Re: Finding fan supplies in a modded laptop (was:Increase voltage using capactior etc)

aosborn wrote:
Hello.
I have a project where I'm making a digital picture frame/media center
out of an old laptop. I took the systemboard/lcd out of the case and
mounted in a shadow box. Everything went well. I made a power button
out of a push button microswitch and mounted a panel-mount audio
connector and usb connector for peripherals.

My issue is with heating. I bought 2 40mm 12v PC case fans to cool it,
one intake one exhaust. Obviously a laptop doesn't have headers for
fans or any molex connectors to plug them in so I'm taking voltage off
the systemboard where I can find it to power the fans.

I found +5V on the serial port (pin 3) so I soldered the black
(ground) wire to that of the fan header. I then found +5.5V on pin 2
of the PS2 port. Both of these ports I don't use. Using my multimeter
I touch the 2 headers and I get +10.6V. That sounds good.



The serial port has no 5V output, you are measuring a data line. As you are seeing more than 5V between it and the PS2 port which *does* have a 5V line, it must be negative, not the +5V you think it is!

I plug my fans into the headers and I get nothing. I barely touch the
fan blades and the fan starts spinning, but very slow. It barely
pushes any air. I plug these into my PC (multimeter reads +12.1V on
the headers) and the fans spin very very fast, normal.
As expected, see below under 'Modders'.

Did I either solder the headers wrong (multimeter shows voltage....)
or can I increase the initial voltage someway to atleast get the fans
to spin up without me manually starting them. Such as a capacitor
which can fill up then release it's energy (as you can tell I'm a noob
with some of this, I'm a Network Engineer, not Electrical Engineer).

One can double a voltage using a switched capacitor converter but not easily for the sort of current a fan needs. Mistaking -5V for +5V does not lead to confidence in your skills with a multimeter and unless you *have* basic electronics skills, building a boost converter, (even from a kit) is not going to go too well. It would use an inductor for energy storage and voltage conversion, not a capacitor. Caps are good for keeping the voltage nearly the same while the current varies a lot, more or less the opposite of what a boost converter has to do. Anyone who's learnt network engineering can learn electronics if sufficiently motivated but this particular part of your project is not a good place to start.

Small kit projects where failure only smokes a few cheap and easily replaceable parts or building circuits from components on a solderless breadboard is a good start, not a project you've already invested a lot of time into where you are risking a laptop main board that you cant easily replace (unless you've got another identical laptop) and whatever camera card or USB stick you are planning to load the photos from).

What is the power supply voltage of the laptop? (I'm assuming you have removed the battery.) With no battery to charge, it should have plenty of spare current for fans, you just need to (A) switch them on when you have +5V on the PS2 port and (B) regulate the voltage so you don't blow them and they aren't too noisy. It may even be possible to wire your fans in series if they are identical and the PSU is in the 18 to 24V range (with a smoothing capacitor across each fan to keep them happy). Linear regulators convert excess voltage into heat so if they can be avoided when you are trying to reduce the heat, it would be nice.

A simple transistor switch in the ground wire to the fan or fans, controlled by the +5V you found on the PS2 port, would switch them on and off with the main power button. You also need a couple of resistors to make the transistor switch operate correctly. More details if/when you reply with the PSU voltage.




I know some PC modders will rewire their fan headers to use the 12v
rail and the 5v rail for ground, yielding 7V, and that makes the fans
work, just slower and quieter (for silent PC solutions). I'm getting
much higher than 7V so I thought my fans would at least move on their
own...

Thanks!

*DONT* COPY THE MODDERS, if there isn't enough load on the 5V rail to handle the extra current from the 12V rail via the fans it will blow the board. Its normally OK on a desktop PC, but I'd still not recommend it. On a laptop its criminally stupid to do it that way, so always run fans between a proper supply and ground. You got lucky the serial port couldn't sink any significant current . . .

--
Ian Malcolm. London, ENGLAND. (NEWSGROUP REPLY PREFERRED)
ianm[at]the[dash]malcolms[dot]freeserve[dot]co[dot]uk
[at]=@, [dash]=- & [dot]=. *Warning* HTML & >32K emails --> NUL:
.

Quantcast