I need to change 12 volts into 5V, and 3.3 volts / How does Ohm's law work?

I am trying to build a power supply for one of my older computers in order for it to run in my car. I was told that I could use some an 8 ohm, non-inductive, 20 watt resistor from radioshack to provide 20 watts of power at 5V, from the 12VDC supply from my car.



There's four parts to my question:

1. If I use the 8 OHM resistors in parallel will I get more amps at the same voltage, or will I just get a different voltage?



2. What are the specifics of Ohm's law, and how does it work in converting one voltage to another.



3. What setup would I need to get a 3.3V line?



4. I'm not sure, but I'd suspect that the line coming from a car's cigarette lighter is not always at 12.0 V exactly; is there any cheap way of regulating it to become within, say +/- 0.1V of 12V?



I'm trying to see if this is worth my time, or if I should just go out and buy a DC-DC power converter. I'd really appreciate ANY feedback you'd be willing to provide, even just a tidbit, or just one segment to my question. TYVMI need to change 12 volts into 5V, and 3.3 volts / How does Ohm%26039;s law work?
You will need a regulated DC to DC converter, The voltage in a car varies quite a bit between engine on and engine off, the voltage is about 14V when the alternator is operating. Also a resistor voltage divider is is sensitive to the load current, if the current varies the voltage will vary too and you can't afford to have much variation in the 5 V and 3.3v supplies



EDIT:

An internet search for ;

dc computer power supply

gets a lot of hits, this is the first I looked at. It costs a bit, but would make things easy.



http://www.mini-box.com/M1-ATX-90w-IntelI need to change 12 volts into 5V, and 3.3 volts / How does Ohm%26039;s law work?
1. parallel resistors have the same voltage and different currents.

2. converting a voltage using ohm's law you'd have to put them in series. and use voltage division. be sure to follow kirchoff's laws when you're calculating these electric designs.

3. to get 3.3v from 12 v, you need to use series resistors with that ratio.

4. buy a voltage regulator at radio shack.I need to change 12 volts into 5V, and 3.3 volts / How does Ohm%26039;s law work?
Ok, here's ohms law: Voltage = amps x resistance

amps = volts/resistance, and resistance = volts / amps.



1. When using a parallel circuit, the voltage in each part wil stay the same, but the amperage in each will go down.



remember: in parallel resistors, 1/Rt (resistance total) = 1/R1 + 1/R2 + 1/Rn ...



So if you have 5 V running through a parallel circuit (with each part containing a 10 ohm resistor) the output voltage will still be 5V, but the resistance will go down to 5 ohms; thus the amps go up.



2. when you talk about going from one voltage to another, your talking about a transformer. a transformer is a device that uses series of coils to modify the voltage (and also the amperage). As the voltage goes down, the amps go up and vice versa.



3. Here's a helpful formual for what you're getting at:



V2 = (n2 / n1) x V1 where V2 is the output voltage, V1 is the input voltage, n1 is the number of turns on the coil ( on the side of the input voltage), and n2 is the number of turns on the coil of the output voltage.



what u do is plug in what's given (12 V and 3.3 V) and come up with the rest. the numbers really don't matter, so long as a balanced ratio between turns and voltage is established.



4. I'd guess that you're right about it not being steady, but I'm not sure. cheap method? the only thing I can think of is a surge protector. that wouldn't really regulate it, but it could cut the power if the voltage got too high. there's also a thing (i think) called dip protectors that will cut the power if the voltage is too low. sorry. I hope the rest was helpful.I need to change 12 volts into 5V, and 3.3 volts / How does Ohm%26039;s law work?
Whoa! Don't do it. Ohm's law says that the voltage at the load will vary as the current of the load. Will your computer always draw the same current. No. Another whoa! The input voltage from the car will be about 12 volts when the engine is stopped, but it will be about 13.5 volts or more when running in order to charge the battery. This unregulated supply is not what you want to subject your computer to.



As computers are so expensive, go buy a regulated supply that will keep the output steady regardless of the input. It will also probably have filtering components to keep noise from the car from entering the computer and causing errors. Or you could get a 12vDC to 120vAC converter to supply 120vAC to your indoor computer supply, but that would be more inefficient in a number of ways. (In electronics, this would be known as %26quot;icky.%26quot;)



And now for something entirely different. When Ohm went before his fellow scientific colleagues with a humongous formula for relating resistance, current and voltage, he was literally laughed off the stage as they thought it was wayyyy too simple an explanation. He went home and rethought the problem and discovered he was completely wrong. It was too complicated! His law says the three are simply proportionally related, with no tricks and lines of squiggles on the blackboard. Thank you, Mr. Ohm, for the added effort. Here it is:



E means electromotive force in volts (voltage)

R means resistance in ohms

I means intensity in amps (current)



E x R

________



I



Cover the desired quantity with your finger to get the formula. For example, to get volts, cover E, leaving R devided by I.

To get volts, cover I, leaving E times R..



The hard part is learning to properly apply it.



Late flash: I just noticed you said 5v and 3.3v. Who has a computer that doesn't come equipped with a 120VAC supply? Are you trying to run the motherboard with no supply? If it's an older PC, you can find used power supplies all over the place; they throw them away! And what's with the 3.3V? Usually that is regulated on the motherboard, with only 12V and 5V provided by the power supply. At any rate, you need regulation in an auto environment.