Controlling single direction motor w/o H-bridge

[BANE]

Hello, I just thought about if it was possable to control a computer fan without an H-bridge using a servo out put.  I'm guessing i need to make a device that can take any servo frequency and use it to open a transistor or something.  Anybody done anything like that? 

Bane

[izua]

computer fans are bldc motors. i don't think it's possible.

[superchiku]

try it out and tell me..

[BANE]

heres an idea.   (see pic from tutorial)  What if i lowered the volts to 6VDC and applied a 5vdc ether A or B from my microcontroller?

[airman00]

heres an idea.   (see pic from tutorial)  What if i lowered the volts to 6VDC and applied a 5vdc ether A or B from my microcontroller?

you only need one transistor . not two

also I would use a MOSFET

[BANE]

I have a Digikey catalog that has a few thousand   (not sure what type to get for v5)
heres one on radio shack, will this work?
http://www.radioshack.com/product/index.jsp?productId=2062618&cp=&sr=1&origkw=MOSFET&kw=mosfet&parentPage=search

Bane

[BANE]

will this work w/o the diode?
what does the diodes in this do anyway?

[izua]

I'm not breaking a party here, but computer fans are ussually BLDC motors (as i said above ).
A brushless motor doesn't use brushes (doh), the movement is controlled by magnetic induction, controlled by a chip. In case of a brushless controller (like in modelling) you can vary the direction and speed by a signal (so the controller will change sequence). If you reverse polarity to a controller (since that's what you supply, anyway) at best, it will do nothing.

Control circuitry is made as simple (read: cheap) as possible for most fans. Unless it has some control pin, chances are you can't do anything else about it.

[BANE]

the fan was just an example of something im trying to build.  To make it simple the circuit, for now , will be controlling just a 6vdc motor from radio shack.  Just out of curieosity, on a h-bridge, you can give it a frequency to control both direct and speed; like a servo?

Bane 

[izua]

Yep, choose a diagonal (direction), and instead of keeping it high, pulse it (speed). There's a whole theory here, for current recirculation, since you can pulse either two opposing transistor, the high side transistor (while the other one is open), or the low side transistor. just pulse one of the two (and by pulseing, i mean PWM pulses) and you'll be fine.

[Admin]

BANE, for that specific mosfet schematic above, use a PNP type mosfet.

as for PWM, it can be used as a rough method of speed control. for direction you need an H-bridge . . . and for servo control you'd need even more . . .