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[SOLVED] Inductor and Capacitor charging time

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Full Member level 4
Dec 12, 2008
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Hello All,
Sorry for asking very silly question.

An RC circuit has RC time constant and RL has L/R time constant. But when i was reading about automobile ignition system, the dwell time do always vary based on how much is input voltage and input current. Where, dwell time is time required for ignition coil to recharge to full.

But according to RL time constant, the charging time should always be constant irrespective of Input voltage and current. If im wrong please correct me. Is charging time of inductor (and also of capacitor) supply parameter dependent (i.e. input V and I) or just time constant is enough to determine charging and discharging time?

This is the dwell time table for inductive discharge ignition i referenced.

Thanks for answers

I believe it should be easy to calculate ignition coil current waveforms and required dwell time to reach a specific coil current.

Historically, cars had mechanical ignition contacts with fixed dwell angle and thus strongly variable dwell time. Coil current at low engine speed was respectively limited by the coil resistance. At high engine speed, dwell time and coil current tended to be too low.

Electronic ignition gives the option to set a constant dwell time.

maybe below equation can answer your question about charging time of inductor:

V=L*(di/dt) and we can change the that into L=(V*dt)/di
we know L will not changed once the inductor was choosen ,so,acording to data from table you provide:

1,if charge current is incresed,the charging time will be getting longer to make the equation aways true.
2,if charge voltage is incresed ,the charging time will be getting shorter to makes the equation aways true.

that's my thougts about your question .

Thanks FvM for answer. I know the dwell angle being fixed while dwell time changes with rpm. But as u said, with modern electronic system, dwell time is more or less constant. But this constant dwell/coil charge time changes with battery voltage and battery supplied current. That is the point i got confused.

Thanks kobeismygod for answer. The explanation is logical and makes some sense. But, can I directly use the voltage and current value is above formula and find the time required, or is there another formula to do that?

I am not sure about what you want to know. For any given coil there is a maximum current , above which the magnetic field saturates. This means that as soon as you have charged the magnetic field, letting the current continue to flow just causes more heating without increasing the magnetic field, or the spark intensity. This is the reason for the dwell angle in the old mechanical system, the current flows for about 30% of the time. With modern systems, such as the capacitor discharge the current flows for a very brief time (100 microsecs?), but still gets to the coil magnetic field saturation level. So they perform well at high RPM as the coils field is always "fully charged".

But this constant dwell/coil charge time changes with battery voltage and battery supplied current. That is the point i got confused.
Do you know for sure that a modern ignition systme does not compensate for supply voltage variations?

A battery doesn't "supply a curent". Instead the current levels in your table refer to different desired coil currents. Battery voltage is the only variable parameter in this table.

Before the electronic ignition, automobiles had a mechanical distributor. Inside is a rotating cam which opens and closes two contact points. When the points open, that is when the spark occurs.

It is important for the spark to occur at the right moment of engine rotation. A mechanic loosens a bolt so he can turn the entire distributor to the right or left, to adjust the spark timing.

When replacing points, they must be positioned so the cam closes them for the proper length of time (dwell angle). The adjustment is tricky. A small distance can change dwell angle by several degrees. Fortunately you'll have satisfactory operation even if you get the dwell slightly below spec. That is if the coil, condenser, etc., are all in good shape.

(I still have my dwell meter although I have not used it in decades.)

Furthermore when you change the dwell angle, it affects spark timing. Timing is much more critical than dwell angle. So a timing adjustment is necessary as the final step.

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