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

 Paul,

With a little finessing, you can fit three CAS pickups spaced 120* apart
into a 20B CAS housing.  That's how I'm triggering the three MSDs that I am
now running.  The CAS rotor turns at 1/2 engine speed, but there are two
pointers.  So, one of the pointers will align with one of the pickups every
120* of crank rotation.  Works for me.

Mark

Mark

Good work Mark. Cool.
So that was the original system you flew with before the RX8
trigger wheel? So Tracy redid his software or you don't use his
ignition wiring? I am glad you have some base line
fuel burn with that system.

Looks like it will work fine. At that diameter one must be careful
to get it at the exact angle. Did Bob set it up on his rotary
table?

I'll do some thing similar for the nose of the e-shaft
as soon as I get the multi-strike going except I will
need only one magnet and it will be larger in diameter
easing the angle problem.

Here is a picture of a stock 2nd gen CAS.
Got any pictures of your modification?

Is the high speed miss now gone?


BTW I think I found the ideal hot tip part. It has individual control
for both upper and lower FET. Just like the opto version but
no worriies about high voltage opto's. It is called the L6388E.
If anything it is simpler to use than the L6383E I have now
and the IR2155 used in the Aussie system.

Here is the pdf and jpg for the L6388.

Paul Lamar