So i finally built my little stereo and put 2 cooling fans in it, problem is these suckers are way to loud. Each fan is 12volts .6 amps, and its running off of a 10 amp 13.8volt power supply.
I figured if i dropped the voltage down to half, say 6 volts, i would quiet these fans down quite a bit. heres how i figured everything...
13.8v source - 6volts = 7.8 volts to be dropped
R=V/C so:
7.8volts/.60amps = 13 OHM
P = C x V so:
.60amps x 7.8 = 4.68watts
I figured a 12ohm 5watt resistor for each fan would do the trick...seem correct?
Also...i wsa toying with the idea of using a petentiometer so i can turn the speed up and dwon as needed but i cant seem to find on that will suit my needs, any ideas?
thanks
Replies (5)
audeogod on 02/11/2005 23:10:58
Ok, it's been a while since my electronics training so if I say something wrong, you'll just have to forgive me.
It seems to me that you are talking about putting a 12 ohm resistor in series with the fan. Right?
Now if I remember correctly, this is going to drive the total ohm load up, and therefore bring down the current level in the circuit, because there is still 13.8 volts on the circuit. Ohm's law says that if resistance goes up, and voltage remains the same, current must go down.
If I go off of the fan's ratings, I can figure it's resistance is about 20 ohms. 12v/0.6A = 20 ohms. I'll use the power supply voltage of 13.8 volts and figure that at 20 ohms, the fan gets 0.69A. This is a wattage of 9.522 watts on the fan.
Now if you put a 12 ohm resistor in the circuit, the total resistance goes up to 32 ohms, and you now have a voltage divider circuit, like you wanted. You now also have only 0.43125 amperes (exact number) in the circuit, cause you dropped the current flow by increasing resistance.
Now we use the voltage divider formula of Vx = (Rx/Rt)Vs and we figure that the voltage on the fan is now 8.625 volts. At 8.625 volts, and 0.43A, you have a wattage of roughly 3.72 watts. That is a 2.5 times reduction in wattage to the fan, which to say the least will make it turn much slower.
It may not turn at all though, if you do this. Might just set there and build up heat until it burns out on you.
On the bright side, a 5 watt resistor would be plenty for this cause it would only get less than 2.5 watts on it. That's not a very bright side if the fan won't work, is it? Sorry.
You should use a smaller resistor. A 5 ohm resistor will let you get more than 6 watts to the fan. Not so much of a drop that way. It will decrease by about 1/3. Might be enough to quieten it down a bit.
swez on 02/12/2005 09:13:58
Am wondering if an L-pad may help here. These come in many wattage ranges and 8 or 16 ohm impedence matching ability. It is a variable resistor and you can dial back the power to the fans to maintain adequate fan speed and minimal fan noise. Works like a light dimmer pot. I use one on my shop venting fan. Works like a charm.
Suggest a pair of 8 -16 ohm, mono L-pads here. This way, you can accurately match fan speeds. The stereo version may work as well. But the concern I have, is calibration on a ganged stereo L-pad. If the resistance values are not the same on each pot, fan speeds will be out of sync and subject to a new noise problem... an oscillating fan noise like a warbling sound.
Check these out:
http://www.partsexpress.com/webpage.cfm?&DID=7&WebPage_ID=3
Swez
Buickman94 on 02/12/2005 14:31:24
Thanks for the advice guys. Swez, the link you gave me takes me to PE but gives me an error for a search result. Would you mind listing the specific part number for the L-pad that woul suit my needs?...id really appreciate it...
One other option i considered is using a resistor on a switch and just jumping it out when i needed high speed. Still not sure which route i wanna go here. The l-pad is looking like a good idea though.
swez on 02/13/2005 03:40:36
A pair of these will do the job and look nice in a mount.
http://www.partsexpress.com/pe/showdetl.cfm?&DID=7&Partnumber=260-250
Part number: 260-250
Swez
Buickman94 on 02/14/2005 17:20:29
I think im going to use those then. Not that I am second guessing what you are saying in anyway Swez, but I emailed PE tech support a few days back asking if they had a variable resistor or anything that owuld work for me and i got this reply:
Mike:
I do not have a product that will control the fan speed or work for your application but doing a quick searchon google I was able to pull up some different ways to accomplish this. I can't really recomend and one over the other but there is a lot of informatiuon available there.
I assume i got this answer because the L-pads are mainly used for audio and they may have been overlooked for this kind of application?
I figured if i dropped the voltage down to half, say 6 volts, i would quiet these fans down quite a bit. heres how i figured everything...
13.8v source - 6volts = 7.8 volts to be dropped
R=V/C so:
7.8volts/.60amps = 13 OHM
P = C x V so:
.60amps x 7.8 = 4.68watts
I figured a 12ohm 5watt resistor for each fan would do the trick...seem correct?
Also...i wsa toying with the idea of using a petentiometer so i can turn the speed up and dwon as needed but i cant seem to find on that will suit my needs, any ideas?
thanks
Replies (5)
audeogod on 02/11/2005 23:10:58
Ok, it's been a while since my electronics training so if I say something wrong, you'll just have to forgive me.
It seems to me that you are talking about putting a 12 ohm resistor in series with the fan. Right?
Now if I remember correctly, this is going to drive the total ohm load up, and therefore bring down the current level in the circuit, because there is still 13.8 volts on the circuit. Ohm's law says that if resistance goes up, and voltage remains the same, current must go down.
If I go off of the fan's ratings, I can figure it's resistance is about 20 ohms. 12v/0.6A = 20 ohms. I'll use the power supply voltage of 13.8 volts and figure that at 20 ohms, the fan gets 0.69A. This is a wattage of 9.522 watts on the fan.
Now if you put a 12 ohm resistor in the circuit, the total resistance goes up to 32 ohms, and you now have a voltage divider circuit, like you wanted. You now also have only 0.43125 amperes (exact number) in the circuit, cause you dropped the current flow by increasing resistance.
Now we use the voltage divider formula of Vx = (Rx/Rt)Vs and we figure that the voltage on the fan is now 8.625 volts. At 8.625 volts, and 0.43A, you have a wattage of roughly 3.72 watts. That is a 2.5 times reduction in wattage to the fan, which to say the least will make it turn much slower.
It may not turn at all though, if you do this. Might just set there and build up heat until it burns out on you.
On the bright side, a 5 watt resistor would be plenty for this cause it would only get less than 2.5 watts on it. That's not a very bright side if the fan won't work, is it? Sorry.
You should use a smaller resistor. A 5 ohm resistor will let you get more than 6 watts to the fan. Not so much of a drop that way. It will decrease by about 1/3. Might be enough to quieten it down a bit.
swez on 02/12/2005 09:13:58
Am wondering if an L-pad may help here. These come in many wattage ranges and 8 or 16 ohm impedence matching ability. It is a variable resistor and you can dial back the power to the fans to maintain adequate fan speed and minimal fan noise. Works like a light dimmer pot. I use one on my shop venting fan. Works like a charm.
Suggest a pair of 8 -16 ohm, mono L-pads here. This way, you can accurately match fan speeds. The stereo version may work as well. But the concern I have, is calibration on a ganged stereo L-pad. If the resistance values are not the same on each pot, fan speeds will be out of sync and subject to a new noise problem... an oscillating fan noise like a warbling sound.
Check these out:
http://www.partsexpress.com/webpage.cfm?&DID=7&WebPage_ID=3
Swez
Buickman94 on 02/12/2005 14:31:24
Thanks for the advice guys. Swez, the link you gave me takes me to PE but gives me an error for a search result. Would you mind listing the specific part number for the L-pad that woul suit my needs?...id really appreciate it...
One other option i considered is using a resistor on a switch and just jumping it out when i needed high speed. Still not sure which route i wanna go here. The l-pad is looking like a good idea though.
swez on 02/13/2005 03:40:36
A pair of these will do the job and look nice in a mount.
http://www.partsexpress.com/pe/showdetl.cfm?&DID=7&Partnumber=260-250
Part number: 260-250
Swez
Buickman94 on 02/14/2005 17:20:29
I think im going to use those then. Not that I am second guessing what you are saying in anyway Swez, but I emailed PE tech support a few days back asking if they had a variable resistor or anything that owuld work for me and i got this reply:
Mike:
I do not have a product that will control the fan speed or work for your application but doing a quick searchon google I was able to pull up some different ways to accomplish this. I can't really recomend and one over the other but there is a lot of informatiuon available there.
I assume i got this answer because the L-pads are mainly used for audio and they may have been overlooked for this kind of application?
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