Why can't you just use a coil transformer, like when you want to get 240 volts out of a car battery instead of a boost converter? I found a thread that sparked my interest - How to transform 5V into 12V?. I want to do a similar thing so I can charge things like a laptop from a portable 5V phone battery pack.
Just to be clear: we are talking about converting 5V DC into 12V DC.
The 2013 question is about DC-DC. Plus, there is a dc-dc-converter.
For a transformer to function, a varying magnetic field must be created inside it. The varying current in the first winding (the so-called "primary" winding) creates a varying magnetic field, which in turn creates an electromotive force in the secondary winding. This does not happen when the primary is fed with pure DC.
Nevertheless, it is possible to make a transformer-based DC-DC converter. DC is chopped so that the transformer is fed with varying current. This is normally done with semiconductor switches. The output of the transformer secondary is rectified, which produces a DC output voltage.
For a small increase in voltage (factor of 6x or less), an inductor-based boost converter is usually more practical than transformer-based (in general, although there are important exceptions).
You're probably wondering why your Falstad circuit seems to work. The problem is not with the simulator. CircuitLab shows the same behavior.
This happens under the following conditions:
- The DC source is ideal (no resistance).
- The transformer is ideal (no loss or primary-side resistance).
When you apply a voltage to an inductor, the current increases over time. If there's any resistance, the current will eventually level out (DC), at which point the inductor does nothing. But with no resistance, the current can rise forever.
In an ideal transformer, this endlessly rising current produces a DC voltage and current on the secondary side. So ideal transformers do work at DC. But there's no such thing as an ideal transformer. Add a small resistance like 0.1 ohms between your voltage source and the transformer, and you'll see the secondary-side output decay pretty quickly.
I'm guessing this has been asked before, but I didn't find it in a cursory search, so I'll give a short answer here.
Coils only effectively transfer energy while the current through the coil is changing. In a dc circuit, the current through the coil(s) only changes noticeably immediately after the circuit is first closed, then again right after the circuit is re-opened. The current builds up until the resistive losses in the coil, or the source impedance, waste all of the available power, with no meaningful energy transfer from one coil to another.
For a (probably better) more in-depth explanation, this article looks interesting.