How do you convert 12 volts to 4 volts?

Answer 1

Using a voltage regulator (LM78XX come to mind), or through Pulse Width Modulation.

In the first case, the typical output power will be at 4W tops (LM78XX are typically rated at around 1A of current). In the second case it's up to the design of the PWM circuit, I guess.

A simple PWM circuit would include an oscillator (or a square signal generator, doesn't matter) driving a transistor in a predictable pattern (turning it on for 1/3rd of the time), the aforementioned transistor, and a capacitor to smooth the ripples out. If for 1 out of 3 seconds it's 12V, and for the remaining 2 seconds it's 0V, then (12+0+0)/3 = 4V. The current rating would depend on the load and the transistor's ability. For higher loads it may be advisable to drive a second-stage, high current transistor. Also, saturation concerns come to mind, but that's a different story.

There is a third option, but it's overkill in this situation -- using a transistor and a capacitor to form a resonant circuit driving a transformer. However, this type of design is typically found in applications where the voltage needs to be stepped up, not down (ie. LCD/keyboard backlights, cold-cathode lights and the like).

Answer 2

You cannot convert 12VDC to -12VDC. However you can get these two voltages simultaneously by using a centre-tap transformer secondary. One half of the centre-tap, depending on the transformer ratio, will give you a +12VDC and the other, -12VDC, all relative to the centre point.

Answer

+12 V and -12 V are potentials measured with respect to some reference point. In the case of a vehicle, for example, the negative terminal of the 12-V battery is connected to the metal parts or chassis (a situation called a 'chassis earth' or 'chassis ground') of the vehicle. All potentials measured with respect to the chassis will then be +12 V. If, on the other hand, the positive terminal was connected to the chassis, then all potentials measured with respect to the chassis would be -12 V.

The only time we assign 'plus' or 'minus' to potentialdifferences (voltages) is to indicate their sense or their direction. If voltage A is taken as the reference voltage, we allocate a positive direction to it; if voltage B acts in the same direction then it, too, is allocated positive; if voltage B acts in the opposite direction, then it is allocated as being negative.

Answer 3

If you're talking about AC, use a transformer with a turns ratio of 3:1. If you're talking about DC, use a resistor network and/or a 4-volt Zener diode.