The fans I bought had tach outputs, its just too slow for the PSU to be happy. If the fan work like I think the tach output works then its 2 signals per revolution. If thats the case then the PSU is probably looking for a 16khz signal or something close to that. (fans are 8800rpm)

But since I don't have oscilloscope I have no way of measuring the fans output. They look like normal 3 wire 12v DC fans for PCs though. I will have to look at a 555 circuit.

At this point I may just modify an old tape carrying case. I did a quick test, its so quiet when the lids closed. But I need to drill holes and use fans for circulation. Might work for this weekend, but I don't want to make the box a permanent solution. Will have to look at building a 555 circuit.

If you have a multimeter that has a frequency display, you could use that to get an idea of the frequency; sometimes frequency measurement is a mode of AC voltage measurement. This won't tell you the duty cycle though, for that it will just take some experimentation. It would be much faster to borrow a oscope if at all possible.

I'd start by trying to nail down the frequency by starting with fan rpm - 66Hz for a 4000rpm fan. 4000 rpm (rev/min) is 66.67 rev/sec (Hz=rpm/60), and then double it if that doesn't work, and so on. Once you get the frequency, start with a low duty cycle (very narrow pulse) and work your way up to 50% duty cycle.

The 555 timer will allow adjustments in both frequency and DC% (duty cycle). However, there are a few things to consider. IIRC, a 555 timer will have limits on DC%, if it won't get as narrow (or wide) as you need, you may need to use a transistor configured as an emitter-follower which will basically "invert" the output. Also, adjusting the resistances tends to adjust both freq and DC% at the same time, so sometimes it is easier to use two 555's in series, the first one as the astable oscillator to generate the right frequency, the second as a monostable oscillator to adjust the DC%. More complex but much easier to control freq and DC% independently. The 556 timer is just two 555 timers in one IC case, so that might be a better way.

I know you want to replace both fans, but how loud is just one fan by itself? Because like I said, you could tap into the tach signal of one fan and feed it to the other input and then use whatever fan you want as the second one if you need it.

If you still want to go with building your own oscillator, here are a couple datasheets for the 555 and 556 timers. Each has an applications section at the end with circuit examples: 555 Timer, 556 Timer.

I will have to look at that, possibly also using a thermister to control fan speeds. Read up a bit on that last night. Never really got into building circuit boards, but I can follow a diagram and do the math. So I'll eventually do that. Kinda tapped out for money currently.

In the mean time? I present a PSU.....in a box. This box was meant for the safe transport of LTO3 tapes in a very sensitive fashion. Which means the plastic is quite thick, and the open cell foam padding makes an adequate sound insulator.

I drilled a few holes in one side for the cables. Though I will visit radio shack tomorrow an pickup some insulated terminals for banana plugs and mount those to the box so I don't have to feed the charger leads through the air hole I made. I used a 92mmn 44cfm 41db PC fan for exhaust. I may add another 92mm fan to the front of the case of the same type if this proves insufficient. Using the left over padding I created and air dam to force air through the PSU.

So far its been running for about 1hr now and the air coming out of the fan vent is luke warm.

Before the box the PSU was about 74db, with the PSU in the box and lid closed its averaging 48db. Not a bad change for free.

http://farm9.staticflickr.com/8190/8...ea7fa002_c.jpg
psubox by thirdgen89gta, on Flickr

Looks like im going to be converting one of these guys soon. Got a 200w charger coming from HK and im going to need it to charge my new 6s packs aswell. I got an old PSU in my spare computer parts that im going to tear into and configure.

I've been running my PSU's like mad and they just keep on going. What a bargain heh!

WOOT! I'm not sure if this has been posted yet. A way to quiet the fan & adjust voltage!

I was working on my HP 575w 12v supplies and came across this page - https://sites.google.com/site/tjingu...projects/HP47A

I just connected 4 to 8 (using 6-8-10 to power up)(so I connect 4, 6, 8, and 10 all together) and lone behold the fan didn't ramp up yet I still got power! :yipi: I haven't loaded it up yet to see if the fan turns on when needed. I'll report back if I can

And just extra reading- this guy's page on putting them in series - https://sites.google.com/site/tjingu...ts/diy-24v-47a

I've got a stupid question. I'm finally getting around to building one of the 47a power supplies like on the first post on the first page.

Do I need to wait until I plug in the power supply to 120vac before I connect the 3 wires, or can I just hard wire the 3 wires to the pins and just plug and unplug the power supply from the wall?

Just hard wire them..

Thanks man.

Thank's for posting that. It is much more pleasent charging my batteries without my power supply sounding like its about to take off.