Here's another thought, with a isolated ground probe, why not just measure the voltage directly across the coil's '-' and '+'? This would show exactly what the voltage is doing across the primary inductor, right?

Seems to me that simple test would tell us exactly what we want to know, how much voltage is at the coil when it first starts to charge, then as it charges the voltage drops till it saturates or gets current limited. I envision this scope trace would start at a voltage around 10-12v (just as the coil turns on) then slope linearly downward toward 0v and at some point it (as it approaches saturation or current limit) it will flatten off likely below 5vdc?

Thoughts?

I found a coil that is being sold by ************ that should be satisfactory with my original CDI unit and points system on my 1973.5T 2.4/CIS. They claim it should be a perfect replacement for the older and reliable black Bosch coil. I will remove the MSD blaster and make the switch. If I can make long runs without a shut down, then I will be happy. I will give it a go.

Bob

Bob,

I think you are referring to this coil?
https://www.************.com/p-1586-cdi-ignition-coil-by-************.aspx
Designed to replace the original bosch coil 901.602.502.00

Yep, that's the one. They claim it is compatible with the three pin CDI unit and has the same internals as the original. They sell a new wiring harness for my model that replaces the wiring from the CDI to the coil and distributor. First I have ever seen one available. My wiring in the 1973.5T is mostly original. With all that heat over the years between the CDI, coil and distributor something has to weaken. We'll see how it goes.

A differential measurement across the coil will just be battery voltage minus the driver voltage (which is what you have with the lower trace on your scope) so not much info.

To see if the coil starts to saturate you want to see a linear ramp from 0 to 8 Amps or at least as linear as can be with V(1-exp( -Rt/L)) with a very small R (0.6).

This is to see if the DME specific coil core is close to saturation to imply that a CDI coil designed as a transformer would probably saturate. I'm betting it is not.


I wonder how much it would cost (and how big) to wind a coil on a powdered iron toroid with a distributed air gap? We could dream up some marketing hype and sell the monster cable of ignition coils.;)

Based on your previously posted data, you've determined where coil saturation occurs.
Why waste more of your time on this? I'm sure you have better things to do, right?

Because he doesn't know where (if) the coil core saturation occurs he just knows when the driver starts to current limit.
It current limits either because it reached its design set point or the current suddenly went up because the core won't support the current. Doesn't know which one but I'd bet the former.

Rick,

Read my post #73 and then look at the video. I think what you'll see in the video is that the coil saturates first then a tad later current limiting kicks in. I'm not 100% certain but that's what I think the scope is showing as I increase the dwell time. See the 3min25sec mark in the video. What do you make of that voltage trace?

i know someone else building the CDI harness. :)
http://forums.pelicanparts.com/porsche-911-used-parts-sale-wanted/740894-building-sc-mid-year-engine-harnesses-17.html#post8637389

Does the scope trace look like this? This is a simulation with an ideal inductor that will not saturate.

http://forums.pelicanparts.com/uploa...1455052764.jpg

Yes, it looks a lot like that, even the voltage is very close to that. Take a peak at the video.

In your picture, at what point do you consider the coil nearly saturated? I assume right at the knee at the 3.4ms mark? Right before that super sharp rise in current? Correct?

The ideal inductor in the simulation will never saturate.

From 0 to 3.4 mSec the current ~ linearly rises to 8 amps. The voltage rise over that period is due to the sense resistor and the resistance of the drive transistor.

When the current gets near 8 amps the amplifier in the DME reduces the drive to the base of the Darlington and Vce rises, the slope based on the gain of the differential pair amplifier.

The plateau is the voltage required to maintain di/dt = 0 (Vind = 0) across the copper loss in the coil.

Rick,
I think I understand, in this simulation the coil never saturates it simply stops charging at 8AMPs. The very sharp upward part of the trace (after the knee) is simply the voltage across the transistor's 'ce' junction as it starts to turn off and limit current.

I think that is correct.

The other interesting thing is that given those dwell times you said the DME uses for differing battery voltages, using the inductance and R values in this thread, it looks like the current always bounces off the current limiter. I wonder if they did this to make the design of the coil easier (cheaper)? Fixed dwell would require a lot higher inductance to support the current level over all battery voltages.

That had to be quite a departure from the standard ignition design of the early 80's

http://forums.pelicanparts.com/uploa...1455125405.jpg

EDIT: higher inductance has a lot of other baggage associated with it
EDIT Again: Larger inductance or larger core

Rick,

No, the stock DME dwell times in the DME don't get high enough to activate the current limit. For me to hit that knee I have to increase the dwell times about 10%.

Basically the stock dwell time is about 3.8ms at 15v and I have to increase it to just above 4.0ms to hit the knee.

Hope that makes sense.

Where are these numbers from?
EDIT with the known coil parameters it should reach 8 Amps at under 3 milli Seconds at 15 Volts.

Rick,

Those numbers are from bench testing the 964 coils with the 964 BIM firing the coils.
The supply voltage was set as stated and the ms time is right before the knee of the trace occurs. Remember that the voltage is the supply voltage and not the voltage at the coil, it does not account for the voltage drop of the BIM module.

The values are exact amount of time it took to hit the start of the knee. I've mentioned this before, I have never seen a formal BOSCH spec for the coil, that 3.6mH value is something that's been found on the internet I simply would not put all my faith in that coil spec. But those values you listed are taken from real bench test done on the 964 coils with the BIM 124 firing them. I documented how I came up with those values in my video.

OK
I thought the 2,3,4,5 mSec numbers were from where you disassembled the assembly language code. The DME does have a way to read battery voltage.

I can calculate the dwell from the dwell map as well. But the dwell map in the DME is not ms based it's crank angle. The processor in the 84-89 DME is not that fast, they did everything based on crank angles (degree of rotation) and crank speed (RPM).

But even checking the dwell time in car shows the DME dwells for 3.8 to 4.0ms or till it runs out of time at hi-RPMs.

The DME has a dwell map with 2 axises: RPM on one axis and Battery Voltage on the other. Then the values in the map are crank angle numbers, it's a flywheel tooth count number that basically says dwell for 'x number of teeth', it's very simple and basic.

I just crunched the numbers from the dwell map.

At 2000RPMs for these given voltages you have these dwell times:

voltage : ms time
5.5v : 9.4ms
7.3v : 9.4ms
9.2v : 8.3ms
11.0v : 6.2ms
12.8v : 4.8ms
14.7v : 4.0ms
15.1v : 2.3ms (this is for any voltage above 15.1v)

Note that at 2000RPMs crank turns 1 rotation in 30ms so we have just 10ms between spark events, this is why the dwell can't be more than 10ms. You see in the above data that they don't ever dwell past 9.4ms or 94% of the available time. They seem to leave 6% of available time for the actual spark event.

Those calcs are very much in line with my bech test results.

Hope that makes sense.