Direct Coil Driver?

[wes kiser]

How well does this work, and what is the criteria for whether or not it will work with a particular coil? I read that the vb921 current limits to 7 amps, so does this mean that a 2 ohm or greater coil will work?
The reason I ask, is I am having low rpm missfire problems with my tfi setup (will handle no load below 1500 rpm, but is fine above, even at 20psi), and am just exploring other options. I will likely eventually convert to EDIS, but drive the car to work everyday in the process. I have also looked at running an ignition box, but by the time I spend that money I could have EDIS. Thanks.

[Bernard Fife]

the vb921 current limits to 7 amps, so does this mean that a 2 ohm or greater coil will work?

Wes,

No, the chip limits the current to 7.5 Amps, whether the coil gets to that level depends on the coil's resistance and impedance, as well as the dwell time (since it is an inductor, not a simple resistor).

So the 7.5 Amp level (which should be more than sufficient for most coils you are likely to use) is only used to limit the current if the dwell is too long.

The time it takes for an inductor to charge up to a certain current (assume t=0, I=0) is:

T = (-L/R) * ln( 1 - (R * I / E))

Where:

T = time (sec)
L = inductance (H)
R = resistance (Ohms)
E = voltage (Volts)
(ln is the 'natural logarithm', often available as 'LN' on calculators)

For example, the values for my 7-pin HEI large cap set-up with red/yellow leads are:

L = 6.18 mH
R = 0.4 Ohms
E = 12.0 Volts
I = 6.0 Amps

This gives a calculated T of 3.44 milliseconds.

In the car, I found that 3.4 worked, but set it to 3.5 to be sure. At 3.3 or less, I would get misfires, especially under 1000 rpm. So it certainly seems to confirm the theory.

Another example is an 8-pin module/distributor and external coil. This external coil has an inductance of 4.285mH and a DC resistance of 0.4 Ohms. So for dwell:

T = (-L/R) * ln( 1 - (R * I / E))

= (-4.285/0.4) * ln (1-[0.4*6.0/12.0])

= -10.7 * ln (0.8)

= 2.39 milliseconds

On the car 2.5 milliseonds works well. Note that both of these are drawing 6.0 Amps peak, and both are well below 1 Ohm in DC resistance. If the dwell was increaed a lot, the current would rise, and if it got too high it could damage the coil. The VB921 would not let it rise above 7.5 Amps though, which is probably safe for these coils (certainly for short periods and small duty cycles).

Lance.

[wes kiser]

Yes, I remembered the time based inductor equation from the Intro to EE class they made us mechanicals take, but thought maybe there was "a factor of safety" based on a worse case scenario. So practically it can be used to adequately drive most coils with minimal issues? Thanks.

[krisr]

Just for my own satisfaction of knowing I learnt something today - How are you coming up with the 6A figure? I run an MSD Blaster SS Coil and MSD rates it's peak current at 300mA. Does that sound right?

Primary Resistance: .355 ohms
Peak Current: 300mA
Secondary Resistance: 6.2K ohms
Spark Duration: 180uS
Maximum Voltage: 48,000 volts
Primary Inductance: 6mH
Turns Ratio: 70:1

[Bernard Fife]

krisr,

6 Amps is a published figure for the GM coils, I don't recall specifically where this came from though. A search on google ought to turn it up.

I suspect the 300 mA figure for your coil is the secondary current (i.e., the current across the spark plug), not the primary current. Typical values for peak secondary current are 150 to 350 milliAmps.

Lance.

[krisr]

Ahh sweet, thanks Lance.