Which resistor should I use to get a 9V current to 3.7V?

Question

If I have a 9V current that I want dropped to 3.7V, how many ohms should my resistor be?

Extra credit: What will the bands on this resistor look like?

Answer 1

What 2 Resistors should I use?   hmmm lets see so lets say we use .001Amp of Current in our circuit,so 9Volt divided by .001 Amp equals 9000  Ohms in Total or 1000 Ohms per Volt,9 subtract 3.7 equals 5.3 therefore R1 equals 3.7 times 1000 and R2 equals 5.3 times 1000

Answer 2

First of all 9V is not current. Volts measure voltage. With the information youve just given me Id say a 2.43 ohm resistor would do the job, that would be a red yellow gold gold resistor.

Answer 3

depends on what the current is.

9V is a voltage, not a current.

If you know the current you are using, you can calculate the resistance needed, R = (9-3.7)/I

But if the current changes the resistor will not work, the voltage will be above or below 3.7 volts.

The only way to get 3.7 volts that stays at 3.7 volts is to use a 3 terminal regulator IC.

edit: "I'm planning to charge my phone using a 9V battery and a car charger."

huh? a car charger needs 12 volts to run, not 3.7 volts.

It may run from 9 volts as is. Try it, or look up the specifications that should be in the documentation.

Answer 4

The charging current must need to know first, and I do not think you are able to find it without using a current meter. I assume it is 500ma.  If you use 9 volts...

R = ( 9 - 3.7 )/0.5 = 10.6 ohm     get a 10 ohm resistor is good enough. Next you need to know the wattage of this resistor,otherwise, it shall smoke and burnt.

W = 0.5^2 * 10.6 = 2.65 watts ,

choose  a  5 watts or more ,10 ohms resistor.

Next, if you use the car battery charger. It usually has output at least 15 volts.

R = ( 15 - 3.7 ) / 0.5 = 22.6 ohms

buy a 20 ohm or 25 ohm resistor is good enough.

Now calculate the wattage...

W = 0.5 ^2 * 22.6 = 5.65 watts

Choose a 10 watts or more , 25 ohms resistor.

These power resistor make of ceramic  casing,about 2 inches long. white or brown color,inside wound with  resistance wire. cheap to buy....10 to 50 cents US

Answer 5

It seems like that you have to learn basic electronics first or just buy you a phone charger. Voltage and current are not  same thing.

Answer 6

>I'm planning to charge my phone using a 9V battery and a car charger.

OK, so your phone power inlet needs 3.7V?

The elegant way is to use an adjustable 3-terminal regulator, but you need to choose resistor values for that, and build it onto a circuit board of some kind. If you don't know voltage from current then I'm guessing that may be a problem, so let's not go there.

Instead you could use a zener diode to drop the voltage. Choose a 1.3 watt rated one with a voltage rating close to Vin - 3.7. For 9V the nearest zener voltage is 5.1V. For a car battery, which is about 12.5V, use an 8.2V zener. Car battery voltage can change a lot though, so you may destroy your phone if you try this!

Wire the zener diode in series from battery +ve to phone input +ve, with the cathode of the zener (marked by a band) toward the battery.

Answer 7

It sounds like you want to power your electronics, and since the current can vary, a resistor is not the best solution.  A better solution would be a voltage regulator.