Using 3.3V for fan operation

fmah · Post by **fmah** » Mon Apr 14, 2003 7:46 am

Well just looking around and I think I will try to use the 3.3V output on the Aux Power Connecter (6 socket in line connector) and see if that will drive a fan. Sounds good if I can get them to turn, if they don't turn I will manually switch between 12 and 3.3.

Anyone else tried using the 3.3V?

fmah · Post by **fmah** » Mon Apr 14, 2003 4:34 pm

I just tried it today. I was able to run 80mm, 92mm, and 120mm fans at 3.3V. They all had a light sounding clicking noise though, since that was probably toward the bottom end of the voltage they can accept. Interesting.

ez2remember · Post by **ez2remember** » Mon Apr 14, 2003 4:57 pm

Oh I think I got one of those Aux connectors too. Which pins are for 3.3v?

3.3v is extremely low, but I think it would be useful for 120mm fans, where 5v is just a little noisy.

Many fans don't start @ 3.3v. Shame there is no easy way of getting it to start at 12v for a few seconds and then turn down to 3.3v. That would be perfect!

fmah · Post by **fmah** » Mon Apr 14, 2003 5:05 pm

http://www.formfactors.org/formfactors/atx_ps.htm

Look at the spec on page 28.

ravton · Post by **ravton** » Tue Apr 15, 2003 7:52 am

The majority of fans I've tested will only start interrmittently at 3.3V but will run fine once started. It takes very little to make the fan stall though. Unfortunately a large portion of these fans have problems with the tachometer output at 3.3V.

tibbar · Post by **tibbar** » Tue Apr 15, 2003 11:05 pm

this is what i posted a msg about recently. i want to make a circuit to start a fan at a higher voltage and then bring it down to a set level.

i.e. with a 120mm fan there's no way you can start it at 3.3v, it'll need at least 5-6v initially, but once it's spinning you can bring it down.

any ideas how to do this??

quix · Post by **quix** » Wed Apr 16, 2003 7:56 am

i remember there was a very simple circuit in the faq of the [h]ardforum. using a diode and a capacitor. was meant for 5volts but should work with 3.3 too. it feeds the fan with 12 volts, until that cap is loaded, after that 5 volts! problem is, forum was down the last 6 hours...

tibbar · Post by **tibbar** » Wed Apr 16, 2003 10:58 am

that sounds promising. i'll see if i can find the page you mentioned.

cheers

quix · Post by **quix** » Wed Apr 16, 2003 12:24 pm

hmm, [h] is online again, and it doesnt look like it's there, must have been some other forum, sorry! it's in a faq, on some hardware site forum, in the overclocking/cooling/modding or something forum. everything was on one page, lots of posts... that's what i remember... will try to find it

abeatlehead · Post by **abeatlehead** » Fri Apr 25, 2003 9:14 am

You could also use a time delay relay to wait a certain amount of time before switching between 12 and 3.3.

fmah · Post by **fmah** » Fri Apr 25, 2003 8:31 pm

That's what I was also thinking, but those of us without design skills would need the schematics.

herosformula · Post by **herosformula** » Fri Apr 25, 2003 9:19 pm

The decay of a RC circuit is very well defined. This page describing 555 timers may help http://www.uoguelph.ca/~antoon/gadgets/555/555.html

fancontrol · Post by **fancontrol** » Sun Apr 27, 2003 9:35 am

Something like this should* work:

/* edit - this is a simpler version that does the same thing */
Select C1 and R4 to get the time you like. t ~= .7(R4)(C1).
Note that by making R4 a pot you can adjust the timing.

so for example pick R4 = 50k to keep current low,
time = 2 seconds, then C1 = 58uF.

Making R2 larger will allow the use of a smaller C1. For exampe,
if R2 was 50k C1 would be 22uF.

R5 is optional; it allows adjusment of voltage (like a rheobus).

*by that I mean someone should check my work. I'm not that
smart. I look forward to people pointing out my errors.

Using 3.3V for fan operation

Using 3.3V for fan operation

getting fans to start

capacitor/diode circuit