> I need to produce a single spark at a very precise time
 > My problem:  The FET heats up beyond reason.
 > How would I trigger a single spark from a coil, using a 555 timer,
 > based on a repeating trigger (The trigger is located on a rotating
 > shaft which houses the sparkplug... but I only need one spark).

 Try a 25-30 amp FET. The on state resistance should be about 0.06 ohms
 and will dissipate only 2 watts at 6 amps.

 Or, you can use a CDI arrangement, but you need a high voltage supply
 which could be made from a 555 and a buffer to drive a transformer.
 The supply impedance (R) needs to be high enough so the SCR turns off
 when the coil discharges. Otherwise it will trigger once and leave a
 short on the supply.

                          R         1-2 uf 400 volt non polorized
  +300 volts DC <--------\/\/\---+-----+----| |----+
                                       V           |
                                      ---          )
  Trigger pulse <--------\/\/\-------/ |  10 amp   ) Ign coil
    (+12)                510 ohms      |   400     ) primary
                                       |   volt    )
                                       |   SCR     |
                                       |           |
                                       |           |
    common negative <------------------+-----------+


Date: 1 Dec 1995 16:57:31 -0500

Original Subject: Just one spark!

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