anyone know what value resistor to use to get 9vdc from 12vdc? any help would be great
Cherokee Kid, I'm going to be a typical Australian here and not give you the answer, what I will give you though is a hint on how to work out it yourself....
Using several equations of Ohms Law, it will give you the correct value of the resisotr required.....
Sorry, but it's something you need to work out for yourself.
Raymond Wilkinson-Reed
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Check all advice given with a meter
And I, being a typical American, will say I agree with the Ausie answer above that you need to understand the theory for yourself, but it never hurts to have a shortcut:
https://ourworld.compuserve.com/homepages/Bill_Bowden/r2.htm
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Support the12volt.comIt's easy to use two resitors to get from 12v to 9v using a voltage divider, but the circuit will change as soon as you put your load on it. This diagram assumes that the load impedance will be much greater than the resitors used thus the effective parallel load will not be affected. However if your load impedance is low enough and close to the value used in R2 you will get much less than 9v.
You'll need to know your loads impedance in order to do this properly. Once you measure make R2 at least 10x your load impedance.
Using Ohm's Law, you can find the resistance..
Volts equals Amps times Resistance
If you want to drop from 12 volts to 9 volts, you'll be dropping 3 volts. If you know the current draw of your device you can figure resistance. Assuming 500mA current draw, your resistance would be 3 volts divided by 500mA.. the answer would be 6 ohms.
Depending on what you want to do, you'd probably be better off using a voltage regulator.. describe what it is you're trying to do.
Jim
One bleeder resister in series with a zener diode rated at 9v in parallel with your load will give you what you need. Go to Radio Shack, tell them what you're doing, what you need, and they'll walk you through the rest. The resistor you use will need to safely handle the current draw of the piece of equipment you're trying to power.
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My ex once told me I have a perfect face for radio.