Im being thick Variable Resistor to Rheostat

[Pjalchemist]

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

[Stocker]

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

[Pjalchemist]

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

[digits]

just get a pc fan controler....

[sofaspud]

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

[Pjalchemist]

Originally Posted by digits

just get a pc fan controler....

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.

Originally Posted by sofaspud

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

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

[sofaspud]

@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).

[Pjalchemist]

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!

[sofaspud]

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.

[Pjalchemist]

Originally Posted by sofaspud

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