# Electronic Circuits and Projects Forum

## How to Step down from 12v - 9v?

1. You don't need to think. You just need to learn Ohm's Law and do a little simple arithmatic.

2. This sucks. I used a 1 K ohm resistor - about the value it says to use. The 5v fan doesn't even turn. I tried connecting two 1k resistors in parallel, thinking maybe I could get more power that way, of course at this point I'm just experimenting. I don't know. What type and size of resistor should I be using here? The voltage source is 19V, and I want to bring it down to 8 or 9v using a cheap simple solution.

3. To step down 19V to 9V at 22mA then the resistor is (19V - 9V)/22mA= 455 ohms. I have never seen a fan that draws only 22mA.

To step down 19V to 5V for a fan then you need to know how much current the fan draws. If it draws 200mA then the resistor is (19V - 5V)/200mA= 70 ohms. The resistor will dissipate (200mA squared) x 70 ohms= 2.8W so a 5W resistor should be used.

4. Many of these fans aren't labeled, so how do I determine how many ohms the fan is? A 5W resistor should be used? How many ohms? 70?
Here is one of the fans that I plan to run.. It's rated at 5V / 0.38A . I know I shouldn't operate it over 5V, but I would rather throw away a \$30 fan in a year or two than a \$250 motherboard. So, I'm going to tap into the 19V rail off of a ceramic cap, then step the voltage down to at or around 8 volts.

5. Use simple arithmatic:
The resistance of the fan is about 5V/0.38A= 13.2 ohms.
The current with an 8V supply is 8V/13.2 ohms= 0.61A. The resistor value is (19V - 8V)/0.61A= 18 ohms. It dissipates (11V squared)/18 ohms= 6.7W so use a huge 10W resistor.

6. Umm...I wouldn't just go tapping into power from a random place on the motherboard! Your motherboard should have some connectors for plugging in fans.

If not, you could also use power from one of the outputs from your PSU. A normal Molex connector has the 5V you need coming from the red pin. Both of the blacks are ground.

7. ............................

8. Glad you got it working!

9. ## Concepts

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10. Originally Posted by audioguru
Use simple arithmatic:
The resistance of the fan is about 5V/0.38A= 13.2 ohms.
The current with an 8V supply is 8V/13.2 ohms= 0.61A. The resistor value is (19V - 8V)/0.61A= 18 ohms. It dissipates (11V squared)/18 ohms= 6.7W so use a huge 10W resistor.
Originally Posted by audioguru
Use simple arithmatic:
The resistance of the fan is about 5V/0.38A= 13.2 ohms.
The current with an 8V supply is 8V/13.2 ohms= 0.61A. The resistor value is (19V - 8V)/0.61A= 18 ohms. It dissipates (11V squared)/18 ohms= 6.7W so use a huge 10W resistor.
That's awesome. I guess what I was worried about was, how do we know this is correct, if we don't know the Amperage of the 19V line? Is that irrelevant? I think you only need one (V I or R) to determine the other two.

I broke this down to make sure I understand the concepts behind it. I think I'm starting to understand.

Fan volt rating is 5v / .38A

5 divided by .38 is 13.157. Round it to 13.2

Goal is to run on 8 volts
8 (voltage you want) divided by 13.2 (ohms of device) is .60606 - round to .61 A

Tapping into a 19V rail. We don't want 19 volts. We only want 8 .

19 (voltage you have) minus 8 (voltage you want) is 11.
11 (voltage you want) divided by .61 = 18 Ohms (This is the resistor size needed)

To determine the wattage - square your new voltage, and divide it by the number of ohms
11 volts squared is 121. 121 divided by 18 (Ohms) is 6.72
So, we need a resistor capable of dissipating more than 6.72 watts. A 10 watt resistor is sufficient.

I read that there is another way to determine wattage. Current times the voltage at a given point.
The current here is .61 amps. Multiply that by 11 (the amount of voltage we plan to dissipate.)

.61 times 11 is 6.71! Everything checks out!

I'm also going to keep reading Ohm's law definitions and tutorials until it really clicks.

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