Ok, this should be relatively easy for you people I think...I don't know much about electronics...hell, I connect wires with glue and duct tape... ANYWAYS, here's my problem. I recently got a bunch of new fans for my comp...and I have a fan controller that shows the temp and RPMs and all that stuff...but see, I have 4 groups of fans...two have only one fan, one has two, and one has four. The ones with one fan are fine, the ones with two show low values for the RPMs (around 1500) which move around a lot (I stabilized that a lot with a 1K resistor), but the group of 4 moves from around 60-600, and keeps setting off the alarm. After trying resistors and batteries of various amounts, I decided to use google to see what I need. And it appears that the sensors use AC volts to determine the RPMs. Now, here's the problem. How the hell do I increase AC voltage? I'd prefer something that converts from DC batteries, AAA or AA, but I might be able to use a molex plug from my comp...not sure if I have an extra...but anyways, I just need some simple way to convert DC to AC, preferrably parts I could find at my local Radioshack. A description of how it works would be cool, but mostly I just want how to build it. Anything past just wires and resistors starts to confuse me....
Well, this is going to be a bit more difficult than you imagine, mainly because your logic is flawed here. First, the fan RPM signal is not a variable AC voltage, but rather a pulsed DC signal, which is at +5v twice per revolution. The ICs on the motherboard essentially divide this number by 120 to get the RPM figure and send that to the SMBus, which programs can access. However, I really don't like the fact that you're trying to avoid the issue, that being that your fans aren't running like they should. Maybe if you told us how you wired the fans and what controller you're using.
I'm not avoiding anything...perhaps you didn't exactly understand what I have here...I've got a set of four fans in the front, two in the back, one in the side, and another one in the top of the front. The one I've got the biggest problem with is the set of four in the front. Since my fan controller only has enough plugs for four fans (and I've never seen any with more), I've cut the wires and wired each group of fans together...and with each added RPM wire, the value recieved gets lower. So basically, I've got 4 RPM meter wires connected together, which gives me a reading so low the alarm starts going off. With just one fan though it works perfectly...and even two it's decent...
Oh, so you're saying that the fans are all running as fast as they should, but the RPM signalling is off? Well, the simple solution would be to not tie all of the RPM signals together and just attach one to the motherboard. See, the reason that the readings are so low is that because the only time a half revolution is being read is when all four RPM signals are low, which means that each fan has to be at roughly the same position at the same time. Besides that...I can't think of what else you'd do.
You've only two choices here, do as whypick1 says and just use 1 fan sensor wire to your controller, or run each separate wire through a rotary switch or solid-state equivalent so you just measure 1 at a time. The controller measures how long the sensor signal is at a low of around 0V, so with your square-wave pulses from each connected fan overlapping, it's low for longer.
