I read about people 7 volting fans, but I didnt understand how to do that, maybe im just stoopid. I want 7 volts so I can power 2 3.5 Volt LED's without the need of a resistor. So can someone explain how to take 7 volts from a molex connector.
dunno if itll work for LEDs, but to 7v my fans I used the 5v line as the ground. and powered off the 12v. some weird electronics law (magic, really) makes it turn into 7v
You'll need those resistors though, because you need to also limit the current running through them [ that mA thing ]. Might as well just get a 0 Ohm, 1/4 or 1/2 watt and still do the 7v trick. The way I know this is because I also thought I could go resistor free, and over time the LEDs would get really hot then dim, and dim, and eventually die out.
This is how you do it... connect both in parallel on the 5V line as there is no need for 7V so we know they are 3.5V, so thats 5 - 3.5/0.02 (0.02 == 20mA) 75ohm resistor per branch (in your case two)... 0.5W fusible (blue color) are perfect for the job. Why can't you not use a resistor? You can but as someone else said they will eventually die, as there is nothing to limit the current as the leds arn't perfectly matched, whats making the leds 100% 3.5v each and resistance in wiring, soldering etc etc?????!???! However I tend to do this a little (with out resistors) if I use matched leds and got a good reliable powersource! We can't use a resistor if you got 7V [from the 5v/12v from a molex as 12 - 5 = 7v] as you can not allow for the voltage drop across the resistor AND the leds. Hence parallel comes to the rescue, hope this helps! Code: -----\/\/\-----|>|----- | | 5V-------- -------------grnd | | -----\/\/\-----|>|-----
How about I put 3 in a series to make it glow better and use a 75-80 ohm resistor 12 V 3.5 from each led 3 leds 20mA =75 ohm resistor needed I can get some 82 ohm ones from http://lsdiodes.home.comcast.net/resistors.htm
Here’s a quick and easy little guide to knock a 12volt molex down to 7volt. click the picture to see the whole guide
Hrm, that would work, but I tend to make my own custom cables...neater in the end with heatshrink and maybe sleaving. Gonna sleave all of it in my current mod.
That would work, an 82 would however mean the leds will run at 18mA, killing the brightness a tad. Most ultrabrights dont 'dim' but switch off it they dont get the minimum they need which is usually 20mA but check the datasheets.
I have an old powersuply thing they are called at or sumthing the ones that have a machanical switch, it is like 500 wat so i use it to test thing before i put them in my rig and i just shorts out and shuts down when I try and do the 7v trick. Any ideas why?
Is there anything else connected to it and any other possible obvious shorts? Have you wired/soldered the leds the right way around : 12v--- resistor --- +ve -ve +ve -ve +ve -ve -----5v Some AT PSU's need a proper load attached iirc....
Yea everyhting is connected right but and it has a load on it, i am gunna have to have a depper look as to why it does this.
