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Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
I calculate the time constant of the primary (L/R) and then set pulse width to two or three times this figure, so the core is almost - but not quite - fully magnetized.
For example, I have an ignition coil whose primary I measured at 684uH and its ohmic resistance at 1R4, which by L/R gives a time constant of 489uS to build the magnetic field to 63.2% of its maximum possible value.
On the basis that there are always other delays in a system, I will ascribe a practical working time constant of 500uS to the primary of this coil.
The length of five time constants - 2.5mS - will bring the field up to more than 99% of its maximum possible energy storage, so in theory this is the maximum pulse length for use with this particular coil and corresponds to excitation with a square wave of 200Hz.
Choosing a pulse width of 2.5 time constants still gives about 90% of the theoretical maximum energy storage in the magnetic field, and corresponds to squarewave excitation of 400Hz.
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ZVS ignition coil driver!
