Wiring Diagram: Check my work and fill in the blank, please.

[MotF Bane]

Two requests:

I would like to know if the wiring is correct so far,

and what should I fill in the blue circle with.

Using change in voltage over amperage yields desired resistance, I came up with a 5.55 ohm resistor, to take the fans from 12V to 7V. Each fan has a maximum input current of .3 amps. Can it be wired with just that 5.55 ohm unit in a single line, or do I need a 16 ohm resistor on each fan line, and diversify the 12V earlier?

[tdunks]

connect the ground of the fans to the one pin of the switch (the out pin) and the psu ground and psu 5V line to the other 2 pins. no resistors needed. just do not put a hdd after this before the psu.

[yngndrw]

Would have been useful if you labeled what each part is, seems as you're not using the proper symbols.

Anyway, you can use either a single 6Ohm resistor for all the fans, or you can have a 17Ohm resistor for each of the fans. It doesn't really matter which option you choose, except that using a single resistor could cause some fans to spin faster than others due to the manufactoring tolerances.

;)

[davmax]

1 Running the fans on 5 volts is a good suggestion. Have you tried it?
2. You have not considered the heat in the resistors. Using a single 6 ohm it should be rated at 5 Watts. Using 3 only 16 ohm rate them at 2 to 3 watts.
3. Is it a three way switch? The diagram shows a single pole two way (normal speed and slow speed). There is a switch version with centre OFF but this does not make it three way.

[JfRsQ]

Look attachments for 5-12V or7-12V switching. Don't care about the wires colors, I didnt bother and did it fast :ROTF:

[Oc-Ghost]

like tdunks said:

+12V to Fans,
Fans to the switch,
to the switch ground&+5v

[davmax]

JfRsQ. Great input . Beat me to it.

The 7 volt operation has some cautions.

1. The current drawn on the 5V rail must be greater than the the combined current of the fans. Should be no problem if installed in a running computer.
2. It would be a good idea to connect the + of a 2200uF 16V cap to the centre switch terminal and the negative to ground to help with fan switching noise.

[JfRsQ]

I've also been using a combo of 2 switches for OFF-5V-7V-12V operation without problems for 2-3years. How it works?
Switch1 ---- Switch2
OFF ---- OFF = Off 0V
OFF ---- ON = 5V
ON ---- ON = 12V
ON ---- OFF = 7V

[little_scrapper]

You also going to want to install a couple zeners for voltage regulation and some capacitors (low-pass filter arangement) for line filtering so that your fans arent picking up any noise.

Shoudlnt take more then 20 minutes to put it all togather once you have the parts :)

And if these are case fans you can add a series of photo diodes that look through the fans as pickups. Run those pulses into a simple counter/multiplexer chip preprogrammed with the number of fins per fan and it can output to a 3-way 7-segment diode arangement to give you exact rmp counts of all your fans.

I would personally go for an LCD controller chip for LCD diplays look better then a dozen or so 7-segment diodes.