Im being thick Variable Resistor to Rheostat
 

Im being thick!

Have 8 fans, spec are 12v 0.70A! From what i've worked out thats 8.4W (watts) per fan.

Ether working the 0.70a out again for 8 the totaly power draw of all 8 running is 67.2W(watts) or 5.6A.

I need a way i can control the voltage to these fans. There all wires into each other with just + and - cable feeding them 12v.

Now how do i work out what size i ohm or resistance i need?

Basicly im wanting to be able to control the fans up and down in terms of speed, idealy voltage seams the right way

You also need to tell us what sort of power supply you intend to use.

12v DC, 1200w. It's in a PC

just get a pc fan controler....

I recommend a basic PWM controller. You can build one easily with a 555 timer and a FET, an IRF510 or better.

Doesn't work, all the fans are wired into one cable as the setup is external and no pc fan controler support the power draw.

I bought this 12V-24V-36V 10A DC Motor Speed PWM Control Controller | eBay UK

Doesn't seam to function properly, @ 7v the fan grind so i think the power is up and down

@7v???
A PWM should deliver a constant voltage; only the width of the voltage pulse is changed (hence the name). So it controls the fan speed by turning the power on and off at an adjustable rate. I can't tell what the eBay circuit is, but it looks more complicated and expensive than it need be (but admittedly, there's no shortage of that sort of thing 'round here).

Yeah when i turned it down to 7v that with a multi meter on the output the fans just grind like hell. Yet when i run them off a 7v molex no problems.

I think it's the fact of the pulse is the problem and i need something that doesn't do that at all!

You can't really measure the PWM output with an ordinary multimeter.
A 7v molex? What runs off a 7v molex? What is spec'd for 7v??
IMHO the pulse is not the problem. PWM fan controllers are a well proven technology in widespread use.

You can take a PC molex adapter and mod it to output 7v as the lines on the psu allow this. Google it'll show you loads.

Well on straight DC they work perfectly, even @ 5v, 7v

Fans in a pc normaly 12v, servers are 24v. For silent you can run @ 7v and some fan even spin @5v. Lower the voltage the more silent they are.

This the point as the DBA is 50 per fan @ 12v and i've 8 of them so it's pretty loud. I wanted to be able to control them so if benching or gaming can have full power and if im just browsing or a film can lower so it's silent.

The controler off ebay looks the part and all but just doesn't seam to be working correctly