Shedding Voltage?

[AmEv]

Because voltage is relative.


Physics class is teaching me that right now.
(need to pay more attention as to why...)


And resistance is futile

But, yes, 12-5=7.

Still hasn't explained how they get the -12v rail working...

[Omega]

Heh - I know - it's odd and tbh I don't know the science behind it, but if you put +12v thru the positive and +5v thru the negative, you end up with +7v - if you doubt me, get a multimeter and stick the prongs into the red and yellow connectors on a molex.....

Black and Yellow

Black and Red

Red and Yellow

You have just seriously confused the **** out of me, though I suppose with AmEv's explaination that voltage is relative, it makes sense that with 12V on one side and 5V on the other you would get 7V across the component, 'cause that's how voltage drop works. Then the problem becomes how to wire that...

Perhaps I could have constant +12V going to the fan, then have the switch in the ground connector going between a -12V rail and a +5V rail. I really need to buy a cheapo PSU to use as a bench unit...

online calculators say 51Ohms

Its big and gray. Green/Brown/Black/Gold

Thanks. I'll look in to this too.

[Stonerboy779]

Green, brown, black, gold

5, 1, x1, 5%

51ohm resistor with plus or minus 5% error.




12v - 5v = 7v

It's the difference here and for 3 fans on a decent PSU you should have no issue with wiring it like this, however if you wanted to do more fans like this you would want to grab resistors.

Oh and do not use the -12v when wiring up.

Wiring up should be 12v (yellow from molex) to positive (red on 3pin fan connector) and 5v (red molex) to ground (black 3pin)

[slaveofconvention]

The wiring aspect would be pretty simple - you'd wire the 12v from the molex directly to the 12v on the fan(s) then get a SPDT switch and wire the common terminal (which one that is depends on the type and make of switch) to the ground on the fan - the two other poles on the switch are where you'd wire the molex ground, and the molex 5v - this would mean that the switch would be selecting either ground or 5v to go to the fans while the 12 is constant - in essence you'd end up choosing between 12v-0v=12v (Ground) or 12v-5v=7v

I think I have all the parts needed for this here at home so if I get time tonight I might breadboard it up to try to make it a lil clearer if you like....

[diluzio91]

Yeah. lol... way way way easier than using resistors. Just think of it mathematically, if you have 12v pushing one direction, and 5v pushing against it, then 7v will spill over, although i'm not sure of the actual physics, but resistors just complicate what you're trying to do. and the yellow/white wire is to report the speed of the fans.

[slaveofconvention]

OK so here goes - I hope this makes a little more sense....

This assumes you use a Single Pole, Double Throw (SPDT) switch, and the centre connector is the common connector with the two outer ones being the switchable ones (this is usually the case). (NOTE: Corrected from DPST switch - thanks xr4man)

The yellow wire coming out of the fan is for RPM monitoring and doesn't need to be connected to anything.

The two black wires on the molex are both ground and it doesn't really matter which one you use (9 times out of 10, they're actually wired together inside the PSU anyway).

As for more than one fan - that's not a problem - you just connect them in parallel - wire all of the red wires from all of the fans together and connect to the yellow from the molex - similarly wire all of the black wires coming out of the fans together and connect to the centre connector on the switch...

Clear as mud?

[xr4man]

wouldn't that actually be a single pole double throw switch? otherwise that circuit is correct.

[Fuganater]

Double throw sounds right. Middle is off, Up is 12v and down is 7v/5v right?

[xr4man]

pretty much except there wouldn't be an off position. i think to have an off position in the middle, you'd need a triple throw switch. (is that even a real term?)

[slaveofconvention]

wouldn't that actually be a single pole double throw switch? otherwise that circuit is correct.

I freely admit I'm not 100% sure but I was always under the impression that the number of poles represents the number of options, while the number of throws represents the number of circuits the switch can control. The switch in the circuit only has two options, so it goes from 12v to 7v without any central "off" position.

I was going to actually build this but when I went thru my parts bins, I only have SPST (on or off) switches, or DPDT switches which have six terminals - now I could have used that and ignored half of the contacts but I didn't want to confuse things, and I figured the diagram would probably be clearer than a messy breadboard lol

EDIT: Turns out you're correct - see here - http://en.wikipedia.org/wiki/Switch - so it IS a SPDT switch