Please help me with a USB 5v to 3V to off toggle switch...

I am doing a mod to add a fan to my laptop and am drawing power from the USB port.

I am using a 5V 60x10mm fan. It is a bit noisy and adds some vibration, and obviously only want to use it during certain conditions, like gaming. I am hoping/assuming that (1) the fan will run at 3V at reduced speed and (2) fan will still cool decently at the slower speed.

I don't want to solder to any pads inside the laptop because I am connecting the fan to the bottom of the bottom panel that I can swap out with a regular bottom panel if desired.

What I am looking for:

(1) Pass-thru USB connector so it doesn't hog an entire USB port like this one: http://forum.notebookreview.com/har...t-upgrades/668015-usb-passthru-connector.html

(2) Three way toggle switch for 5v, 3v, off. Variable voltage would be fine too. At worse case I can use this switch for on/off.

I am willing to build the voltage regulator but would prefer something more or less off the shelf.

Thanks for your input.

 

You are going to want a voltage regulator for that . That should be the most efficient way to do it from a power consumption point of view.

Actually, you could put a resistor in series with the fan and combine that with a switch of sorts (I'm sure there's a three way switch you could find...) to toggle between 5V pass through, 3V with series resistor, and off. Might have fun finding the right resistor to put in series though. If you go that route pay attention to the resistor's current capacity.

 

Thanks. I guess that's what I'm looking for, someone educated enough in this to hand hold me through the process a bit. Plus if I build it myself it will likely be larger than I'd really like it. But I may not have a choice.

EDIT:
Hmm, seems I should be able to string some diodes together to drop voltage 0.7V for each diode. So 2 diodes to 3.6V, 3 diodes to 2.9V without affecting current. Now I just need to find a 3 way toggle switch.

 

to find the correct resistor, you need to know how much current your fan will draw from the USB port. let's call it I (measured in Amps). then, for a 2V drop (from 5V to 3V) across the resistor, its resistance has to be R=2/I (measured in Ohms). for example, the fans in my CoolerMaster NotePal U2 notebook cooler are rated at 0.25A of current @ 5V. so if i want them to run at 3V, i would have to use an 8 Ohm resistor.

there is the issue of start voltage though; a lot of fans need 5V to start moving, and after that, they'll just rotate slower when the voltage is lower.

 

Thanks darxide sorcerer. I think I'll try the diode route because it offers a voltage drop independent of current.

And yeah, I've messed with 12v fans trying to get them to start at 5v and they didn't like it. Sometimes I could just flick the fan and it would go, but that's not ideal. I'm also hoping that with the diodes at 0.7V drop with each, try one at a time in series and go for best cooling vs dB. Maybe only 0.7v is required to get noise reasonable.

 

Yeah, the more I search the more I see the 1N4001 seems recommended. I ordered some online, pack of 10 for like $3.50 shipped. Also ordered a three way slide switch, so will try my hand at a 5v, 3v, and off. I just have to find a spot to mount the switch. It's quite small. Hope this works! But won't get parts until middle or end of next week, so will just have to wait.

 

use a variable resistor so you can control the speed much better than just a switch and it will be easier to do.

something like this: http://www.radioshack.com/product/index.jsp?productId=2062355

 

Vicious,

That won't work very well, the resistance is to high. You would be using 1/20 of a turn (of the shaft) to get any usable range. You would want something like maybe a 500 ohm so that you have full range of the pot and also you need to make sure it has enough wattage handling capability.

 

I said something like that as in device type not that resistance value

He needs to take a multi meter and measure his specific project to pick the right resistance.

 

Well, I just picked up some 1N4001 diodes from Radio Shack and it works great. But variable would be best.

How about this one: Amazon.com: 500 Ohm Potentiometer: Electronics

edit: NM, it's only 1/2 Watt. I need at least 1.5W. Can't find Amps of the fan, but even assuming 0.15A x 5V = 0.75W and it's likely closer to 0.2A or more. The closest I can find is a 1W 500 ohm potentiometer at digikey but costs about $20. Too much money.

 

This is a really informative thread. I got pretty tired quick searching for this info on electronics hobby fourms, where the only answer you get it "that way of doing it is not perfect".
Bookmarked.

 

Yeah, lots of threads are like that, and hard to dig out the right info.

My next question though is about soldering to the USB pad inside the laptop. If I draw a power and ground off the pad, would that be considered running in series or parallel to a device plugged into the external port? I don't want my voltage cut in half if I do this with another device plugged in. Granted it will only be my mouse usb plug, but still. I'm not real smart when it comes to this stuff. Well not smart in pretty much any respects, but this I just feel dumb.

 

I have a surplus store called Ax-Man. They are awesome. I'm sure you could find want you need for about 1$. Unfortunately I don't think there are so many electronic surplus stores in other places across the country.

 

Thanks, duh to me. After thinking of it a bit I see that, so Voltage should be constant but current shared between whatever devices attached correct? That is good. I think I am going to solder a 3pin male fan connector to it, because basically that's all I've done now. It's a USB spliced to a 3-pin (only using 2 of course) fan connector to hook up the other fan. This way it is removable if needed and not taking up a USB port.

 

Hmmm, tricky tricky, but good idea. Not sure if I have room for that though.

I did learn one thing though, that diodes are directional. This may be all basic stuff for most, but I'm learning as I go. I probably learned this in college at one point, but haven't touched it in years, decade even, lol.

 

You know what I wanted to do to solve this problem, find a USB device like a toy or something that has some kind of a dimmer like for LEDs, and then hook it up to your fan and find a way to hack the software and link it to your temps for automatic fan speed.

Never was able to find something, but I didn't look very hard.

 

Anyone know where could find a y-cable for the 3-pin mini laptop fan connector? It's really small, only y-cables I can find are the traditional 3-pin fan ones.