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Master_Scythe

Whats the amperage of a 120mm fan?

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Well im hapy to inform you all, those who didnt know: a 120mm generic $5 fan from MSY is more than happy to spin to quite a decent speed at 5v.

 

(and correct me if any of this is wrong)

 

Now all i need to know, is, are 120mm fans typically less than 250mw each? (there being 1000mw in an amp?) meaning i could run 2X sans off a single USB port?

 

Im building a USB cooling pad. for mothers laptop. also a cooling fan for my Wii and modem. I like cooling.

 

Are my facts right? Been a good couple of years since i did this. (last time i could see), lol. so now i can again, projects are back.

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How did you figure that 1w = 1A? There's 1000mA in an amp. What is the amperage of a 120mm fan? It depends on the voltage for one and the power used for another.

 

Have a read.

 

To work out the current draw of the fans concerned, you need to know both the voltage and the wattage. Or a multimeter.

 

Let's take a look at this fan, for example.

 

12VDC, 3.12W. 3.12/12= .26. That's .26A.

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First PC cooling fans are 12v normally and USB is 5 volt.

So the fans will run a bit below half speed, which may be no bad thing for this job.

As far as usb pinouts see here

http://www.hardwarebook.info/Universal_Serial_Bus_(USB)

Max draw is 500mA total across all ports on that hub.

As 1shot said a multimeter in series is probably the best way to measure the current draw.

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*face* i used wrong terms, that my fault.

 

Though probably good i did as it leads me to another noob question.

 

USB provides 500mA at 5v i knew that (why i thought mw, i dunno)

 

the fans i see are usually somewhere around 12vdc @ 0.12A (thats 80mm though) hence asking for the 12cm measures.

 

12v @ 0.26A (X2) should just run... do i actually have to take wattage into account? Sorry its been a while. My basics have slipped :( I hope to build a wooden laptop stand with a 2 120mm fans @ 5v (tested one at 5v using the 5v rail on a PSU, runs at a fantastic speed\air flow).

 

Thanks for any help :)

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12v @ 0.26A (X2) should just run... do i actually have to take wattage into account?

Yes, you do. At a lower voltage, the current draw will change, depending on the load. What you're after tho', should work. Just make sure you do your sums, is all.

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Ok just checked a Spire and a Glacialtech 120mm fan. Both are rated at 0.25A @12v.

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Ok just checked a Spire and a Glacialtech 120mm fan. Both are rated at 0.25A @12v.

Thank you all.

 

Excellent i thought 0.25A was the norm, at least i remembered that. lol. 2X120mm fans, here we come. Lucky little laptop.

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12v @ 0.26A (X2) should just run... do i actually have to take wattage into account?

Yes, you do. At a lower voltage, the current draw will change, depending on the load.

 

I'll swallow pride, Why...

 

I read that link, which joggel a lot of memories. However, if watt = volt X amp, then as long as the amperage doesnt add up to be more than the provided amperage, and obviously they're running at 5v, why do watts come into it at all?

 

Also, just for my own mind, If i had one of various components inline that would drop voltage to the fan, would that change the watage draw? or since, lets use resistor, gets hot, is the excess voltage just becoing heat?

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Let's assume that a fan has a load of 3W.

 

At 12VDC, the current draw will be 250mA. (3W/12V=.25A)

 

At 5VDC, the current draw will be 600mA. (3W/5V=.6A)

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well a USB port can only provide about 2.5W according to my calculation.

 

and if dropping volts ups required amperage, and a USB port can only provide half an amp. Why do you say "what im after should work"

 

a single fan alone is above 500mA let alone 2.

 

im not attacking, im just trying to understand. I should really go do a cert in tafe on this sort of basics.

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Why do you say "what im after should work"

Because I don't know the wattage of your fans. Remember when I said earlier that you need to take the wattage into account? You don't need to do a TAFE certificate for this, it's basic mathematics.

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I get the math, but its just not working in my mind i guess.

 

by those sums, a USB port can provide 2.5w of power.

 

if on average, a fan pulls 3w, its already too much.

 

also i didnt mean a tafe cert for the math, i can do math ><

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if on average, a fan pulls 3w, its already too much.

That's right. But you don't know the wattage of your fans. Got a multimeter? Connect it in series between the fan and the PSU, that'll tell you the amperage (current draw), with the amperage and the voltage, you can work out the wattage.

 

I know you didn't mean a TAFE certificate for the math. I've not done a TAFE certificate and I have a very basic understanding only of electronics. Substitute watts, amps and volts for letters of your choice and it's just algebra.

 

a/w=v, where v = 5 and a=.6. What's the value of w?

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ah ok.

 

So lets see if i did this right.

 

with the multimeter set to 'A' (amperage), and the switch to 200m ( so logic says 200ma), it reads 150 (or 180 when i manually stop the fan).

 

so, 0.15a X 5v = 0.75.

 

So, 0.75W for that fan? (meaning 2 will be ok)

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I hope you don't mean to tell me that you had a fucking multimeter all along!

OK I wont tell you that i had a multimeter all along.

 

so anyone for cake?

 

lol, on the plus side, assuming i used said multimeter (which im not telling you i have) right, this will work.

 

I'll say thank you for all the effort you put in, but i still think im gonna be slapped :P

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If the figures *roughly* fit, just do it... the worst that will happen is the USB controller will shut the port down to protect itself.

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Great idea there MS ... I have a Vantec Lapcool3 with two 80mm fans. I think two 120mm fans would be even better (and quieter).

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If the figures *roughly* fit, just do it... the worst that will happen is the USB controller will shut the port down to protect itself.

Ummm, no... the worst that will happen is the USB power will die.

 

You HOPE it will shut itself down in self-defence...

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yeah being an older device it could die.

 

Thats OK I have a PSU hooked up testbench style, so i can hook both fans up in paralel and test the amperage, if thats below 500mA, i'll then do the math X5 and get the watts.

 

if thats below 2.5w, well, we have a winner.

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yeah being an older device it could die.

 

Thats OK I have a PSU hooked up testbench style, so i can hook both fans up in paralel and test the amperage, if thats below 500mA, i'll then do the math X5 and get the watts.

 

if thats below 2.5w, well, we have a winner.

My Vantec Lapcool3 with 2 x 80mm fans (at 1500 RPM) draws < 0.9W so I think it should be OK.

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I shall buy fans when i head out today then.

 

Have to love MSY. $5 fans. brilliant.

 

and at 7v they're silent in a case, and at 5v they're silent all together. cant complain at all.

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