motorcycle these were usually damped with a 0.1 uF paper capacitor ..........
not how the switched coil works.
The coil is an inductor passing current when the points (switch) are closed.
The points open on the cam and break the inductor current. That starts a back emf and that emf is multiplied by the other part of the coil into kV of emf that jumps the gap across the spark plug.
If that was all there was to it, that final spark is so short that it does not reliably ignite the mixture.

So they add a cap in parallel to the coil primary. That creates a resonant circuit and instead of damping, it multiplies the output but much more importantly it oscillates and creates a series of sparks . It's the relatively long duration of the series of sparks that ignites the mixture reliably.
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Old 14th September 2012, 05:19 PM   #112
nigel pearson is offline nigel pearson  United Kingdom
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Good point if the unintended pun can be allowed Andrew . I suspect I even knew that once upon a time as I used to read endlessly about HT coils . My Honda 900 was said to work better if given 2 extra volts . There were expensive coils that worked on 13.8V as replacements . It was said Honda were being crafty and using it to restrict performance ( the 107 BHP originals were trouble I read ) . I can believe it as with new plugs mine would approach 130 MPH ( in Germany , honest ) . When the plugs were a bit used 110MPH was more typical . Bike magazine raced one and used the extra battery cell . They switched to standard for road use .

The difference of note is the use of a snubber on a switch . The points is just a capacitor ( ? ) and as you say a resonant circuit . Anyone have views on ideal snubber circuits ? 0.1 uF and 100 R is common .

My observation was the spark levels give a clue as to how healthy the relay is in use . The devices are very similar . Relays say > 50 000 operations . A typical motorcar will do that in a few minutes . I suspect relays are good for millions of operations if used with care .
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