Ignition power consumption

[Paul Marx]

Old topic .. but interesting info.

Marco Patzer (from Sachse) came back to me (some time ago, sorry for the delay) with the dwell time information for the ZDG3 ignition module.
-> 3.5ms (fixed).

As Red wrote, smart electronic systems, like the Ignitech one, will adjust the dwell time to ensure that enough magnetic field is built up for a good (ignition coil) charge.
Modern coils draw about 4-6 Amps while the points are closed (for Marx)
So all data needed here to do the maths.

Ha ha.
The only bike where I worried about that was my TT2 without a charging circuit, so running total loss on a small lithium battery but with Nology coils. The Ignitech for that is still on the shelf. I need to work on that bike so as to make it forget that somebody trid to burn it before throwing it in a pond.
Otherwise, no need to worry about electrical consumption, a dynamo delivers plenty enough.

Paul

 

[KERMIT1200]

Hi Steph,
When I decided (years ago 1980's!) to run total loss on my Monty for racing I measured the currant drain of approx 1 amp.
4-6 amps just seems too much!

Andy

 

[bazzee]

Old topic .. but interesting info.

Marco Patzer (from Sachse) came back to me (some time ago, sorry for the delay) with the dwell time information for the ZDG3 ignition module.
-> 3.5ms (fixed).

As Red wrote, smart electronic systems, like the Ignitech one, will adjust the dwell time to ensure that enough magnetic field is built up for a good (ignition coil) charge.
Modern coils draw about 4-6 Amps while the points are closed (for Marx)
So all data needed here to do the maths.

So at 8000rpm, 1 crank rotation takes 7.5ms. A dwell time of 3.5ms means the coil is conducting current for 46.7% of each crank rotation (assuming the points or pickup are operated from the end of the crank so there will be a wasted spark).

If the coil has a resistance of 4 ohms, then it will conduct a current of 12Volts/4Ohms = 3 Amps maximum.

So at 8000rpm the average coil current will be 46.7% of 3A, which is 1.4A per coil as a first estimate. At lower rpm the current drain should be proportionally less.

If the pickup is operated at camshaft speed the average current will be halved from this calculation.

There are other considerations: since the coil is inductive as well as resistive the current does not immediately jump to 3A when power is applied, but ramps up from zero to the peak value of (say) 3A. This will effectively reduce average current drain.

Cheers,

bazzee

 

[Dellortoman]

There are other considerations: since the coil is inductive as well as resistive the current does not immediately jump to 3A when power is applied, but ramps up from zero to the peak value of (say) 3A. This will effectively reduce average current drain.

Spot on baz. You could make a reasonable inference that the boffins who designed the Sachse ignition chose a dwell time of 3.5ms because that's how long it takes for the coil to saturate. I expect the current will increase at a steep rate when the coil is first energised, then level off as it reaches saturation.

You could probably approximate the current draw with a quadratic or sine curve. Then do some integration to work out the area under the curve, etc. But I reckon an approximation would be near enough. My guess would be that the average current draw would be about 2/3 of what you figured out above.

 

[CLEMTOG]

I have a meter that measures dwell angle, a much more acurate way of setting the gap in points ignition than using a feeler gauge, othert than that i have lost the plot on this one, way over my head

But I can also tell you that when I bought my 1958 AJS, I did not know that it was not charging, because the ampermeter showed that it was, but not getting to the battery, the bike ran well for about two months (6volt) but got harder to start, and then wouldn't take full throttle, and finally limped two miles on merely a sniff of throttle (any more and it died) back to the campsite in the IOM, next I found zero compression, when the boys arrived about a week later we stripped the motor in Toms awning and the inlet valve guide (which has its own supply of oil metered by an adjustable needle valve, total loss) had seized on the valve, and had dropped into the combustion chamber, fortunately without damage, but enough to hold the valve off the seat, makes me wonder how it ran, or rather kept running, a restart was not happenning. but it did, obviously low voltage to the coil can do more than you think! its never runs since.

CLEM