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Wire up LEDs to PC

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  • Power Supplies
  • Fan
  • LED Monitor
  • Components
Last response: in Components
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July 20, 2012 12:44:14 PM

Hi all

I have done some searching but no results for what I need.

I have a number of fans that I want to be able to turn off as and when I want. I dont want to buy a fan controller as i'm working on my own front bezel mod.

So far so good - I have wired up a switch from Maplins to a fan, running from a Molex. Now I want to put an LED in line so that it lights up when the fan is on i.e. the switch will turn on the fan and led at the same time.

My problem - I don't know which LEDs and resistors to get. I'm guessing if I pop down to Maplins and buy the first LEDs I find, and solder them in line with the fan...something will go pop because of voltage/amp ratings...but I don't know.

So on the surface it's quite a simple mod...I just need to get the right parts.

Can anyone provide any advice?

Many thanks

More about : wire leds

a b ) Power supply
July 21, 2012 1:50:18 AM

You need some spec's and some calculations. You will need to connect a LED and a resistor in series, and that pair then between the +12 VDC output of the switch and Ground. The trick is: what resistor? You will need to calculate both resistance and power rating.

First, you need the spec's on the LED. Most are for +5 VDC, but get the spec for yours. Call that VL. You need to size the resistor so that it drops the voltage from 12 to VL, so the voltage drop across the resistor must be (12-VL) volts. To figure the rest out, you need the current spec for that LED. It will be in milliamps, or mA. Call that I, in amps. That same current must flow through BOTH the LED and the resistor. So now you can calculate the Resistance needed in Ohms, as R=V/I = (12-VL)/I. And remember to change the mA spec to amps by dividing it by 1000.

Next, the power rating. P=VI = V^2 /R. The V here is the voltage drop across the resistor, (12-VL), and it has to be squared in the formula. R is the resistance in ohms of the resistor, as you got it above. The power you get is in Watts, so you need a resistor able to handle at least this much.

As an example, suppose your LED is spec'd at 5.0 VDC and 4 mA.

R=(12-5)*1000/4 = 1750 Ohms

P= 7*7/1750 = 0.028 Watts

So a 1.8 Kohm or 2 Kohm resistor will do nicely, and the power dissipated is very small - about 1/40th of a Watt - so you will have no problem finding a resistor - they are commonly available in 1/4 watt, 1/2 watt, and up. If you want, use a slightly higher-value resistor than you calculate, and the LED will just glow a little dimmer.
July 21, 2012 2:37:10 AM

That's perfect thankyou

What do you think of this wiring method? The aim is to not take any power away from the fan:

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a b ) Power supply
July 22, 2012 1:46:18 AM

Yes, that's right.
July 23, 2012 12:14:18 AM

Paperdoc you're a legend - thanks, it worked, and here is the first phase of my project - switch 1 complete:



!