How do you convert 120 volts to 12 volts?

Answer 1

Stepping down AC voltage

To step down 12 VAC to 3 VAC, you could use a step-down transformer with a winding ratio of 4:1 between primary and secondary.

Stepping Down DC Voltage
There are a number of ways one can step down 12 VDC to 3 VDC, but they basically fall into three categories:

• Unregulated voltage dividers
• Linear voltage regulators
• Switching voltage regulators

Voltage Dividers
A voltage divider is two or more resistors connected in series with a power source, which causes the voltage to be divided between them. For example, if you were to connect a 30 ohm and a 10 ohm resistor in series across a 12 volt supply, the voltage across the two resistors would be 9 volts and 3 volts, respectively:
__________________VT = 12 V _____________________
|...................R1......................................... R2........................|
+12V----| 30 ohm |--------+3V----------| 10 ohm |-------Ground
|...................................... |................................................... |
--------------VR1 = 9V-------------------------VR2 = 3V--------------
RT = R1 + R2
VR1 = VT R1 / RT
VR2 = VT R2 / RT

Note that trying to draw power from the point between those resistors would quickly cause the voltage to drop because of the parallel resistance of the load. Instead, substitute the device as one of the resistors. So, to get 3V to your device, you could measure the resistance of the device, then pick a resistor that is 3 times the resistance of the device, and connect the device to 12V in series with that resistor. The resistor will have to have a wattage rating of at least 3 times the power used by the device (or 9 watts divided by the resistance of the device), to keep from overheating. Also, If you are using batteries for power, make sure that the resistors you connect are of a high value in the order of kiloohms or else the resistor divider network could drain your power source.


The problem with this approach is that if the input voltage (12V) changes, or the resistance of the device fluctuates, the voltage to the device will fluctuate. Basically I would only use this method for simple resistive loads such as light bulbs. Linear Voltage Regulator
A linear voltage regulator is a circuit that acts like a voltage divider, but automatically changes resistance to maintain a constant output voltage regardless of change in the current drawn from its output, or variations in the input voltage.
While the circuit itself has a bit of complexity, integrated circuits that perform this function are commonplace and inexpensive. One the cheapest and most readily available is the LM317T ($2.29 at Radio Shack) regulators, which accepts an input of up to 40V and has an output voltage of 1.2V to 37V, determined by a pair of resistors attached to the regulator. This regulator can provide up to 1.5 amps of output, more with the addition of external transistors. I've posted a URL to the datasheet for this regulator in the Links section.

Linear shunt regulator
A shunt regulator made from a resistor and a zener diode is another type of linear regulator. Connect a resistor from 12V to the cathode of a 3V zener diode, and the anode to ground, then connect the load from the cathode to ground. The zener diode will draw current as needed to hold the load voltage near 3V. The output voltage will vary with load current, although not as much as a voltage divider. It will also vary with temperature. The zener diode and resistor must be sized correctly to handle the currents involved. Switching Voltage Regulator
One of the disadvantages of linear regulators is that they are very inefficient, especially when drop between input and output voltages is large, because the extra voltage has to be dissipated away as heat. In the case of regulating 12V down to 3V, you are basically throwing away 3/4 of the power being used, meaning an efficiency of less than 25%. In some applications this might be acceptable, for example in an automotive application where plenty of current at 12V is readily available. In other applications such as battery powered devices, such a waste would be a bad thing.
Switching voltage regulators work on a much different principle, switching the power on and off at a high rate of speed to regulate the output voltage (see the URL for DC-to-DC Converters in links for a more detailed explanation). They offer efficiencies of up to 95%, and can even output higher current or voltage than the input. They are more expensive than linear regulators, but offer more performance per buck - for example, for $9.25US through Digi-Key the LT1074 (see link) can provide up to 10 amps of output, and accept input voltages up to 60V.



Actualy two types passive and active. No matter what names to add to that.

Answer 2

You buy a voltage converter or power supply that takes 120 VAC as an input and provides your desired 12 VAC or 12 VDC as an output. You need to know your current requirements and size the converter or power supply to provide the required amperage.

Answer 3

Use a 'step down' transformer. you can make your own from a bar magnet, some copper wire and a length of adhesive tape.

Wind one piece of copper wire around the bar magnet ten times - leaving a little space between the wire as you go. Connect both ends of the wire to the power source.

Wind the second piece of wire around the magnet BETWEEN the wires of the first coil (without touching) but only wind FIVE times. Connect the ends of the smaller coil to the equipment.

Once you've got the coils wrapped correctly - secure everything with some adhesive tape.