12v down to 9v

i have a wireless camera i'd like to use on my car and use the 12v from the car to power it up..the onlything is that the camera needs a 9v power supply..my question is how do i go about scaling down a 12v to around 9v power supply..

thanks

Quite simply. Just make a voltage divider. A simple one would be a 3ohm and a 9 ohm resistor. If you hook up the 3ohm reisistor to 12v then the other side of the resistor to the 9ohm reisistor then to ground, the voltage between the 9ohm and 3ohm resistor will always be 9v. Obviously you will want to use larger resistor values so you dont have huge current drain through those resistors, just make sure the resistors can handle the amout of current you require of them and they are in the same ratio of the 3 and 9 ohm resistors. Ex: 300ohm and 900ohm, 30ohm and 90 ohm, 300000ohm and 900000ohm, etc

hope this helps

"Quite simply. Just make a voltage divider. A simple one would be a 3ohm and a 9 ohm resistor. If you hook up the 3ohm reisistor to 12v then the other side of the resistor to the 9ohm reisistor then to ground, the voltage between the 9ohm and 3ohm resistor will always be 9v. "

 Until you try to pull some power from that junction point.. any draw at that point will increase the current through the 3 ohm resistor and decrease the 9v that you desire. Using your example of a 3 ohm and 9 ohm resistor voltage divider, and considering a current draw of 500mA, what would be your voltage?

 The math.. 1.5 amps through a 3 ohm resistor would calculate to a 4.5 volt drop. So, instead of your 9 volt, you've got 7.5v.

 Also keep in mind what happens to your voltage divider once the input voltage goes up as the alternator starts charging..

"Obviously you will want to use larger resistor values so you dont have huge current drain through those resistors, just make sure the resistors can handle the amout of current you require of them and they are in the same ratio of the 3 and 9 ohm resistors. Ex: 300ohm and 900ohm, 30ohm and 90 ohm, 300000ohm and 900000ohm, etc"

 Good that you considered the power rating of the resistors, but increasing their resistance will make the voltage drop even worse.. using a 300 ohm/900 ohm resistor voltage divider circuit means that..

 more math.. with no load, the voltage divider is flowing 12v/1200 ohm = 0.01 amps, or 10 mA. Attempting to draw 500mA from the junction point (between the two resistors) would result in a voltage drop of 300 ohm * 0.510 amps = 153 volts.

 Voltage divider circuits have their uses, but this isn't one of them. Best bet is to use a RS DC adapter, or build something using a voltage regulator.

 Regards,

 Jim

 Well, when you consider that a 9v regulator costs about a buck

https://www.partsexpress.com/pe/showdetl.cfm?DID=7&Product_ID=11444

and can provide up to 2 amps when properly heatsinked... well, what other choice is there?.. <grin>

 True, a current requirement hasn't been provided.. and if the current requirement doesn't vary, and if the input voltage didn't vary so much, and if the current requirements were fairly low (<0.1 amp), then just maybe a voltage divider would be acceptable.. they work well as a voltage reference, but not very well as a power source. Just the cost of a 9 ohm resistor in the watt size required would be more than the cost of a regulator.

 Can we get a current requirement from the OP?

 Regards,

 Jim 

 A zener and resistor would be a great improvement over a voltage divider circuit..  Here's what National says..

"The LM78LXX series of three terminal positive regulators is available with several fixed output voltages making them useful in a wide range of applications. When used as a zener diode/resistor combination replacement, the LM78LXX usually results in an effective output impedance improvement of two orders of magnitude, and lower quiescent current."

 As I said a few posts above, the main problem with the voltage divider circuit in an automotive application is that the input voltage varies so much.. if it wasn't for that then, as I said, just maybe a voltage divider circuit would be acceptable.

 More than anything I was amused by the person who originally stated that

"Obviously you will want to use larger resistor values so you dont have huge current drain through those resistors, just make sure the resistors can handle the amout of current you require of them and they are in the same ratio of the 3 and 9 ohm resistors"

 Now understand, the larger you make the resistor value, the more voltage variation you'll see when you try to draw power from the junction. I did the math in an above post.

 A couple years ago I needed an alternate power supply for a digital camcorder.. I was taping a wedding and needed hours of battery power. So I used an adjustable voltage regulator and a number of 12v 7ah SLA batteries. It worked extremely well with varying current draws from as little as 300mA to over 1 amp. I'll admit that I was a little apprehensive the first time I connected the camcorder (I had bought it a year earlier for $800) but things worked out well. I did some power measurements (power in vs power out) and don't remember any horrible no-load current draws.. any way you look at it, the voltage regulator method will be more efficient than a voltage divider circuit.

 My intention was to point out the downsides to using a voltage divider circuit for this application.. the person who initially presented the idea didn't seem to understand the principles involved..

 On to other things.. Mad Scientists Club

https://www.madscientistsclub.com/

 I read them when I was just a kid.. they were recently reprinted by purple house press..  great stories about really smart kids.

 few months ago the HVAC system in the house stopped working.. I came home Saturday afternoon to a hot house. Not good. With all the cr*p I bring home (as my wife puts it) I knew that if I could get the AC working again using stuff I had 'in-stock' then life would be good in the future with regard to electronics stuff being brought home. I ended up using a variac to generate the 24VAC and ran it through a Valhalla Scientific Wattmeter to monitor the voltage and watts. Now the Wattmeter sells used on the Internet for about $900 and the variac probably goes for a couple hundred. So I've got around $1000 in gear replacing a $10 furnace transformer.. but it worked.

 I've got a pretty good workbench setup.. I can generate up to 300Vdc @ 7 amps, and on the AC side I can get about 280Vac. Lots of meters and other test equipment. Just stuff I've collected over the years.

 Regards,

 Jim

xetmes wrote:

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I didnt say a voltage regulatior is not a better idea, but to say a voltage divider unusable is conjecture. Why not use a zener and a resistor? Depending on current requirements power loss may be less. Why not a switching regulator? Linear is watefull.

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