When a car ignition coil increases the voltage from 12v DC
to 10,000v DC, what happens to the value of the current? Voltage and
current have a direct relationship. Does the current increase also?
Nope. As voltage is increased, current DEcreases. What stays the same is power,
which is voltage times current.
Richard Barrans Jr., Ph.D., M.Ed.
Department of Physics and Astronomy
University of Wyoming
No, Paul, it is the inverse, for transformers.
what is conserved is power.
So when voltage goes _up_ by a factor of N,
the current goes _down_ by a factor of N.
But I should tell you that is for an efficient AC transformer,
which transfers energy continuously, and is never suddenly interrupted.
Ignition coils waste some power and are also calculated differently.
They store and discharge energy.
While the 12v is applied and the primary-coil current ramps up,
energy is being stored.
The energy stored is E = (1/2) * L * I^2,
where L is the inductance of the primary coil, and I is the current.
(It is exactly like the formulas for
energy on a charged capacitor: E = (1/2) * Capacitance * Voltage^2
or kinetic energy of a moving mass: E = (1/2) * Mass * velocity^2. )
After the current is ramped up,
all further time running high current adds no more magnetic stored energy,
it is just wasted in the resistance of the wire, making the coil get hot.
(Many gimmicks on market to minimize that.
Capacitor-discharge ignition is one of them.)
Then a switch (the distributor points) opens and stops the primary current.
That forces a corresponding current to flow in the secondary,
about 1000x less current, but capable of charging the spark cable
to 1000x higher voltage. More, actually, if the cable's capacitance is small.
That stored energy insists on getting out somewhere,
and merely charging the cable to high voltage is not quite enough.
At some point the spark-plug reaches its breakdown voltage,
and all the stored energy is discharged into that spark.
End of story until next time the points close, re-starting the 12v charge-up.
It is hard to get a good value of L of an ignition coil,
but E = (1/2) * Voltage * ChargeTime * FinalCurrent also works.
Charge-time is the time mentioned above,
the time during which the primary coil current is linearly increasing.
(if it's not linear, rather an exponential decay to he final current,
extrapolate the initial ramp rate with a straight line
to where it would reach the final current, and read that time interval.)
It's some milliseconds; you can see it on an oscilloscope
when hooked up to measure current in the primary side of the coil.
And I think the actual turns ratio of wire-windings inside an ignition coil
may be less than 1000:1, maybe it is around 200:1.
The top 5x or 10x of voltage increase happens because of
the inductive voltage surge that always happens
when an inductor with current is suddenly open-circuited.
The peak voltage the ignition coil can reach
must always be higher than what the spark-plug requires to spark over.
Maybe 2x higher.
All that being said,
thinking of it as a transformer still kind of works,
including the inverse relationship of voltage and current.
Yes they have a direct relationship, but not in the manner you see with typical
transformers in steady-state applications. In a step-up transformer, output
voltage is greater, but at a lower current than the input current (conservation
of energy). On the 12V side of an ignition coil, current ramps up over time (inductors
resist changes in current) until it reaches a few amps, slowly building up energy
in the magnetic field. When it gets disconnected, the magnetic field collapses and
creates "back EMF" at a much higher voltage on the secondary side because of the
turns ratio. There is no current until the voltage reaches the dielectric strength
of the air-fuel mixture at the spark plug, at which point the gasses ionize and form
nearly a short circuit. At this point there is a very brief spike of current
(around 10-100 microseconds long) that spikes and decays. The current can be quite
high (nearly a tenth of an amp), but being relatively brief, does not violate
conservation of energy.
Yes, voltage is directly related to Current times Resistance. (Ohm's Law)
When you have a step up voltage coil what happens is the voltage increases
But the current decreases.
In physics you have the law of the conservation of energy (Joules).
Power is rate of energy consumption (Joules/sec)
Electrical Power (Watts) is the product of current (amps) times voltage
So if you increase the voltage, the current decreases unless you have what
is called an "active device" to add power,
Like a battery, alternator/generator, or Power company adding power to the
"Passive devices" just consume power, "active devices" add power to a
system. (but they really don't add power they just convert power from one
form (chemical power in batteries) to electrical power.
Click here to return to the Physics Archives
Update: June 2012