Pages: 1

 

2007-02-09 08:23:38
Stormscythe
Aiming for the head
+88|7012|EUtopia | Austria
Hi folks,

so I had this old case fan with blue LEDs which I exchanged for a silent one. I said to myself: Hmm... why shouldn't I remove the LEDs and put them into my case? Well, I did it, soldered the wires off the cooler, looked up the voltage on the molex cable that went to the fan (contacts for fan/LEDs were the same) and soldered the wires together again, so that I had 4 LEDs on a string, connected to some 12V molex.
Now, that I'm not a complete newbie with electronics I thought, hum, this would work quite well. Gnah!
The first thing for me to notice was that the fuse for my room blew up, so I fixed this (I experience this quite often when re-connecting my PC, actually) and tried to switch the PC on again.
Well, what can I say: All fans rotated for about 1-2cm and then stopped, my CPU wouldn't even do a single operation, neither would all the other components.
After 20 minutes of struggle to get that cheap, crappy molex cable off my beautiful PSU cable (yes, I have blisters on my fingers and a completely destroyed, cheap molex cable now, with some fancy LEDs that won't work) my PC worked again.
I'm not only totally puzzled but also relieved - my PSU obviously didn't take any damage.

Still, I wonder...


PS: I did some amateur-stealth-wiring, took me about 45 mins just to have that stupid light shine on all of my shoddy components. Nevertheless I hope, the airflow has improved a little by this.
2007-02-09 08:41:10
LT.Victim
Member
+1,175|7026|British Columbia, Canada
... You Connected some LED's to a 12 V rail?
2007-02-09 08:42:52
heggs
Spamalamadingdong
+581|6851|New York
something tells me you need some resistors in there to limit the amperage. you were probably creating a short somehow. here's a link i found that basically explains what you should ahve done. what this guy is doing is a little not straight, but very similar to what you were attempting.

http://www.emanator.demon.co.uk/bigclive/trink.htm

here's another one further explaining what you should have done.

http://www.metku.net/index.html?path=mo … /index_eng
Remember Me As A Time Of Day
2007-02-09 08:55:18
Stormscythe
Aiming for the head
+88|7012|EUtopia | Austria

LT.Victim wrote:

... You Connected some LED's to a 12 V rail?
Yes, they were connected to a 12V rail parallel to the fan they lightened up before, so why change it??

As I said, I'm not quite as bad with electronics as it may seem now and of course checked that there were resistors pre-mounted to that LED rail.

What I forgot, though, was that with all that connections on this cable (2 DVD drives, floppy drive, 2 fans + 4 Leds (also parallel d'oh ^^) it might have just lowered the overall resistance too far.

Hell, now I'm even more relieved my PSU didn't take any damage...
2007-02-09 10:56:13
Airwolf
Latter Alcoholic
+287|7183|Scotland
Man I had a problem like you do. I had these LED's on the front of my case under a cover. One time I decided to scrap the fan that were running in parallel with it and connected the LED's straight to the molex.

To my utter disappointment, I connected the molex upside down (somehow) and the LED's blew.

*pop*

lol. I finally replaced them and always keep them in parallel with fans because the current splits between components


EDIT: use 56® resistors for maximum luminance

Last edited by [E.F.L]Airwolf (2007-02-09 10:57:22)

2007-02-10 06:01:03
Stormscythe
Aiming for the head
+88|7012|EUtopia | Austria
Okey, no that I've tried the leds with a 9V block battery I've come to the conclusion that the manufacturer of the LED rail was incapable of distinguishing between "+" and "-"... I still wonder why it worked on the fan then.
2007-02-10 14:05:52
CrazeD
Member
+368|7136|Maine
It worked on the fan because there is a resistor on the fan.

LED's are very sensitive to voltage, so a 3V LED is going to fry hardcore on 12V.

Here's how you calculate what oHm resistor you'll need:

Voltage supply - LED voltage divided by current.

Example: 12v (your molex) - 3V (the LED) \ 0.020A (current) = 450, so you'd need a 450oHm resistor. You sometimes can't always find the exact oHm resistor, so allow around 100oHm difference. So in the above example a 350 - 550 oHm resistor should work.
2007-02-11 02:44:52
Stormscythe
Aiming for the head
+88|7012|EUtopia | Austria

CrazeD wrote:

It worked on the fan because there is a resistor on the fan.
The fan is the resistor.

CrazeD wrote:

LED's are very sensitive to voltage, so a 3V LED is going to fry hardcore on 12V.
Uhm...

Stormscythe wrote:

As I said, I'm not quite as bad with electronics as it may seem now and of course checked that there were resistors pre-mounted to that LED rail.
Well, I should've taken a picture.
I've already written that I tested each LED with a 9V block and all of them work. It's just one strange thing that the red wire (connected to the yellow wire on the molex cable) and connected to the positive end of the LED rail needs to be connected with the negative end of a battery to make the LEDs work. Strange, hm?

And still I wonder why an infinite resistor in a parallel circuit could cause the PSU to fail, I mean - if the resistance is close to infinity, there'd just be no current, but that'd go only for this part of the circuit...

 

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