I recently became interested in doing a Molex connector mod. This mod consists of soldering a LED light onto the 5v (red) wire and the ground (black) wire. This makes your molex connector light up which can look very interesting.

Take a look at this website for How To on molex modding, and images of the final result.
http://www.metku.net/index.html?sect=view&n=1&path=mods/molexlight/index_eng

The main problem you encounter when doing this mod is, it is hard to find an LED that is small enough that runs on 5volts. I'm not even sure a 5volt 3mm LED exists. Because of thise you need a resistor. The resistor is either going to stick out of the molex connector a bit or your going to have to figure out a way to fit the resistor+led inside the molex. The method I am choosing to use is to just led the resistor stick out a bit, and place an inch of heatshrink around the wires outside of the molex (since I am going to be doing that anyway after installing wire sleeves, it all works out).

Well, today was my very first time ever soldering. I think I did pretty good. I ran into a small problem with this though. It's hard to get the molex pins back into the molex after doing this. You have to make sure there is no connection between the 5v wire and the ground wire other than the LED light or you'll end up shorting/frying something. You also have to eliminate part of the "wall" inside the molex connector that is between the 5v/ground wires so that you can fit the LED inside. Not sure what is a good tool to do this though -- if you can think of one let me know. You need to eliminate about 75% of the "wall" .. the other 25% needs to stay there so your two molex pins dont end up touching each other as a result of taking out the plastic wall.



my first soldering job (look below)

Make sure you have a fire-extiguisher handy when you come to test that mod.

Pretty intereseting. To my opinion, the result is awesome. Just a quick note, I have a Nokia cell phone. It's got a few blue leds that make a hell of a light. I'm sure that that's what you would need. They make a strong light and are small enough. Again, as you said, they are rated 3.6v. I know you can overvolt a led a little, buy is it possible to put 1.4v molt to a led without burning it? And if you could then we wouln't know the lifetime of that led....

The series resistor is required for this mod to work. Basically you need to know the normal operating voltage and current for the LED that you are using.

Next you subtract this operating voltage from the supply voltage (5v in this case) and you now know how much voltage must be dropped across the resistor (supply voltage minus the voltage dropped across the LED), and current (same as for LED).

You can now work out what size resistor you need to connect in series from R=V/I.

Note: Because the LED is a 'diode', it will maintain a near constant voltage drop across its terminals, thus any remaining voltage is dropped across the series resistor. You can therefore 'increase' the resistor value for a dimmer light if required (same voltage @ reduced current), but you should not 'decrease' the resistor value as then you might blow the LED (too much current), or cause other problems with the PSU or the device that the power connector is attached to. Have fun.

Well I know I need to drop 2.5 volts... 5volt supply minus 2.5 volt LED. My "guess" is I need a resistor of about 125-150 ohms.

Not sure yet, but after speaking with some people on here I think I am just going to install 2 LED's instead of one, thus eliminating the need for a resistor. (2.5 x 2 = 5)

Don't do it! You need to have a resistor to take up any slack in the voltage output, otherwise your PSU will 'fight' to try and maintain the 5v (if your LEDs only require 4.9 v) and the LEDs will burn out in a short time. Some LEDs have built-in current limiting resistors, but most don't, so unless you are certain you should not leave out the resistor.

Here is a useful link on LEDs ...

LED Information and Technical Data

Quote: "... a resistor must be used in series with the LED to avoid destroying it ..."

Hmm I'll take a look at that tonight, gotta leave for work soon. Thanks

Acid, I thought you said these LEDs were max 3.5v or something like that, if so, you can use two with no problem, otherwise Nic has a point. Anyway, it shouldn't be too much trouble to find out the current of those LEDs, you should be able to get that info from their packaging or from a datasheet(if you got them from Mouser or NTE or somewhere like that). from that you can find the resistor size you need, just as Nic outlined in a previous reply. According to how many of these things you are building, you may also need the current of the 5v rail as we were discussing in the IRC channel. This wouldn't really be necessary unless you are going to run a bunch of them and already have other devices running off that same rail.

They are 2.5 volt LED's with 3volt max. I tested one with 3volts and it burnt it so I dont think 3volts will work .