Driving an MSD Ignition Box

[Michael F. Sargent]

> Probably the easy way would be to call Autotronics and ask them.  I'd be 
> curious to find out what it takes to fire those.  Good Luck!

So, I called MSD. They're very cooperative.

The white wire (points) trigger is connected to a 40 ohm pull-up to +12
volts. Connecting a transistor between the white wire and ground will
trigger the box. The transistor should be rated between 30 and 50 volts,
and will need to be able to handle at least 300 mA. The engineer I
talked to suggested an MJ520, but the requirements are so easy to meet
that there are a large number of transistors that will do the job.
Because the input is purely resistive, there is no need to add
additional components to handle back EMF, etc.

As for the trigger pulse, he suggested that the pulse be a positive
going pulse of at least 0.5 mS duration. The duration is needed because
(as Bruce suggested) there is a debounce circuit in the MSD box. So,
normally the trigger will be at ground level, going high when the box is
to fire, held high for at least 0.5 mS, and then returns to ground.

Ed Lansinger was asking for some more information about the MSD boxes,
so I'll try to summarize some of the information from the catalog.

Specifications (for MSD-6A, -6AL, and -6M-2):

Operating Voltage:		12 VDC
Operating Current:		10 A @ 10,000 RPM
Maximum RPM:			10,000 (8 cyl)
Spark Duration:			20 degrees (8 cyl)
Energy Output:			600 milli-joules (Cap. Discharge)
Output Voltage (to coil):	450 V
Output Voltage (coil out):	40 kV
Weight:				Varies by model between 2 3/4 lbs and 4 lbs
Size:				Varies by model; max. 8"L x 4"W x 2 3/4"H

A scope trace diagram (without units) shows the multi-spark output
versus the output of a conventional ignition. The MSD box is shown as
producing 5 sparks for each one produced by the conventional ignition,
at 1,000 RPM. Each of the multiple sparks is shown as having similar
voltage and duration to the conventional spark. As RPM increases, the
number of sparks decrease. I can't find anything in the literature to
confirm this, but I have heard it said that above 3,000 RPM, the MSD
boxes are only giving one spark.