# Using DC in transformers?

I read in many websites that transformers can only be used, to step voltage up or down, with AC current (which is why AC is preferred for power transmission since they can transfer huge power in the form of high voltage over thin wires instead of thich ones then they can be stepped down again), but then I started studying car mechanics and I discovered that the ignition coil also acts as a transformer and can step voltage from the 12V battery to ~30kV, but from the DC car battery, so the question here is: Is a transformer only used with AC? And if so, how does the ignition coil step up voltage. And if it can be used with DC too, so why use AC current in the first place??

• Turning DC on and off quickly is a form of AC too. That’s what switching power supplies do. – Michael Oct 31 '16 at 10:54

Transformers are AC only.

Running DC through a transformer basically gives you a heater.

Critically, transformers work through the fact that a change in magnetic field induces a voltage in a wire. The critical portion is that the change is required.

In an ignition coil, the change is created by simply connecting and disconnecting the ignition coil from the battery. The disconnection of power from the coil produces a collapse of the magnetic field produced by the current-flow through the coil, and results in a high-voltage pulse on the output, and subsequently a spark.

The connection and disconnection of the ignition coil from the battery converts (some of) the DC battery voltage to AC.

• AC and DC superimposed, actually... – Level River St Oct 31 '16 at 2:07
• Wow, nice answer, this actually explains more than the question that was asked. – Jimmy Knoot Oct 31 '16 at 12:47

Details of a homemade experiment in "Old Sparky": my ignition coil circuit. Diagram from the link:

The vibrator generates the AC that feeds the primary of the transformer.

• Any idea if there was a need to have the vibrator separate from the ignition coil, or if magnetically-operated contacts could have been incorporated into the ignition coil? – supercat Oct 31 '16 at 16:15
• @supercat, possible ≠ useful. In a real car synchronization with pistons is required. See en.wikipedia.org/wiki/Ignition_coil and en.wikipedia.org/wiki/Contact_breaker. – Martín-Blas Pérez Pinilla Oct 31 '16 at 23:02
• If one were using the crankshaft or camshaft to trigger one precise sparks per cylinder per rotation, I don't think one would need the vibrator. I've seen a bullet chronograph design that used an ignition coil from an old vehicle as a source of rapidly-repeating sparks, which would suggest either that the spark timing was loose, or else that the vibrator was wired in series with rotation-operated switch that wasn't expected to stay closed long enough to operate the vibrator, but the vibrator would limit current if the switch did stay closed. – supercat Nov 1 '16 at 13:33

@Connor Wolf has the correct answer. But, I would like to add one other image, since you are an automotive type.

An inductor is a lot like a fly wheel. With a flywheel, you can't change the angular velocity (RPM) instantly. The faster you try to change the RPM (accelerate the flywheel), the more torque is required (or released).

Similarly, in an inductor, you can't change the rate electric charge is flowing through the coils (currently) instantly. The faster you try to change that rate, the more voltage is required (or released).

So, just like you have either apply torque to, or get torque from a flywheel to change its angular velocity, so you have to apply voltage to, or see a voltage generated from an inductor when you try to change the rate electric charge flows through it.

Incidentally, a transformer is much like a torque converter in an automatic transmission. The magnetic core of a transformer is kind of like the fluid medium in the torque converter, coupling two inductors / flywheels together.

And some equations... for no good reason. LOL

\begin{align} Accel_{rpm} &= \frac{torque}{Mom\ of\ inertia} \\ Accel_{rpm} &\leftrightarrow \frac{dI}{dt} \\ torque &\leftrightarrow voltage \\ Mom\ of\ inertia &\leftrightarrow L \\ \frac{dI}{dt} &= \frac{V}{L} \\ \end{align}

Like my previous speakers said transformers work with AC. But it can work with DC and vibrator/self-exciting circuit. If you want to get to know more deeply this topic check how works DC/DC transformer converters.

(photo from TracoPower documentation)