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What resistor would I use to reduce 12v to 9v?

6 Answers
What resistor would I use to reduce 12v to 9v? Im making a 9 volt brick for a guitar pedal case. I have a 12 volt 1 amp power transformer and want to convert the voltage down to 9 volts. The brick will have 4-6 output jacks plus 1 LED. What resistor size would I use and/or any other components to give a constant 9 volts. Thanks
P

There are three options for doing this. If the load is constant you can use a resistor to drop the voltage by 3 volts from 12 to 9V.

From Ohms law, the required resistor would be V/I so for a 1 amp current draw

Resistance = R = V/I = 3/1 = 3 Ohms.

The power dissipation would be I^2R or (1^2) x 3 or 3 watts

The resistor would have to be a wire wound type and possibly one you could bolt to a heatsink.

The chances are that the current requirement from the source would not be constant and what you would need is some form of voltage regulator. A linear regulator behaves like a variable resistor, with feedback to keep the voltage constant as the current demand varies.

As Jockto86 suggests, you could use the LM317 variable regulator. If you just need a small current, you could use the LM317T. The LM317K with the TO3 case style has a lower thermal resistance and bigger surface area so is a better job for screwing to a heatsink as it takes two screws. The terminals are easier to connect to as well because they are further apart and thicker and less likely to bend.

This is the datasheet for the device:
http://www.datasheetcatalog.org/datasheet/nationalsemiconductor/DS0090 63.PDF


The problem with the 2 solutions above is that you get waste heat and might need a heatsink to cool the devices. Another option is a switching regulator. These don't drop voltage across a transistor and therefore are more efficient and don't generate as much heat.
This is a typical device:
http://www.newark.com/xp-power/sr05s09/ic-dc-dc-conv-330-khz-sip3/dp/2 4T5071

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J

Unfortunately, it's not that simple. If you were using one device that draws a steady current, you could use a resistor to drop the voltage. If I understand correctly, you want to have options to run up to 6 devices. This won't work as the more current you try to pass through the resistor, the more voltage will be lost across it. In other words, you won't have a steady 9 volts.

Your best bet is a simple voltage regulator circuit using an LM317, 3 terminal voltage regulator and two resistors. They may still have these at Radio Shack or at least they can order one. Shouldn't be too hard to find info about building a simple volt regulator online.

Here's the first thing I found:

http://www.techlib.com/electronics/regulators.html

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F

I am not sure if this is still livebut...

1. Find an old mains adapter that produces close to the current/voltage you need ie one that is say 9v to begin with and has a 2A output. Alternatively you can buy them in any combo you can think of in the form of laptop power supplies and so on.
2.Buy a multi-rated plug. I have one. It is a block that plugs in the wall socket and has little dials on it. You can select 1.5V up to 12V and various current levels in adddition you can select the polarity of the output jack.
3. Just load up the circuit with standard 9V bats or on of those big lamp bats - the big boxy one with springs on top.

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U

I am not sure what works. But I am sure using a simple resistive voltage divider is not going to work and it is NOT a good idea. Reasons are:

1. Resistor(s) taking out 3V is/are going to heat up. It is a fire hazard.
2. The amount of volts the resistor(s) take(s) away depends very much on the load. You want to support variable number of load in your design. And I assume each load consumes different amount of current at different times too. So the 9V coming out of the resistive voltage divider is not always 9V.

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L

You would need a bridge rectifier and smoothing capacitor before you connect the 7809 so that the AC from the transformer is converted to DC before reaching the regulator chip.

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L

You can use a 7809 regulator chip to drop to 9V.

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