Fans and powers

[absolom7691]

As far a noise goes, it is all about RPM. The faster the fan spins, the louder it will be. Larger fans are not quieter per se. The reason they seem quieter is because you can run them at a lower speed and still get decent airflow. I image the larger blades probably give you a lower frequency noise, but regardless, unless you plan to have a BUNCH of 40mms and run them at half speed, you are better off going with a larger fan. Do check the RPMs of the fans. The lower the RPM, the quieter, but at the expense of airflow. If your supply voltage is 15V, try getting two full speed fans and run them in series, that will lower their speed and each fan will only be running at 7.5V.

[lamborgini8]

Just managed to get two free 40mm 12V DC fans, unfortunately though I can only fit one on the scanner...

Im not sure what speed it is or anything so what can I use to step 15V down? I just tried running it on 5V and it did nothing haha.

Edit: Bad batteries, 5V makes it wobble but not spin. It spins after a little encouragement...

[danielbriggs]

What current is marked on the fans?
Use V=IR to find the required resistor to give the right voltage drop.
If there is no current, look for a power rating e.g. 1W or something.
Then use P=IV to find current. Then stick that back into V=IR
To make sure your resistor doesn't overheat, find the power rating and go for the nearest one above that for a bit of a safety factor. Use P=I^2R

OR, if you can't be bothered with all that. Go out and grab a Zalman FanMate II. I think they have them in Maplin for a couple of quid.
(But they also sell resistors for a couple of pence!)

Dan

[lamborgini8]

What current is marked on the fans?
Use V=IR to find the required resistor to give the right voltage drop.
If there is no current, look for a power rating e.g. 1W or something.
Then use P=IV to find current. Then stick that back into V=IR
To make sure your resistor doesn't overheat, find the power rating and go for the nearest one above that for a bit of a safety factor. Use P=I^2R

OR, if you can't be bothered with all that. Go out and grab a Zalman FanMate II. I think they have them in Maplin for a couple of quid.
(But they also sell resistors for a couple of pence!)

Dan

Don't know that much about electronics so half of that made no sence to me haha... The fan has 0.6W written on it though.

I just ran it at 10.5V and its quiet enough for my liking and has a nice cool breeze from it. Because of the position I am putting the fan I need to track down a decent 40mm dust guard or whatever you call them as well.

[colouredmirrorball]

Daniel Briggs, learn your SI!
No offence to you, but you made a huge mistake:
R = U/I (and not P = V/I or something )
Resistance = R in Ohm, Voltage = U in Volt, Currency = I in Ampère
Wattage (sorry don't know word in English) = P = U.I

Your fan needs 15 V (= U) to run at full power and has an offset voltage of 5V or something, but if you connect it directly to a power supply, the cables will overheat. So you need to put a resistor in the circuit. To calculate what type of resistor you need, use the formules of Danielbriggs. Hint: P = I².R <-> R = I²/P (I guess).

Don't they teach you this kind of things in England? Ever paid attention in physics class? This is elementary electronic knowledge! (But I had to look them up too )

[danielbriggs]

Daniel Briggs, learn your SI!
No offence to you, but you made a huge mistake:
R = U/I (and not P = V/I or something )
Resistance = R in Ohm, Voltage = U in Volt, Currency = I in Ampère
Wattage (sorry don't know word in English) = P = U.I

With all due respect, I was working in S.I. units and my formulae were correct.
Assuming your 'U' is Voltage (What I called 'V'). Then both expressions are equal. Must be the way they teach it in Germany...
V=IR is a rearranged form of R=V/I (or R=U/I)
Voltage (Volts) = Current (Amperes) x Resistance (Ohms)

P=VI
Power (Watts) = Voltage (Volts) x Current (Amperes)

But if you substitute V=IR into P=IV, you get P=I^2R
I never stated P=V/I

Your fan needs 15 V (= U) to run at full power and has an offset voltage of 5V or something, but if you connect it directly to a power supply, the cables will overheat. So you need to put a resistor in the circuit.

I don't follow that...
Most case fans are rated at 12V.
The motor will overheat before the cables do. Voltage won't make cables overheat, large currents will.

Dan

[colouredmirrorball]

V for voltage? Now I am confused, they learned it to me like that and told it was SI! This is exactly the situation why they invented S.I., to avoid confusion!
Agree that high voltage doesn't cause overheat, but high voltage implies high current with low resistance. Indeed, MOST case fans run 12 V, but Lamborgini8 stated he used 15 V ones in his first post. Maybe he made a mistake or something.
I'm sorry if I caused confusion. Let's misuse the language barrier again for the reason rather than my lack of knowledge...

[danielbriggs]

Sorry to sound like I'm picking everything to bits, but:

V for Voltage... seems to make sense for me!

but high voltage implies high current with low resistance.

Not necessarily; take HeNe lasers as a topical example, 1000's of volts, very low current, several mA.

In his first post, he states his supply is 15V, whereas his fan is 12V.

Ta,
Dan

[colouredmirrorball]

Yes I know high voltage doesn't imply high currency - think of the Tesla coil etc., maybe I should have added "generally" to my post. Now you say it, you are right, his power supply is 15 volt and not his fan. Did not saw it, I'm sorry.
V for voltage makes also more sense to me than U, wondering where they got that one. It was just confusing me when you said that V=IR.

As for the fans, avoid too small ones! I prefer larger ones rotating slower (with the same airflow as the small ones) just because of the noise. My pre-built cheap Chinese projector has a fan with a horrible noise (could be 40 mm but not sure). Unfortunately I can't replace it because something bigger doesn't fit in.

[lamborgini8]

Did a little homework on the fans, the data sheet for them states that the fans maximum voltage above the 12V is only 13.5V... I am not going to risk runing that at 15V as that will probably burn out.

So, I need to work out the resistor or whatever it was I need/ The fan is 12V DC and 0.6W.