swbtm

iTrader: (2)

Hello,

First off, thanks to Kent for the great write up on how to get the Transistor Trick circuit made for using the second gen coil and ignitor. I'm just about finished with the build, but I've run into a question that I want to make sure I get right the first time.

Where does the external resistor pack get wired into the circuit?
(1, 2 or 2.5 ohm; the one I made is 1.9 ohms mounted to a block of aluminum)

The second gen coils have two different internal wire setups and I'm not sure if the guidance I've found is for one or the other. The instructions that I've followed up to this point are in the attached word document with reference links back to where I found the info.

I'm pretty sure that I just need to take the wiring from the existing 0.35 ohms ballast resistor and connect my external one in its place. If anybody could confirm this for me that would be appreciated.

Thank you

Here's a few pics of the setup

and a few detail shots if anyone is interested.




PS: does anyone know if the heat dissipation from those ceramic wire wound resistors are omni directional? Is one side better to have in contact with the aluminum? The other side is going to be a sandwich of some leftover heat sink compound (the strips, not the paste, from a computer build) and the aluminum cover that comes with that radio shack build box.

swbtm

iTrader: (2)

Wanted to make some type of progress today, so I went with the assumption that the new resistor pack is a direct replacement for the existing ballast resistor. If anyone knows if this is correct or not, please comment. It's raining today so the car isn't getting started anyways, but all the prep work is done. This whole beastie is getting mounted to a section of aluminum box channel that has been opened up and flanges bent out to mate up with the 2nd gen mounting plate. Just made an already large thing even larger. Good thing my ac compressor is long gone. All the connections are wrapped in marine heat shrink, a bit stiff but the adhesive makes them weather tight.

Thanks again

swbtm

iTrader: (2)

After doing some research I still have a few questions for anyone out there, hoping someone with one of the early auto switching versions, Jeff or Kent can chime in on this please.

Reading through the entire development thread:
https://www.rx7club.com/1st-gen-arch...gcdfis-478518/
it stops right around the time when the first buy of the auto switching boards were being delivered.

The understanding I have to date is that the simplified circuit in the archives and attached in the word doc above allows the J109 to vary the pulse width according to rpm and eventually at high rpm the J109 is not able to provide a long enough pulse time to provide a good spark. The fixed pulse width (2ms) was the fix for this and provided enough time for the coil to charge and provide a good spark up to a high rpm (bench tests results are discussed in the development thread but the pics from the oregon state university host are gone, does anyone have local copies of the pictures that were hosted at the osu sight?) but didn't work so well at low rpms, hence the auto switching.

There was a bunch of discussion early on about the 2nd gen coil's ignitor heating up as well. There was one guy who pointed out that this was probably due to the resistor heating up and not the ignitor. I'm guessing this is where the origin of the externally mounted resistor came from, but I haven't been able to track that down.

Most of the people (except one with a turbo that sounded wonky) were reporting good results from the auto switching board.

Found this history of the development work.
Transistor Trick for 2GCDFIS

It mentions that there was a design flaw in one of the later auto switching boards. Does anyone have information on what that issue was? Is it inherent in the auto switching boards that were delivered in the first buy?

Since the internal resistor was most likelt the culprit of the heating, replacing that one with an external makes sense, but what doesn't make sense to me is the recommendation to use a higher resistance value versus using something similar to what was originally there?

After talking with an electrical engineering friend who used to race an FB he pointed me in this direction and got me thinking about it.

The question he posed to me was:
Why would you want to add a higher resistance value in series with the primary coil?

In effect the higher resistance lowers the available current flow to the primary coil and lowers the overall energy available in the system to provide a spark.

In terms that I had to think of it in my sleep deprived state. The existing 0.35 ohm 5 watt resistor allows a certain amount of flow into a bucket per unit of time. When the bucket is dumped out there is a certain amount of energy associated with it (voltage dump to spark plugs producing spark). Adding a 1 ohm, 1.9 ohm, 2 ohm, 2.5 ohm resistor into the circuit effectively lowers the amount of flow into the bucket over that same unit of time and gives you a lower potential of energy.

So then, why not use an externally mounted 0.33 ohm, 100 watt resistor? Similar to this:
HS100 R33 J ARCOL / Ohmite | Mouser

The 100 watt rating is the amount of heat dissipation potential the resistor and heat sink. My EE friend explained the reasoning for using that wattage here:

The resistor in series with the coil makes a voltage divider. There is a voltage drop across the resistor and the rest of the voltage is across the coil. Assuming the coil is running at 6V, there is 6V across the resistor. That would make the power dissipated by the resistor 109 watts. I think the normal duty cycle of the coil being energized is about 60%, so it will dissipate about 63 watts, so the 100W one is actually probably a better bet. If it is actually a .6 ohm primary coil, the coil voltage at .33 ohms is 7.74 volts, so 4.26 across the resistor at 12V. Since running the voltage could be 14.7 volts, 9.48 on the coil and 5.22 on the resistor so the power dissipated is 82.4W so go with the 100W.

This assumes a 60% duty cycle over the entire rpm range, I know this isn't true, using this value to give a safe cushion for the rating.

These are the specs for the 2nd gen coil found in the second gen section,
FC coil: inductance = 3.5mh, Q = 3

Leading Coil:

When testing the primary coil (A to B) winding. Meter should display around (0.6) ohms

When testing the secondary (L1 to L2) winding. Meter should display 12.56k ohms.


The takeaway is that the circuit with the higher resistance will work but not as well. Why try and control a heat problem by lowering the amount of current? Why not address the heat issue directly and maintain the current flow? My guess right now is that there is something in the TT circuit that I am not accounting for in that train of logic...?

Thoughts, anyone? It seems providing more current to the coil (or really the same current it is expecting to see stock, maybe a little higher since that is a .33ohm vs a stock 0.35ohm) would be a good thing when trying to make a consistent strong spark, especially for those running boost.

Either way I have to take apart my contraption shown above, I put the thermal compound on the wrong side of the resistors. It should be between them and the aluminum to fill up the little air gaps between the ceramic and the aluminum.

Also, if anyone has info on what exactly the issue is when using the auto switch that would be appreciated. If it was a later development change and the early auto switching circuits work just fine, will those every be up for sale again? It would be nice to have a pcb or etched one vs the radio shack perf board that I would end up building myself.

t_g_farrell

iTrader: (1)

I have to ask this, and I actually built one of these when it was first being done, why are you trying to use this setup?

Hats off to Kent for doing the legwork but it turned out to have some issues in all versions and
was pretty much dropped after awhile. I don't recall exactly all the issues but one was the use
of the 2G coil and its reliability when used.

I never actually hooked mine up and later proceeded in a different, more COT ignition setup
that has been working well for me the last few years.

Jeff20B

iTrader: (13)

Agreed. DLIDFIS is what I use. It's all I use, because it works.

swbtm

iTrader: (2)

Why?

- Curiosity
- It gives me a simple project to relearn/remember how to solder and gives me a good idea if I can even attempt to take on a megasquirt build.
- Gives a good excuse to learn more about the car and ignitions systems in general.
- Gives me a good background so I can understand the DLIDFIS when this one is done
- Understanding the shortcomings with the 2nd gen coil will give me as much, if not more info compared to something that works.
- It is much easier to find information on successes in the forum, in many hours of searching I haven't found any info on what the problem actually is (I might be using the wrong search terms though)
- Bullets seemed appropriate for this response.
- Well actually they are dashes
-

I think that pretty much sums it up.

t_g_farrell

iTrader: (1)

Ok, cool, sounds like you have the right expectations.

Note that rotaries always have weak ignitions, all the 7s, all generations, weak ignitions. Even
the 8, the first mod most folks do is to upgrade the coils. So any improvement in the ignition is
worth it.