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Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141
Dr. Kilovolt, the 100 ohm and electrolytic are to provide 12 Vdc for the 555
The 0.1 uF and 1k were already in the diagram, they form a 'snubber' Automobile ignition coils based for the Kettering system used to have 0.1 uF direct from the 'switched' side of the coil to 0 V mainly to slow the dV/dT to allow the contact points to open with reduced arcing with a transistor switch it just slows dV/dT for less stress on the transistor. I don't use RC snubbers myself, I prefer a hv diode and TVS//cap.
Theodore, I see that you no longer have the 0.1uF & 1Kohm across the coil. If you do use it be sure that the 0.1uF is rated 400V or more, or it will short-circuit the flyback 'spike' Try adding capacitors across the 1nF capacitor, e.g. 10nF, 22nF etc.
Registered Member #2099
Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716
Dr. Kilovolt wrote ...
Sulaiman, what is the purpose of the 100 ohm resistor and the RC across the coil? We're trying to produce the highest voltage "kick". And with the 100ohm resistor, the output will be almost nothing. ... Also don't forget the possibility that the coil might be fried.
Sulaiman has raw 12V applied to the coil. [oops, I missed his earlier response] The 100 ohm is part of an RC filter delivering power to the 555. (an excellent idea IMHO; voltage regulation is unimportant, while filtering of harsh spikes is of the essence).
The coil can be tested by itself, in a circuit with no semiconductors outside of the 12V supply. Just simulate a breaker point ignition. Please try to understand some reference like before proceeding. Make and break the connection between low side of coil and GND, by manipulating an insulated wire. Doing that without a HV load (sparkable path to GND or +12V) is hard on the coil. The voltage spikes >> 12V appear across coil primary when the connection is broken. Continuous connection of 12V (many seconds) can overheat the coil primary winding.
Note that in normal Kettering ignition, the firing rate is much less than any resonant frequency of coil. The "on" time must be long enough (milliseconds) for primary current to build up to maximum. The HV spark happens at the very beginning of the "off" time, and may be a damped oscillation at frequency that depends on inductances & capacitances of coil (8 kHz?).
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555-based ignition coil, assistance requested
