Dead time is the time between when the top FET switch is
 turned off and the lower FET switch is turned on.
 These are called totem circuits, push me pull me
 or huff and puff if it is a tandem engine airplane :)
 Normally this is controlled by and can be adjusted
 by the designer in the half bridge driver chips.
 It is like you have to turn off the front engine
 before you start the rear engine.

 I am using the AtMel ZBasic computer and opto couplers
 to simulate a half bridge driver chip. With the computer,
 the dead time can be anything controlled by the software
 but in this case I am using just a few micro seconds.

 Half bridge driver chips are normally used for
 switching power supplies that can oscillate at up
 to 100K Hertz (cps). Me and the Aussies are using
 them in a different way for capacitor discharge
 ignition systems. The top FET switch is on the vast
 majority of the time to charge the capacitor.

 When you need a spark, the top FET switch is turned
 off and the bottom FET is turned on for the length
 of time it takes the cap to discharge through
 the lower FET and the coil. Usually less than 100 micro
 seconds. Then it is turned off and the top FET turned on.

 Here is the software to do that.
 ===================================================
 Public x as long
 Public y as long

 Sub Main()
 for x = 1 to 100000000000 'keeps running until
 Debug.Print CStr(x)

 Call PutPin(12, ZXOutputHigh) 'top FET CD 1 on
 for y = 1 to 1000 '.......... .roughly 2.5 ms
 next y '...................... combined with debug.print
 Call PutPin(12, ZXOutputLow)   'top FET C1 1 off
 for y = 1 to 5 '............... dead time
 next y
 Call PutPin(13, ZXOutputHigh) 'lower FET CD 1 on
 for y = 1 to 50 '............. stays low for 100 uS
 next y
 Call PutPin(13, ZXOutputLow)   'lower FET CD 1 Off
 for y = 1 to 5 '............... dead time
 next y
 Next x

 End Sub
 ================================================

 The last thing you want to have happen is both
 FET switch's on at the same time. Our new 300 V power
 supplies have a safety feature so they would
 shut off if that happened for long. They also
 have a low voltage turn off so if the battery
 is less than 9 volts you get no ignition.
 The up side is your expensive ($150) power
 supply is not destroyed :)

 The cap is charging fine at around 16,000 RPM
 through a 2.5 ohm resistor simulating the coil.
 The Mazda 2nd gen trailing plug coil has
 less than 1 ohm DC resistance.

 The scope trace shows the voltage at the top
 of the cap. As you can see it is the power
 supply voltage for the majority of the time.
 It goes toward zero when the lower FET is
 turned on.

 BTW these are SMP60N05 FETs just because I happened
 to have a bunch. The lower the on resistance
 of the FET the better the ignition will work
 and the faster the cap will charge. I can't find
 any data on the SMP60N05 so I don't recommend it.
 Apparently it is obsolete.

 Paul

 Paul,

Will it work at 21000 rpm?  That would be 7000 rpm on a 3-rotor.

Do you really feed 300v into pin 4 of the optoisolator?

Mark

Good observation Mark.
Not yet. I am not sure feeding 300 volts into an opto
isolator will work but Pansonic claims to have some
350 volt versions.  There are some on that DigiKey
page you sent me down near the bottom. (attached)
However the switching time is real slow at 40 us.
That could be a show stopper for the opto version.
Not for the half bridge driver version however.

The current test rig is running on 8 volts.

Regarding this on a 32 rotor:
Maybe with better FETs but you would need a distributor
if you are using only one CD system with one coil.

You could make a distributor that worked off the nose
of the e-shaft. Or get a pre 1989 Mazda distributer.
As I recall those were two levels with two spark rotors so
even those required two ignition systems with two coils.

The real early models, back in the 70's, had two entirely
separate distrubtors. Fortunately by the time
the 3 rotor came out in the early 90's electronic ignition
and CAS's came in to use.

My vision is one of these for each leading plug
and something else for the trailing plugs. So they only
need to go to 7500 RPM.


Paul Lamar