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Easiest way to make 12vdc into 6vdc
It's been a really long time, but how is the easiest way to make 12vdc into 6vdc. I want to run a 6vdc load off of the cigarette lighter in my car. Can't I just run the supply through a couple of resistors in series? Could someone help me with the math?





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What you really need is a voltage regulator. You could design your own, but you can get one off the shelf cheap enough that it isn't woth the bother (IMHO). 






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Let's say that your load has an equivalent resistance of 200 ohms. If you make a voltage divider out of two bigger resistors, say 2000 ohms, then what you end up with is a voltage divider with a resistance of 2000 ohms and a resistance of both 2000 and 200 ohms in parallel. The equivalent resistance of two resistors in parallel is Req=1/(1/R1+1/R2), which in this case would be 1/(1/200+1/2000), or about 181 ohms. Whenever you have a larger resistance in parallel with a smaller resistance, the equivalent resistance is going to be just slightly less than the smaller resistor. Now you've got a voltage divider of 2000 ohms and 181 ohms, which means your output voltage is going to be 12*181/(2000+181) or just under a volt. A 6 volt device isn't going to work very well being powered by one volt. If we use two smaller resistors instead, say 20 ohms, then our equivalent resistance is about 18 ohms, and our voltage divider is 12*18/(18+20) so we'll get 5.7 volts out of our voltage divider, which is close enough for most 6 volt devices so we'd be fine in this case. The problem here is that we have two 20 ohm resistors that are sucking up current all the time. With the load disconnected they are going to draw 300 mA (I=V/R), which is going to be converted into 3.6 watts of heat. Our device, on the other hand, only draws 0.18 watts, so basically our voltage divider uses 20 times as much power as the device we are trying to power. If you try to scale this up to provide a lot of current, you can see that your resistors are very quickly going to become space heaters. (PS  it's late at night, hope I did my math right) 




The use of a voltage divider is unecessay in this case. A series resistor equal to the resistance of your device will give you 6 volts across the device. The current used by the device, or the power (Watts) rating should be listed somewhere on it. Just divide 6 V by the current or 6^{2} by the Watts to get the size series resistor in Ohms.
However, investigate Booker57's suggestion first. A series resistor will be a pain and if considerable power is involved it will be big and will get pretty hot. For example if the current is 1/6th Amp (1 W power) the resistor will also have to dissipate 1 W and something like a 5 W resistor will be needed to avoid high temperatures and even it will get warm out in the open air. 




There are four ways to do it.
1. If the load impedance is constant, you could select an appropriate series resistor and be done with it. (The series resistor and load resistance form a voltage divider.) But this technique is rarely used, because it’s usually the case that the load impedance is not constant, which means the load voltage will not be constant. Note that this is also an “unregulated” approach. 2. To improve voltage regulation at the load, you could use an external voltage divider, and then tap off one of the resistors. But this technique sucks if you’re trying to power something. This is due to the Thevenin equivalent source resistance. Normally, if you’re trying to power something, you want the equivalent source resistance to be low (or ideally zero), and the only way to make it low using a resistor divider is to use small value resistors. But this means the resistor divider itself will dissipate a lot of power. Again, this technique sucks if you’re trying to power something. 3. A linear voltage regulator. A voltage regulator is the same as #1, except that the series resistance is automatically adjusted (using a closedloop controller) so that the output voltage remains constant. Regulators such as the 78XX series work great; they’re simple to use and very inexpensive. The only drawback is that they’re no more efficient than using a series resistor. 4. Switching regulator / DCtoDC converter. These are the hot thing right now. There are two types… some convert the DC to a highfrequency pulsed (AC) waveform, send it through a transformer, and then convert back to DC. Others use energy storage devices (caps and inductors) and a switching circuit. The former offers galvanic isolation, while the latter is simpler, less expensive, and more efficient. At any rate, switching regulators / DCtoDC converters are popular because they’re efficient. But they do have some drawbacks… it’s the most expense option, and the output may have some noise spikes in it. 




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