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Old 02-08-2010, 02:33 PM   #21
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I just wanted to add that turning on the ignition driver is similar to charging a capacitor: the smaller the resistor, the higher the current and the faster the capacitor charges. This means that the full saturation voltage will be reached faster which will charge the coil faster and keep the ignition driver cooler since it's more efficient (less resistance) in fully saturated mode.

It also means that my previous concern about the power rating of the resistor is somewhat unfounded since the actual current will reach the theoretical maximum value only at turn on and go lower from there (and would be almost null at the fully saturated point). So the 1/4W 330 Ohm resistors will probably not suffer since the average current (and therefore power) will be much less than the instantaneous maximum value.

Jean
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Old 02-08-2010, 02:58 PM   #22
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Originally Posted by Joe Perez View Post
Obviously there are a hell of a lot of Miatas running around on 1k pullups, as that was what DIY called out in their original build document. And they run fine. We're gilding the lilies here, I admit, but I like these things to be perfect.

What happens when you use a high value resistor is that it takes longer for the coil primary to charge. The igniter circuit seems to load its trigger input pretty heavily at turn-on, such that the voltage on the trigger line gets pulled down. This seems to correspond to a a decrease in the rate at which current through the primary winding builds.

Here is an example of what I'm talking about:



The yellow trace is voltage on the trigger line, and the blue trace is current through the coil primary. See how when the trigger first turns on, the voltage starts out very low and then builds up, almost as though the igniter itself is reaching a saturation point? As you use smaller value resistors (and thus increase the available drive current) the rise of this waveform is much less retarded.

Where this becomes important is that I observed a correlation between the rise time of the trigger voltage and the rise time of the primary current. It's not a linear relationship, but it does happen. As you make more current available to the trigger (and thus increase its voltage) the current in the primary builds more rapidly. In this picture, you can see that the primary current never did reach full saturation (which would be indicated by the blue trace leveling out and becoming horizontal). When trigger drive was increased, the primary was able to become saturated within the time allowed.

(Keen-eyed viewers will note that the dwell time on this capture is less than what's specified for an NA. I believe that this particular trace was actually taken back when I had my EMU, I just can't find any of the more modern stuff right now.)

Ok, now the practical upshot of all this is that what I have illustrated here (limiting trigger current too much) is roughly equivalent to decreasing your dwell time. Both have the effect of limiting the amount of charge which the coil can build prior to firing. If you have the ability to measure DC current with a scope, you can tune the dwell time of an MS built with the 1k pullups until you reach coil saturation.

Or, you can just install smaller resistors.
Almost* an epic reply

I have a scope, but it's 1 channel and can't pause stuff easily, nor can it dump to a PC, I'll give scoping the coils a go, (as they're coils I doubt anyone else has used so I'm not gonna get the Dwell for them easily!).

But that said I don't think I'm going to rush out to do it

*needs boobies to be epic IMO
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Old 02-08-2010, 03:17 PM   #23
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Quote:
Originally Posted by jbelanger View Post
I just wanted to add that turning on the ignition driver is similar to charging a capacitor
I must admit that I don't fully understand this phenomenon. When I first measured it, I observed that the igniter seemed to be a capacitive load. JasonC SBB here on the forum correctly observed that this was not a fully accurate explanation, owing to the fact that the trigger voltage does not ramp up from 0, but rather jumps straight to about 1.5v and then starts ramping.

Jason postulated that this may be some artifact of a current-sensing scheme in the igniter (clamping trigger voltage to allow the ECU to sense primary current), though I find it difficult to imagine that such a scheme would have been implemented in the 1.6 Miata's ignition system, which is of late 80's vintage in origin.

The truth is that I don't know exactly why the igniter behaves the way it does. Maybe there's some turn-on threshold such that it (the igniter) doesn't start drawing current until a certain voltage is reached on the input. Regardless, the fact is that a relationship exists between trigger input voltage and coil primary current, and it's thus beneficial to allow as much current to flow on the trigger line as practical in order to achieve maximum slew rate on the coil primary.



Quote:
Originally Posted by jbelanger View Post
It also means that my previous concern about the power rating of the resistor is somewhat unfounded since the actual current will reach the theoretical maximum value only at turn on and go lower from there (and would be almost null at the fully saturated point).
Very true. I often find myself thinking that everything is steady-state, but you're absolutely right on that one. This is backed by the fact that even when I've used 100 ohm 1/4w resistors on the 5v setup, they tend to run rather cool.





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Originally Posted by richyvrlimited View Post
I have a scope, but it's 1 channel and can't pause stuff easily, nor can it dump to a PC, I'll give scoping the coils a go, (as they're coils I doubt anyone else has used so I'm not gonna get the Dwell for them easily!).
Unfortunately, to make this sort of measurement you need a DC current probe. You can build a simple shunt-type probe by placing a very low-value resistor in series with the coil primary wire, though the fancy $2,000 inductive probes make life a lot easier. I really need to build one for myself so I can measure the Toyota COPs that everybody uses nowadays.
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