CPU Fan Size, Ampere, Watt Measurement

♦♦♦CPU Case Fan POWER DRAW:
Measured in Amps (A), tells you how much power the fan will use. To convert this figure to maximum wattage, you will want to multiply it by the maximum voltage the fan supports. For example our AF120 Performance Edition fan has max power draw of 0.13A and max operating voltage of 12v.  Now,  0.13a x 12v = 1.56 Watts.

♦♦♦Need to find out how many Amps a 12v 120mm PC Fan uses on average.

It's easy to tell what size pot you need. Ohms of fan = Volt rating of fan / Amp rating of fan . You will want a resistor a bit lower than the resistance (Ohms) of the fan.

--110 kOhms is way too high as most 80x25 12V fans will be 100 Ohms or less. So a 100 Ohm, linear taper pot or rheostat, rated at a minimum of 2 Watts will be usable for most fans that draw up to 0.45 Amp or so at 12V.

--Most 12V fans won't run at less than 7V so check for stall position on the dial and leave a conservative cushion. If your fan draws 0.5 Amp or more, then you will need both a lower Ohm rating and a higher Watt rating.

--I'm thinking of doing a resistance chart for every .05 Amp step of 12V fans - easy to do w/ a spreadsheet (or even a BASIC) proggy. I could probably do it in my head, let's see:
For 12 Volt Fans:

--However if you really need to know you can work it out by measuring the resistance of the fan when disconnected then apply Ohm's law.

I = V/R

Here,
I = currant
V = voltage
R = Resistance


Amp------Resistance---- Rating of------ Fan

==================================
0.10A = 120 Ohms
0.15A = 80 "
0.20A = 60 "
0.25A = 48 "
0.30A = 40 "
0.35A = 34.3 "
0.40A = 30 "
0.45A = 26.7 " At this point only 25% of 100 Ohm pot rotation could be used.
0.50A = 24 " From here a 50 Ohm or less pot should be used rated at 5 Watts up.
0.55A = 21.8 " The 15 Ohm 15Watt rheostat from allelectronics.com
0.60A = 20 " would probably work for all from about 20 Ohms down.
0.65A = 18.5 "
0.70A = 17.1 "
0.75A = 16 "
0.80A = 15 "
0.85A = 14.1 "
0.90A = 13.3 "
0.95A = 12.6 "
1.0A = 12 "
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Note well: Make sure the pot/rheostat is in series with the fan (IOW, you use only two legs of the pot, the middle and one end - not all three), not in the typical series/parallel configuration you'd use in a volume control circuit as you don't want the additional load.


♦♦♦Example: A 120mm 12V case fan. The rating on that one is 1.25 watts. Divide the watts by 12v to get the amps.


♦♦♦ 120mm 0.60A fan power consumption watt:

==========================================
OVERVIEW

Model : XXVVDDSS
Dimension 120 x 120 x 25mm
Speed (R.P.M.) 800 - 2200 R.P.M ± 10%(PWM) / 1400 - 4000 RPM ± 10%(PWM)
Air Flow (CFM) 34.02 – 93.74 CFM ± 10%
Air pressure (mmH2O) 0.43 – 3.30 mmH20
Noise 15 - 38 dB
Bearing Type DynaLoopTM Bearing
Fan Life Expectancy 40,000 hours - 80,000 hours
Voltage 12V DC
Current (Ampere) 0.45A (0.7A MAX) / 0.60A
Power Consumption 5.4W (8.4W MAX)
Weight 102 g
Connector 4-pin
Accessory Rubber x 4
Metal screw x 4
Warranty= N/A

♦♦♦Replacing desktop 92mm cooling fan. When unpackaged the new fan, the sticker on it says DC 12v 0.20A and the one in my case is a 0.50A. What difference will the lower amperage make and should I use it?

That's a 92mm fan that's rated for 0.50A - which equals what, 6 Watts? A fan like that would be thick (thicker than regular 25mm) and spin ~7000 rpm, LoL. Silent model, heheh... 100cfm probably.

The 0.20A is more typical; 2.4W and ~50cfm. Note that this too would be relatively powerful fan, but average.

Here's a Thermaltake SilentBoost 92mm fan I have - they're great by the way, large and moves a lot of air - and it wants 0.25A.

So 0.50A is really a lot! Too much for a motherboard plug, use molex only.
I use a lot of Vantec Stealth fans. Their 92mm is 0.84W @ 1750rpm, which is only 0.07A. A 92mm fan like this is quiet but still moves good air (almost 30cfm).
A faster 80mm fan might claim more cfm, and also use more power.
These Stealth fans are really not appropriate for CPU coolers, at least not in the smaller 60mm and 80mm sizes, but big coolers with 120mm and even 92mm fans will usually work okay (and very quiet!) with the low rpm fan.
The 0.20A fan you mention should be great for CPU or casefan duty. It would be considered typical for CPU cooler and strong as a casefan.
Now I know people love to use motherboard-monitored power plugs, and it's okay! Especially modern motherboards, if your powersupply is 24-pin and you have the 4-pin squareplug (and even molex) mobo connections, no worries.
But nice older rigs with 20-pin powersupplies should always molex all their fans whenever possible, this is important. Resist the urge to monitor these fans and always bypass the mobo, with the obvious exception being the CPU fan.
That 500mA fan you mentioned is pushing the 3-pin spec on any mobo, even the most modern, however. I would definitely molex that sucker!
Perhaps the newest mobos (with 4-pin monitored CPU fan power) will happily supply a 6 Watt fan.

Replacing your fan that is rated to used 0.5 amps of current with a fan that is rated to use 0.25 amps of current means that it takes less current from your PSU to run the fan and therefore will load the PSU less than the old fan. One thing to be concerned about is the CFM (rated airflow) of the fan, lower CFM means it will move less air and will affect your temps.