Im being thick Variable Resistor to Rheostat

Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
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
 
Last edited:
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.

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
 
@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).
 
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
 
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.
No, no, I'll take your word. My 5V works fine thank you. If'n it ain't broke...
Maybe your controller needs 12V minimum and your 5+ amps worth of fan is dropping the voltage output. Have you monitored the input to the controller?
The 555 and FET circuit I mentioned could have been built, cased and installed in the time we've been discussing this.
 
No, no, I'll take your word. My 5V works fine thank you. If'n it ain't broke...
Maybe your controller needs 12V minimum and your 5+ amps worth of fan is dropping the voltage output. Have you monitored the input to the controller?
The 555 and FET circuit I mentioned could have been built, cased and installed in the time we've been discussing this.

11.75V input control set to min

11.51V input control set to max

1.01V output set to min

11.45V out put set to max

Around 10.86V the grinding stops and all fans spin

I've no idea where even begine with a 555, FET and such hense why i bought this off ebay
 
Last edited:
Apologies, but none of the measurements make any sense to me.
Measure the wires you use to power the fans. No controller, no fans, nothing but the wires (that's 1).
Then connect the controller (with the fans connected to it) to the power.
Measure the wires again turning the controller adjustment from minimum (that's 2) to maximum (that's 3).
So we have 3 voltage readings.

Oh yeah. Do tell what is the input control?
 
Last edited:
Hi. If you want your fans to just have two modes, full and quiet, you could just use a switch. In switch position 1 the fans are all in parallel, giving 12V per fan. For switch position 2 the fans will get about 6V each. I say "about" 6V each because some fans will hog a bit more current than others, but it will be close to half of 12. Just an idea. FYI the switch is a DPDT.
 
Then connect the controller (with the fans connected to it) to the power.
Measure the wires again turning the controller adjustment from minimum (that's 2) to maximum (that's 3).
So we have 3 voltage readings.

What do you have connected to the white 3-pin header near the electrolytic caps?
 
Wire them all in parallel to the positive supply and the drain of a large MOSFET, wire the source to ground, wire a 10k resistor from the 12V to a 10k pot, wire the other side of the pot to ground and wire the wiper to the gate. You have to be aware that the FET will dissipate ~20W worst case, so you need some heatsinking.

w
 
Then connect the controller (with the fans connected to it) to the power.
Measure the wires again turning the controller adjustment from minimum (that's 2) to maximum (that's 3).
So we have 3 voltage readings.

What do you have connected to the white 3-pin header near the electrolytic caps?

Measure the same wire i did for the volage feeding the controller? IE 12V input

(2) Control knob all the way to the minimum 12.02v

(3) Control knob all the way to maximum 11.38v

When turning to max it dips to 11.26v these rises to the above

A100K Verable Resistor - It came with the kit
 
Last edited:
Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.