Hello all. Firstly, Lets not assume im a cheap skate here....*cough* I recently bought some blue battery powered LED christmas lights to stick on the back of my tv, to act as one of these fancy back lights...i got a proper set for 30 quid but only ever had it on blue, so i got these for a fiver... anyway, so i wanted this to run off my pc. turn on when the pc came on, then back off again. So i did a bit of soldering, took the battery pack off and put on a USB connector. YEAH IT WORKS! but the problem is the usb is always powered, so it wont turn off. So i decided to plug it into the Fan header on the motherboard. so i got an usb extension, cut the end off and soldered the 3 pin fan plug to the end of the the usb. but now it doesnt work. if i plug into the usb, its fine. but not with this extension. I've done red to red and black to black, and assumed the red wire in a fan is the 5v. the red in a usb is 5v. so i dont get it.... anyone any ideas? should i scrap the extension, take the usb end off and solder on the fan plug?
I suspect it might be something to do with a) current draw (less likely); b) the fact that no 'tacho' is being fed back to the mb; or c) the wiring (and I'm not doubting your soldering skills! But - would an easier solution not be to stick an extension on a Molex plug? That way, no feedback, and a higher current throughput?
Motherboard fan connectors are usually 12v and USB output is 5v, is 12v too much for the LED's? Got a spare molex connector with a 5v connector to use? Also some motherboards have ErP BIOS settings to control what is still powered when the PC is shutdown, check the BIOS.
I always thought pc fans were 12v not 5v? I converted a 80mm fan to usb to run off my laptop with the intention of making a plexi stand with fan and the fan ran much slower than on it's original supply.
most fans are 12v and you use inline resistors or variable speed controllers to vary the fan speed or if they are 4 pin PWM (pulse width modulation) the fan speed can be controlled through bios, If it was a 12v fan and you connected it to a 5v usb it will run slower.
Yeh I expected it to run slower knowing that USB was 5v as opposed to 12v I was just trying to confirm that fans normally run on a 12v supply.
the 3 pin connector i took off a fan had 3 wires coming from it, red, black and yellow. I obviously assumed the red was 5v....so what would the yellow one be then? unfortunatley ive tried and researched different ways to kill the power on USB when its off but with little success. so i headed off looking for different ideas. Also, if i plug the lights directly into the PSU via a 5v connector on a molex, wouldnt that mean they will just stay on unless i switch it off at the wall? ALSO, even if the header was 12v, wouldnt something happen with the lights when i turn them on? i.e really bright or explode into a mushroom cloud? because the voltage is greater than 5v. the battery pack was for 3 AA batteries. total of 4.5v right?
damn clearly i shouldnt be assuming then i shall have ago with the molex tonight then. thanks for your help so far chaps.
You could use logic gates to only power up the LEDs when you have 5v USB and 12v from the fan header. A simple AND gate would do the job I think in this instance. Just make the resistor fairly high (15k or so). When the 5v is high and the 12v low the LEDs will not light up. Once the 12v and 5v go high the LEDs will come on. Simples.
so i tried soldering it to the 5v on a molex connector and it worked. lovely. does exactly what i want it to do now. Tealc, thanks for the suggestion, i would have attempted at this should the molex failed. so thank you for that. its worth people knowing though. Thanks all for your help. problem solved!
