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Coil charging

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injb:
Hi, I have some noobish questions about inductance and current etc.

I captured a trace of my car's ignition coil using a scope and current clamp, and what I found was that it charges up to approx 9A in around 3.7ms or so. The alternator voltage is 13.85 and I know the coil's inductance is 5.85mH. But I don't know exactly the resistance of the whole circuit, and the ECU has some circuitry that limits current to 9A.

Is there a way that I can calculate the coil charge time mathematically? I found this calculator (https://daycounter.com/Calculators/Inductor-Current-Power-Calculator.phtml) which uses the formula:

Ton_max= I_sat * L/V

and it gave me approx 3.8ms for the values above, which is close to the charge time I'm seeing on the scope. But I don't know if I really understand what this formula is saying. Is it "this is how long it takes to reach I_sat, given L and V" ? Is the resistance value implied, because I've specified I and V?

I thought the above formula might just be a rearrangement of:

T = L / R

?

But this formula is supposed to tell me the time it takes to reach 63.2% of maximum current, right? I know that the ECU of the car limits the current to 9A. Does this mean that the current I'm seeing after approx 3.8ms is 63.2% of the maximum that would flow if it wasn't limited by the ECU?

Thanks!



Benta:
Your ECU may limit the current to the ignition coil (to protect itself), but mostly the coils have internal current limiting resistor/resistance.
The 'scope plot you've supplied indicates this as well (semi-exponential shape), where magnetic core non-linearity most likely also plays a role.
A constant voltage applied to an ideal inductor will result in a linear increase of current in it.

injb:

--- Quote from: Benta on May 12, 2019, 07:26:19 pm ---Your ECU may limit the current to the ignition coil (to protect itself), but mostly the coils have internal current limiting resistor/resistance.
The 'scope plot you've supplied indicates this as well (semi-exponential shape), where magnetic core non-linearity most likely also plays a role.
A constant voltage applied to an ideal inductor will result in a linear increase of current in it.



--- End quote ---

Thanks. This coil is an old school coil with nothing fancy in it. It has a resistance of around 0.4 ohms.


Buriedcode:
You can play around with LTspice, providing pulses of fixed periods to an inductance, and change its DCR, inductance etc..  It's free and quite handy for playing around to see how things work:
https://www.analog.com/en/design-center/design-tools-and-calculators/ltspice-simulator.html#

As for inductor "charging" a link I often pull out is this one: https://www.dos4ever.com/flyback/flyback.html

It explains how the inductor works, in both a boost converter and a flyback (ignition coils can behave like flybacks, because current doesn't always flow in both the primary and secondary at the same time).  It isn't my website, but he explains it remarkably well.  The equation I believe you're after is:  I=(V/L)*t  Which provides the current flowing through the inductor, for a given voltage, after a period of time. It does not deal with the inherent resistance of real-world coils, but like most things in electronics, one deals with "ideal" components first, then dealing with the real-world limitations after.

Jwillis:
How "old school are you talking" .The ECM will reduce the voltage in the coil from the expected 12 -13 v.This is done to prevent damage to the coil after the alternator comes "online" .The old way was with a ballast resistor .The coils don't normally run at 12 v . So you need to measure the voltage at the coil and not the voltage at the alternator or battery . The ECM also controls the Dwell Or duration that the coil is turned on or the length of time that current flows through the primary winding of the coil.This has a direct effect on the current level and hence the energy stored.

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