General Purpose Low Level Ignition Drive

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Fred
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Re: General Purpose Low Level Ignition Drive

Post by Fred »

Sounds exactly right to me! This is why this thread was started in the first place... :-)
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Re: General Purpose Low Level Ignition Drive

Post by TonyS »

Doesn't sound right to me either.

The BIP373 appears to have a current gain of about 1,000 (Fred I'm not sure what ignition Darlingtons you checked previously where the gain was approx 100) which should (barely, given the large Vbe-sat of the BIP373) provide enough current to fire with your drive impedance and 5V supply voltage. But of course without knowing the coil's specs (recommended current and specd inductance) or your dwell time, I could be mistaken.

My real question is why would someone use a Darlington device today when there are Ignition IGBTs available now purposely designed for this application?

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Re: General Purpose Low Level Ignition Drive

Post by Peter »

TonyS wrote:My real question is why would someone use a Darlington device today when there are Ignition IGBTs available now purposely designed for this application?
Because MS is very successful at scamming uninformed people by acting like they know what they're talking about.

I've been looking at the datasheet, and the specs I find interest are:

Collector Current Limitation I_C-L = 11.5A when V_CE = 6..10V
Base Emitter Saturation Voltage V_BE-SAT = 3.2 to 4.85V when I_C = 7A & I_B = 12 mA
Input Current I_B = 200mA max

I'm probably pushing around 12A through the coil. I'm surprised that the BIP opens at all with 5V for the MS people. 5V through a 330 Ohm resistor really doesn't have the current capability required to open all three gates properly as far as I'm concerned.
:-p
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Re: General Purpose Low Level Ignition Drive

Post by Fred »

330ohm
0.012A
V=IR
V = 3.96 (drop)
5 - 3.96 = 1.04

1.04 < (3.2 - 4.85) :-p

Reality is a higher voltage at a lower current, as there is no current until the voltage rises somewhat.

100 ohms would likely work with 5V if the drain was 7A, however as Peter points out, many coils are far worse than that.

The Subaru ones that I tested are capable of pulling 24A once saturated. Twenty four amperes! Of course, you wouldn't push them that far, and, you will drop some voltage on your igbt/darlington (all of the older oem external ignitors are darlings) even when saturated/fully on.

14.4V / 0.6ohm = 24A

The trouble is, that the less saturated the ignitor is, the more voltage it drops. They ain't FETs. The more voltage you drop, the lower the peak current through the coil, the lower the energy stored in it, and the weaker and shittier your spark will be.

My answer to Huff's question is this: Margin! IGBTs still aren't cheap, and I bet you that those BIPs were relatively cheap.

Incidentally, the switching time of the IGBTs that I've looked at is borderline when driven with a simple resistor from 5V as we're discussing above. And it becomes worse than border line if you have a ground differential (which you will if you're not brain dead and sharing grounds with sensitive stuff). So even they benefit (a lot) from a higher voltage current limited drive.

This is all 1.5 year old news. My wrist now hurts. I'm going back to bed.

Fred.
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Dan
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Re: General Purpose Low Level Ignition Drive

Post by Dan »

fred you beat me to it!

I had exactly the same calc you posted above written down at work today :-)

it makes perfect sense!
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Re: General Purpose Low Level Ignition Drive

Post by Fred »

By a year and a half ;-) I determined these values empirically with OEM ignitors in Howick, Auckland, New Zealand, and Preston confirmed them in Waiuku at the same time. Two people are now rolling around under FreeEMS power with 12V FET driver drives to their ignitors. Peter on BIP373 and Daniel on OEM ones, different FET drivers, too.
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Re: General Purpose Low Level Ignition Drive

Post by TonyS »

Random comments in no particular order -

1. The BIP373 appears to have been designed for use with 5V logic (first sentence in data sheet and application schematic). But, of course you have to supply the right amount of current (base resistor).

2. IGBTs purposely designed for ignition (ON, Fairchild,...) are currently considerably cheaper than the BIP373 (about 1/3 the cost based on a quick search).

3. I did find that evidentally quite a while back, Darlington based ignition drivers were in use and did require about 100mA drive (I have no idea as to which ignition modules used them).

4. I think that the resistance of the coil is being wrongly used to calculate the magnetic "saturation" level. The resistance is merely the resistance.

5. I'm curious as to how Peter determined that he was drawing 12A through his coil. Does he have a current probe? Did he use the spec'd inductance, resitance, battery voltage, and dwell time to calculate this? Also, from what information I have found, ~ 6 -7A is the "normal" range that ignition coils are designed for.

6. I'm not sure about the "switching time" of IGBTs being "borderline" statement. A more detailed explaination would be appreciated.

7. I am ignorant of the details related to ignition coils but because one of my current work assignments requires me to know some of the details regarding driving them, I found the following link to be very helpful in helping me to understand - http://www.dtec.net.au/Ignition%20Coil% ... ration.htm

Thanks,
Huff
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Fred
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Re: General Purpose Low Level Ignition Drive

Post by Fred »

First real test of the writs rests that just arrived :-p

Random answers in a very particular order:
7. I am ignorant of the details related to ignition coils but because one of my current work assignments requires me to know some of the details regarding driving them, I found the following link to be very helpful in helping me to understand - http://www.dtec.net.au/Ignition%20Coil% ... ration.htm
If you read that link, it answered some of your other/earlier points :-)
TonyS wrote:1. The BIP373 appears to have been designed for use with 5V logic (first sentence in data sheet and application schematic). But, of course you have to supply the right amount of current (base resistor).
Agreed, which is difficult at such a low Voltage. This is compounded by inevitable Voltage drops sometimes causing 5 to become 3. In the general case, not that of the BIP373, a pure UNLIMITED 5V will often NOT supply sufficient current to properly saturate the darlington.
2. IGBTs purposely designed for ignition (ON, Fairchild,...) are currently considerably cheaper than the BIP373 (about 1/3 the cost based on a quick search).
Whose price? If you look around you'll find BIP373 available from, gasp, twice as many places as it used to be: DIYAutoTune and Future Electronics: http://www.futureelectronics.com/en/Sea ... e,Nea:True

DIY is a retail joint, though I'd expect future's price to be lower, it isn't. In any case, you can bet there is good mark up on DIY's price. You can also bet that things have changed in the 4+ years that DIY has been selling them with IGBT prices way lower now than only a few years ago; it's still maturing. Furthermore, are you comparing current limited apples with temperature protected apples? Or cheap unprotected IGBTs with protected BIP373s? This is a consideration.
3. I did find that evidentally quite a while back, Darlington based ignition drivers were in use and did require about 100mA drive (I have no idea as to which ignition modules used them).
Virtually all of them. And "quite a while back" refers to only a few years. Now think "who mods the engine on their brand new car" and the answer is no one. So we have the situation where the newest of DIY EFI installs are still using these things by default when using OEM setups. Also, for anyone who doesn't want to over-complicate their engine retrofit, these are still the go-to choice for coil driving when the coil is a naked coil. This is unlikely to change any time soon.
4. I think that the resistance of the coil is being wrongly used to calculate the magnetic "saturation" level. The resistance is merely the resistance.
Energy stored is determined by inductance and current flow. Higher current flow for the same coil of wire with the same inductance = more stored energy and a stronger spark. DC resistance is the end point of the dwell curve. If you are using too much dwell, your curve flat-lines at the DC resistance level. If you're using the right amount for an optimum spark (keep in mind we want to ignite 30psi 11:1 rich mixtures here) then your curve peaks at and ends exactly at the DC resistance level. If you consider that over dwelling produces no further benefit, and that the best possible spark occurs with maximum current flow, and no spark occurs with no current flow, then there is a curve of usable energy AND Voltage available depending on dwell time for any given Voltage and driver setup. Clearly the coil won't ONLY work at the absolute peak position. In fact, it turns out, they work "well enough" for a normal low pressure low output engine for a large proportion of that curve. The upshot of this is that you can do a fairly shit job of driving the coil and it will still run the engine acceptably. Typical values for modern coils tend to be in the 1.5 - 5ms range at nominal operating Voltage. Old can style coils required longer dwell to operate correctly.
5. I'm curious as to how Peter determined that he was drawing 12A through his coil. Does he have a current probe? Did he use the spec'd inductance, resitance, battery voltage, and dwell time to calculate this? Also, from what information I have found, ~ 6 -7A is the "normal" range that ignition coils are designed for.
I don't know about Peter, but I certainly DID instrument my test setup properly monitoring both Voltage across the coil and current through it under saturated conditions. Remember, correct dwell for an optimum spark is an OUTPUT of the other things, which we aim to meet, and not exceed, and maybe come in a hair under for battery drain reasons. If you're on a vehicle that is electrically limited with a tiny charging system, such as a bike, this strategy may change. If your coil drivers suck and your coil is good, this strategy may HAVE to change to avoid cooking things. It's also a law of diminishing returns situation the closer you get to the DC part of the curve, the more heat you dump in to get further and the less extra results you get for your troubles. So, once again, there is no correct dwell, really. Electrically correct is between 0 and whatever it takes to hit DC draw. Practically correct is between whatever you can barely get away with and still run and whatever you can barely get away with and not melt stuff. These two sets of constraints are true.
6. I'm not sure about the "switching time" of IGBTs being "borderline" statement. A more detailed explaination would be appreciated.
If you drive them from 5V with a simple resistor you're driving a capacitive load, exactly the same as driving a FET. Caps take time to charge. Typical 5V outputs are 10 - 25mA capable. Current limit resistors should be chosen for the short condition. As such the switching time is marginal (for some devices, at least) when the correct resistor is used and the Voltage is 5V.
Thanks,
Huff
You're welcome! Wrist report: Not doing too bad considering the volume of text banged in at high speed above. An encouraging sign.

Fred.
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Re: General Purpose Low Level Ignition Drive

Post by Fred »

Errata: The problem with not driving the darlingtons properly is that you end up with a significantly lower voltage at the coil that, no matter how long you dwell it for, minutes, days, whatever, NEVER produces the current peak that you want. Thus the goal here, distracted by dwell conversations, is to get a minimum V drop through the ignitor in order to make a maximum voltage and therefore peak current AVAILABLE to the coil, allowing a stronger spark with which ever dwell we happen to choose, all inclusive.
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Re: General Purpose Low Level Ignition Drive

Post by Fred »

Why do I feel as though someone is following me around? http://msextra.com/forums/viewtopic.php?f=67&t=44554
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