12v to 6v

[NickCoons]

I want to power a 6v device from my car battery.  Is doing so just a matter of popping a resistor inline?  How do I calculate the amount of resistance that I'll need to introduce?  What information do I need to know?

[wooferhound]

The quick answer is No

The long answer is that a resister will lower the voltage but if the Current draw changes, then so does the voltage through the resister. for instance, a lightbulb or LED will always draw the same amount of current so a resister would work there. However, operating sound equipment or a CB radio will not work as the current it is pulling is constantly changing.

[NickCoons]

I thought that might be the case.  I'm familiar with Ohm's law, and I knew the current draw would vary, so it didn't make much sense.

So the next question would be: What would be the easy way to accomplish this?  The power draw would be extremely low, and it would be coming from a running vehicle, so I'm looking for the easiest-and-cheapist-to-build solution, not ncessarily the most electrically efficient.

[JW]

Generally when this need arises for me 12vdc to 6vdc, I use a regulator, usually series 78XX or some simular family. Using the regulator semiconductors a heat sink is needed. However regulation can be variable from between/or lower than 12dc from source anywher from 3 to 12volts direct current. in general using one of these regulator chips 7volts direct current can be precisly regulated to the current specific load/capacity @ 6volts, so the regulated range can be between 7 and 12 volts, thats quite a range... Simularly a 24volt dc input can be regulated anywhere from 24volts tom 7volts(thats an even better range). So, the regulated output can renain mono-stable thruoght the performance range of the amp-hour capacity of the AH load of the battery, throught the discharge range[7 volts]. Anyone care to speculate on the minumuim state of charge(for the batt bank) when the regulator circuit exhausts it head room? Id say off hand you could utilize the batt state of charge down to 33 and 1/3rd from a full charge. If that not maximum potential, I dont know what is. So basically yes, voltage regulator chips regulate voltage precisely using PWM, without voltage drop, within a given current rating...

JW

[johnlm]

The next simpelest method after a single resistor is a simple Zener diode regulator.

These work well with a very low power application as you suggest.  Ill try to explain in lieu of a schematic.  To your basic series resistor between the 12V source and the 6 V object you are powering there is a  6V zener diode after the resistor to ground.  To determine the value of the resistor use the following example.  If your 6 V device uses say 10 mA Max and you are dropping 6 V (12V to 6 V; 12-6=6) so your resistor value is 6V/10 mA which = 600 Ohms.  To keep the regulator always operating in the Zener breakdown reagion (dont worry not used in a destructive sense) you need to add a bit of current to the 10 mA max, say 10% to 20% so instead of using the 10 mA as the denominator use 12mA so 6V/12mA = 500 Ohm.  Use a 1/2 to 1 W Zener (very common costing maybe as much as $0.50 at Radio Shack). You can select the next lower or higher std value of resistor say 470 Ohm or 520 Ohm 1/4 W size is fine.  This cheap regulator has good enough input line regulation and output load regulation for your application.  Total cost if you have to buy all the parts less than $1.00.

Johnlm

[srnoth]

Hey guys,

Why not use a variable voltage regulator such as the LM317T:

http://www.radioshack.com/product.asp?catalog%5Fname=CTLG&category%5Fname=CTLG%5F011%5F002%5F017
%5F000&product%5Fid=276%2D1778

It's pretty simple actually. I made a circuit with one recently. Basically all you need is one of those chips, two variable resistors (or two combined as one such as those stereo volume adjusters) to adjust the output voltage. You should also have a suitable resisistor to keep a very minimal load on it, or the voltage will rise. I didn't have a suitable resisitor, so I just uses a coil from an old relay. Works great.

Seems a better idea to me, as it keeps the voltage constant even when the load changes, or when the input voltage changes, such as 12v when operating on battery and 14v when the engine is on.

Cheers,

Stephen.

[wooferhound]

If you can work with 5 volts

then you could use a 7805 voltage regulator

it's a 5 volt output that's not adjustable

and current carrying up to 1.5 amp

It doesen't need but one or two other parts to make it work

If it needs to be adjustable then use an LM317

it needs about 5 other parts to make it work

Both are available at Radio Shack

[Opera House]

The real question is how much current do you need.  If you are powering a small transistor radio that draws less than 0.2A then something like a 7805 would be just fine.  These are quite adjustable as long as the minimum voltage is greater than 5V.  Put a 1,000 ohm resistor from the output to the common pin.  Then put a 220 ohm resistor from the common pin to the negative for the battery & load.  That would give you 6.1V.  A 270 ohm would give you 6.35V or a 330 ohm would give you 6.65V.   Just don't forget the caps on the input or output or the regulator will start to sing.  Above this current you should think about using a switching regulator unless you have power to waste.

[K3CZ]

There is another alternative to you, which is available from watts to kilowatts - a DC to DC switching type voltage converter.  I used to build those things with germanium switching transistors and hand-made transformers on hand wound on mag tape ring cores. This could still be done, but there are commercial alternatives(some are advertised in Home Power).  Very practical, rugged and reliable, too.  Directions can be found in the ARRL handbook, among other sources.             K3CZ

[twidget]

There is a low tech way to do this.

When I was a kid I had a VW Beetle and had to put new backup lights on it. The only thing I could find in the junk pile was a couple of 6v tractor lights, Needles to say I was a poor kid on a farm. My dad showed me that you could drill a small hole and use a sheet metal screw to tap into the top of the battery and pull a 6v tap off of it.You dont want to drill verry deep! I probaly wouldnt do this a a brand new battery, expecialy if it has any warranty. Nowdays I would use a 7805 voltage regulator with a diode in series with the ground pin to raise the regulated voltage up to around 6 volts.

[ghurd]

I would go with a 7806.

They can be had to a rating of at least 5 amps.

Keep it heat sinked. Maybe a metal box.

The caps are not always needed. Sometimes they don't work right without them.

3 parts. Cheap and simple.

G-

[richhagen]

A neat little trick I learned a while back is that you can add a diode between the ground pin of the 78xx and the ground, in effect making it see the ground as about a half a volt, and the regulator will output a nice an extra half a volt for you.  In the not so good photo of an example below, I've attached two diodes across the ground, which in this case makes the 7805 see ground as 1.08 volts, so the output voltage to the two diodes connected in series is held at about 6.08 volts.  Rich Hagen