LED/resistor question

[artoodeeto]

Anyone ever seen UV LED's? I know I can get UV CCFL's, but I'm wondering if there are any LEDs out there that are UV.

Also, is there a resistor that can drop a 12V line down to 2V or lower? Or if not, then what's the lowest I can safely drop a 12V line? I have 2 fans on the outside of my case that are purely cosmetic, and they spin fast on 12V and make a fair bit of noise, so I want to slow them down. Preferably a lot.

And I had one other question that now I've totally forgotten :facepalm: oh well, I'll think of it eventually.

[x88x]

Anyone ever seen UV LED's? I know I can get UV CCFL's, but I'm wondering if there are any LEDs out there that are UV.

Yes. http://www.mouser.com/Search/Refine.aspx?Keyword=uv+led


Also, is there a resistor that can drop a 12V line down to 2V or lower? Or if not, then what's the lowest I can safely drop a 12V line? I have 2 fans on the outside of my case that are purely cosmetic, and they spin fast on 12V and make a fair bit of noise, so I want to slow them down. Preferably a lot.

I forget what the current level on those 12V lines are, but the equation you want is:
V = I*R
V = voltage drop over resistor
I = current through resistor
R = resistance rating of resistor

I seem to remember the current level might be 0.5A for the peripheral 12V lines, so in that case, you would need a 20 Ohm resistor rated for at least 12W.
http://www.mouser.com/Passive-Components/Resistors/Power-Resistors-with-Leads/_/N-2gra1?P=1z0x8bg


And I had one other question that now I've totally forgotten :facepalm: oh well, I'll think of it eventually.

Can't help you with that. ;)

[slaveofconvention]

UV Leds are definitely available, I have a tray full myself so there's no doubt on that issue.

Regarding your 120mm fans - I'd be very surprised if 2v would be enough to start them running never mind maintain them. Before you do any soldering or cutting, I'd strongly suggest having a play around to see exactly how low you can get away with. 5v is easily available from your power supply, you can also try simple AA batteries to create 4.5, 3.0 and 1.5v test currents. At least this way you can 1. See how quiet/effective the fan is at each voltage, and 2. Find out what voltage you actually need to get the fan to run... Pretty much every 120mm fan I've ever come across will run at 5v and is effectively silent at that voltage.

EDIT: you can also safely create 7v from your PSU by simply connecting both 12v and 5v lines to the + and - of the fan - it might sound odd but it'll give you a stable 7v

[x88x]

Doh! I can't believe I forgot this, but you might not need to do much at all in the way of wiring. If you look at a SATA power lead, you have 12V (yellow), 5V (red), and 3.3V (orange) leads. 3.3V should probably be low enough, and if it's too low, you can go up to 5V.

Alternately, if you can't get the motors to spin slow enough for your liking, you could always get some cheap continuous rotation servos.

[artoodeeto]

Regarding your 120mm fans...

whoops - I didn't think to specify. The fans I want to slow down aren't 120mm, not sure what size they are. They're small, just guessing they're probably under 50mm, and specifically designed for the...northbridge heatsink on my motherboard. they also happened to be visually perfect for a spot on the outside of my sandcrawler, so I got a couple more and used them up there. They don't cool anything off, so as long as I have enough voltage to get them turning, that's all I care about. Oddly it'll also make them more noticeable, right now they spin so fast that unless you're paying attention, you can't really tell they're doing anything.

thanks for the voltage experimenting tips too - that'll be helpful before I go spend loads of small change on resistors :D


Doh! I can't believe I forgot this, but you might not need to do much at all in the way of wiring. If you look at a SATA power lead, you have 12V (yellow), 5V (red), and 3.3V (orange) leads. 3.3V should probably be low enough, and if it's too low, you can go up to 5V.

Alternately, if you can't get the motors to spin slow enough for your liking, you could always get some cheap continuous rotation servos.

I'll keep that in mind for future applications; in this specific scenario I have these fans wired to the 12V lines (in between the ceiling and roof of the crawler) that run the lights, so the 3.3V and 5V lines aren't up there at all. Good idea though, it woulda been way easier, 3.3V would be perfect probably.

[SXRguyinMA]

I've got a ton of spare UV LEDs laying around. wire up 4 in series for 12v operation, or put a 470 ohm resistor on each one for independent 12v operation :up:

[artoodeeto]

I've got a ton of spare UV LEDs laying around. wire up 4 in series for 12v operation, or put a 470 ohm resistor on each one for independent 12v operation :up:

sweet! thanks! I like the 4 in series idea, as I have a number of spare 1ohm resistors sitting around. I'll post an update in my worklog after I figure out how many I need and get them in the computer :D

[Oneslowz28]

I am about to build a cheap variable PSU that can be built for less than $10. You could power it on your 12v line and adjust it all the way down to 1v or less by simply turning a knob on a POT. If you are interested in it just let me know.

[SXRguyinMA]

that sounds interesting

[Oneslowz28]

I will try to get it built and posted by next wed. Still have a lot of work to do on other things here behind the scenes at TBCS.

SXR it uses a LM37T voltage regulator, some caps, resistor, POT and diode

[SXRguyinMA]

sweet keep me informed!

[artoodeeto]

I seem to remember the current level might be 0.5A for the peripheral 12V lines, so in that case, you would need a 20 Ohm resistor rated for at least 12W.
http://www.mouser.com/Passive-Components/Resistors/Power-Resistors-with-Leads/_/N-2gra1?P=1z0x8bg

Actually I need to know how much current the fans draw...which of course I can't find anywhere. I'm sure it's not a lot...do most fans tend to draw the same current level?

[x88x]

Ah, right, forgot about that. If you look at the sticker on the back of the fan, it should tell you. Looking at a few of mine, two newer 120mm fans say they pull 0.16A, and one older, heavier duty 120mm says it pulls 0.33A.

[artoodeeto]

yeah, and given that my fans are these little guys (found the pic online, not my system): http://www.xbitlabs.com/images/mainboards/asus-p5e/fan.jpg
I would imagine they draw even less current, maybe even as low as 0.1A. I was trying to avoid pulling the top of the case off to look at the fan, but looks like I'll have to.

[artoodeeto]

I found a photo of the backside of one of these fans - their current draw is 0.08A. apparently they get up to about 3,000 rpm on that. Looks like I'll need a 100 ohm resistor rated @ 1W. :) course, that's according to an LED calculator, which isn't designed for a fan, so if anyone sees this and there might be an issue, let me know...

[x88x]

If my maths are right (and it's been a long time since my Circuit Analysis course, so they might not be), you should need a 150 Ohm resistor rated at 1W.

[artoodeeto]

seems I neglected to mention I want to knock the current down to 4-5V rather than 2. I realized that I'd initially touched the fan wires to a six 2V LED series on the 12V line, which meant everything was receiving just under 2V, and the fans didn't spin. That tells me they need probably at least 2V, so I was thinking of knocking it down to 4 or 5 to be safe. However, your point about the lower voltage lines in the power connectors got me thinking of just running a couple extra wires up from the plug. The part connected to the PSU is standard molex and fully intact; it's just the custom plug in the upper part of the case that has 12V wiring only. it wouldn't be hard to add a 5V line to that and run those wires to the fans, and that way I don't have to go buy anything. :)

[x88x]

Ah, yeah, for 4V you would want a 100 Ohm. If you're going to run another line up, I would recommend testing it with both the 5V and 3.3V line before running anything, to make sure you'll get the effect that you want.