How do you convert 12 v DC to 5 v DC?

Answer 1

There are many ways this can be done.
How to do this depends mostly on how much current you need.
It also depends on how accurate your 5v need to be.

Method 1: Magnetizm (and transistors).
One can create circuitry for generating AC from the DC and then feed this AC into a wire with many turns around something that is easily magnetized.A second wire with fewer turns around that something, normally a ferro toroid, will provide you with second voltage. (Half the output voltage, half the input turns) The output voltage also need to be rectified.

There is quite a bit of math involved in order to create the "perfect" circuitry that balances this properly, but in many cases the benefits of this approach outweigh the disadvantages as in costly and timeconsuming to make.

Method 2: Transistorized.

Just think of the water in your tap. Consider the pressure to be very very very high, and that you can only turn it fully on or fully off. This would be your 12v source, and destroy equipment designed for only 5v.

Now consider the ability to turn this tap on and off millions of times per second, maybe even billions of times per second.

That will give you the ability of a transistor in the world of electricity.

It is easy and cheap to make.

Just think of something that adjust the flow and is also able to measure how much is flowing through equally as fast and regulate up or down depending on this measuring and a fixed pressure to compare with.

Now, think of the 12v as a big river. It flows into a lake through this regulator-circuitry, and causes certain fluctuations and waves in this lake. The bigger the lake, the smaller will the fluctuations and waves be.
In the world of electricity, these fluctuations and waves are known as ripple.

Tiny inaccuracies in the "close to perfect" regulated voltage (in this case 5 v).

Likewise, the lake is the capacitor in the electric world.

If you need no more than approx 1 Ampere and a tiny ripple is acceptable then the easiest solution would be a single IC-circuit in TO220 package named 7805.
You add 12 Volt to one leg, GND to another leg, and the third and last leg provides you with 5 v. You should also add a capacitor of approx 100-1000 uF between 5v output and GND, and a capacitor of 1000 uF between 12 v and GND.
You need to add a heat sink to this IC.
Disadvantages: Most of the voltage drop will be transformed into heat. You do not get more current than your original 12 v source can give.
Advantages: Very easy to obtain. Easy to use. Overload protected. Heat protected.

Other chips similar to 7805 exist. Some with the possibility to adjust the voltage with a couple of resistors.

Method 1, more:

Method 1 can not be made without transistors. it only involves magnetizm as well as everything else.
It is possible to make a high frequency step down converter. Such a converter will give you approx twice the current when transformed down to 5v.
Example: You have a transformer giving you 2 ampere of 12 v DC current. By using a high frequency step down converter, this will provide you with approx 4 Ampere of 5v DC current.
Disadvantages: Some electrical noise. Many more components needed in order to build one.
Advantages: More current, low loss factor, less heat.

A highly stable voltage of 5 volt with hardly any measurable ripple can be made using a couple of OpAmps, a Zener Diode of 5.6 Volt, an NPN small signal transistor, A PNP transistor capable of 3-10 Ampere and a few resistors.

Most 5 v equipment does not need a highly stable voltage though.