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Anyone want to check my math for me? (12V to 9V)

post #1 of 11
Thread Starter 
I want to take a 12 V fan connector and convert it to 9 V to power some LEDs. A friend of mine drew this up for me:


Basically on the positive lead there's a 1 ohm resistor and on the negative lead a 3 ohm resistor. Does that sound about right? It doesn't have to be exactly 9V, but it needs to be about in the middle of 7V and 12V. My 12V line is really more like 12.6V but for this I'm not looking for accuracy. According to the website, the lights use 2 Amps. http://www.hitlights.com/ls3528-cw.html
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post #2 of 11
I think you find this website useful - if you know all the values of the devices you want to use it will show you a circuit and tell you the exact resistors needed.

Cheers thumb.gif

http://ledcalculator.net/
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post #3 of 11
Quote:
Originally Posted by MME1122 View Post

I want to take a 12 V fan connector and convert it to 9 V to power some LEDs. A friend of mine drew this up for me:


Basically on the positive lead there's a 1 ohm resistor and on the negative lead a 3 ohm resistor. Does that sound about right? It doesn't have to be exactly 9V, but it needs to be about in the middle of 7V and 12V. My 12V line is really more like 12.6V but for this I'm not looking for accuracy. According to the website, the lights use 2 Amps. http://www.hitlights.com/ls3528-cw.html
That will not work the way you want, for a few reaons:
1. That is a voltage divider circuit. The voltage at the output only works by the calculation when the current through both resistors is the same. If you start trying to power LEDs from it, the current in the top resistor will be more than the bottom resistor. This will drop the voltage at the output.

2. The drawing has 4 omhs to ground from the 12v power, so unless you want to draw an unnecessarily 3amps from the controller (basically a wasted 36 watts and might possibly burn out the controller), then you need a higher resistance.

You don't need to do a circuit like the drawing. Just put a resistor in series with the LED that is calculated based on the forward voltage drop and current draw of the LED. The link that Fan o' water posted is great for that thumb.gif
    
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post #4 of 11
Thread Starter 
Hmm ok, that website doesn't make much sense though. I have a strip of leds, not individual ones.

http://www.blackfiveservices.co.uk/fanspeedcalc.php?Voltage=12&Current=2&Target=9

What about this website? It's for fans though.
Edited by MME1122 - 1/19/13 at 9:51am
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post #5 of 11
Quote:
Originally Posted by MME1122 View Post

Hmm ok, that website doesn't make much sense though. I have a strip of leds, not individual ones.

Their resistance adds up to the one of the individual diode at the operating voltage.

For high power LEDs, resistive voltage control is not the best idea. But if you insist, get a couple of 78S09CV linear regulators, put them in parallel, and bolt a small heatsink to them.

But as I said, I don't recommend going resistive due to the heat emitted by the regulators.
   
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post #6 of 11
Thread Starter 
Quote:
Originally Posted by Artikbot View Post

Their resistance adds up to the one of the individual diode at the operating voltage.

For high power LEDs, resistive voltage control is not the best idea. But if you insist, get a couple of 78S09CV linear regulators, put them in parallel, and bolt a small heatsink to them.

But as I said, I don't recommend going resistive due to the heat emitted by the regulators.

What else could I do then? I don't understand why this is different from lowering fan voltage with a resistor.
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post #7 of 11
V = I x R
12 = 2 x R

Thus R = 6 Ohms

As in the fan calculator you need a minimum 5W, 2ohm resistor. Beware of this getting hot..........

HTH

dunx

P.S. PWM control could reduce the output, but adds cost.
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post #8 of 11
Thread Starter 
Quote:
Originally Posted by 2002dunx View Post

V = I x R
12 = 2 x R

Thus R = 6 Ohms

As in the fan calculator you need a minimum 5W, 2ohm resistor. Beware of this getting hot..........

HTH

dunx

P.S. PWM control could reduce the output, but adds cost.

I don't think PWM control can work with LEDs can it?
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post #9 of 11
Depends on the frequency used, most automotive LEDS are pulsed to reduce power consumption, so they appear to strobe on TV. My kitchen LEDS stobe as well when dimmed.

dunx
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post #10 of 11
Quote:
Originally Posted by MME1122 View Post

I don't think PWM control can work with LEDs can it?

Yes. And in fact it is the most used way to do so, when buck converters are out of question due to price.

Just be sure you switch them at their maximum allowed frequency, you don't want any strobing to be apparent.

If you're crafty, you can do a PWM driver with a quad opamp monolithic package such as a LM324, some resistors, and a small polymer capacitor, with a transistor to drive the LEDs. All of that adds up to less than 3 dollars.

I'm buidling one for myself, so if you're a bit patient I'll post a working schematic.
Edited by Artikbot - 1/20/13 at 3:03pm
   
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PowerCaseOther
PicoPSU-80-WI-25V AIO Aluminium Handmade TP-Link Archer Something Something Wi-Fi AC 
CPUCPUCPUMotherboard
Core2Duo E6400 Core2Quad Q6600 Pentium Dual Core E5200 AsRock 4COREDUAL-SATA2 R2.0 
GraphicsRAMHard DriveOptical Drive
A dumpload of ancient AGP cards Kingston Value DDR2-667 CL4 2T @CL3 1T Seagate 160GB 7200.10 LG IDE DVD-ROM 
CoolingCoolingOSMonitor
Ghettomade CPU waterblock 49cc 2stroke engine copper radiator WinXP SP2 32bit ProView 17" 
PowerCase
Tacens Radix V 550W Ghetto aluminium bench 
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Overclock.net › Forums › Volt Mods › General Volt Mods › Anyone want to check my math for me? (12V to 9V)