CDI current
 

Does anyone know how much current a 3 pin CDI box draws as max?

Roughly 3.5 amps

But rises with RPM.

How much at maximum rpm?

The OP asked for max current. As stated it is around 3.5 amps at typical high revs (7000 RPM). With the ignition on but not firing, the boxes draw just under 2A IIRC.

Gentlemen, thank you very much.

OK so a range of 2 amps up to 3.5.

This is an absolutely ignorance based question. What makes the box draw more amperage?

The simple answer is "RPMs increase and it draws more power because it needs more".

I struggle with this because I am an accountant that fiddles with cars.

Where is the deficit? How does the deficit cause the "box" to suck more current but stops far short of what is available?

Has to apply to all circuits.

My brain pictures a ten amp fuse. It blows for safety reasons but if working properly, the box will only draw 2 to 3.5 amps. Six to seven more amps available but it does not take it. Analog device says "Nah, don't need it. It's there but I don't need it."

What is the basis for this? Can this be explained to a noob?

Is it a simple as "I take what I need"?

Each spark means power is being consumed, the more sparks/minute the higher the input supply current will rise. Additionally, the higher the cylinder compression is the higher the HV must be/go in order to jump the spark gap.

7000 RPM with the engine under load will result in more power consumed vs simply 7000 RPM engine unloaded or test bench.

At idle with low effective cylinder compression "THE" capacitor discharge will not be as complete as might be with the engine under load at the same RPM.

The more of the capacitor charge level that is used to fire the plug the more energy will be required of/from the inverter to bring it back to full charge.

Also, any energy left in the coil once the spark extinguishes will be returned, mostly, to "THE" capacitor charge. Another reason to use a high "Q" E & I transformer type ignition "coil".

^ Cylinder pressures have nothing to do with current draw of the CDI.

The CDI box is a power supply that produces 360V from 14V. That part of the circuit requires about 2A, hence that is the consumption without the engine running.

The supply is used to charge a capacitor that is used to energise the ignition transformer (coil) primary winding. The secondary winding steps the voltage up (x100) enough for a spark to jump across the plug gap down to ground. Of course, now the power supply needs to charge the capacitor again....

Every charge requires a certain amount of energy. Double the rpm and you double the number of charges thus double the current required.

I just use the same 8 amp fuse the car came with when I upgrade my cars. I put one in both 75's and me Race Car has never been so happy! The same fuse used Da Pumpkin that uses the same CDI boxes. Well I've repaired a few HOBBIE and use a 10 Amp Supply and works just fine and I rev the box to 8000RPM
And the CDI works just fine. There is an average current, but there is a peek current and unless Porsches come with SLOW Blow fuses as ALFA's do not, we got those junkie ones, 8a is perfect! Our cars are old.

Thank you.

"Cylinder pressures have nothing to...."


You might try telling that to all the Ford F150 EcoBoost owners who had to replace spark plugs, spark plug mounted coils, due to spark flashover arising from extreme cylinder pressure.

I don't see how that it revelent to the topic at all. The topic is current draw.

The energy required to charge the capacitor is the same whether the cylinder is pressurised or not. You made it sound like the CDI draws more current in high pressure conditions. It doesn't.

If the pressure is too high or the plug gap too wide, you could get 'flashover', but it would have no affect on the current draw.

Stick around. Rational feedback/push-back is a good thing.

The higher the pressure is in combustion chamber the higher the coil secondary voltage must rise in order to jump the plug gap, the period of the gap current flow will be much the same regardless. More pressure = more spark power consumed, leaving less power to be returned to "the" capacitor

As I said, once the spark extinguishes the coil's magnetic field begins to collapse and the energy from that is returned to "the" capacitor.

Less power available? Meaning more current? Or should I shut up on this one?

Inherent in a proper CDI design would be an inverter section robust enough to recharge the capacitor in worse case, maximum RPM, Maximum combustion chamber pressure, and minimum power returned to the capacitor.

Actually, the capacitor is charged in the reverse polarity at that point requiring
the inverter to overcome the negative voltage of the capacitor from the remaining
energy of the ignition coil, i.e. no remaining coil energy is returned to capacitor
for use in the next spark.

don't forget total resistance. a plugged in 100W light is using 100W. many more W available, but not used :roll eyes: here we go

RED...current flow path.

AS the coil magnetic field collapses, as well it must, the capacitor is partially recharged.

Note that the inverter secondary is effectively SHORTED until the coil field is fully collapsed.

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