How to Undervolt a Fan Inside a Small Device?
I would like to know the way to reduce the speed of a fan inside a
small device in order to cut down the noise from the fan. If I understand this correctly, I am supposed to do this by undervolting the fan from 12-volt it is now to something like 6-volt using some kind of "inline resistor" or someting. I can soldering wires together. But I really don't know much about resistor and such. I am hoping someone can give me the "exact" information about the way to do this. The small device is a LinkSys gigabit switch that has a very noisy 40mm fan in it. I have replaced it with a slightly quieter Papst 60mm fan. My intention is to oversize the fan and then reduce the fan speed; then I will be able to maintain the same air flow as the 40mm fan but with less noise. Now, I have the oversized fan mounted inside the gigabit switch. And I can feel that its air flow is definitely stronger than the 40mm fan. This means I can go ahead to reduce its speed without worrying about the possibility of not getting enough air flow. I need to figure out how to reduce its speed. My questions a - I am under the impression that I should use something called an "inline resistor". Is it the right product? - Because the gigabit switch has very limit space inside, the "inline resistor" must be small enough to fit inside. Will it be small enough? - The fan is rated as 12V and 0.6 watt with two wires. What type of "inline resistor" should I use? I am under the impression that inline resistor is rated by "ohms". Can someone give me a RadioShack product-number or something like that? - Which wire should I solder the inline resistor to the fan? Red-wire or the blue-wire of the fan? - Does this matter which way I orient the inline resistor? Thanks in advance for any information. Jay Chan |
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snip Well, at least you know you don't know anything about electricity. If the figures you give are accurate (which they aren't, they are probably maximums) you want about the same ohmage as what the fan appears to be. This would be about 12 v. / 0.05 A = 240 ohms. All that really does is give you a ball park number. You may want anything from about 100 ohms to that. They should probably be at least 2 watt rated for safety, but have virtually no tolerance requirements. Get a set of 2 watt carbon resistors, worth about 5 to 10 cents each (but expect to pay more unless you can raid someones junk box), say 100, 150, 220, 330 ohms and try them out. No, it doesn't matter which lead they appear in, nor which direction they are connected in. Ohms and watts are an adequate specification, and you don't need anything better than 10 or 20% tolerance. Just wondering... When trying to slow a fan in a PC, which is better - using 5v and 12v to get 7v to the fan, or adding a resistor inline? I've got a Media PC that's very quiet except for one case fan. Thx |
On 15 Mar 2005 20:18:20 -0800, wrote:
I would like to know the way to reduce the speed of a fan inside a small device in order to cut down the noise from the fan. If I understand this correctly, I am supposed to do this by undervolting the fan from 12-volt it is now to something like 6-volt using some kind of "inline resistor" or someting. I can soldering wires together. But I really don't know much about resistor and such. I am hoping someone can give me the "exact" information about the way to do this. Take original fan and cut it's power + lead in half, maybe in the middle so you have enough slack to work with. Next look at the length of the resistor you'd use, accounting for a few (maybe 3) millimeters on each of the leads, and cut off that much more from one of the ends of the wire you'd just cut. Essentially you're shortening the lead such that both leads will be same length after the resistor is soldered on, which is theoretically unnecessary but looks better. Get a 2W resistor, the value depends on the fan but a ballpark range would be 47-200 Ohm. A value of about 100 Ohm is a good first guess with no further info. Since you don't yet know what the resultant RPM & noise will be at "6.0V", it could be that you don't actually want exactly 6V? Another alternative could be to start out with a 200 Ohm (or make-do with whatever is available) rheostat, dialing in the resistance value you find to be optimal then measuring the resistance, to "size" the resistor value you need. Then choose the closest commonly available value (or whatever you have on hand). So you've found the resistor value you want, simply slip a length of heat-shrink tubing on each of the cut wire ends, back as far as possible away from the heat of soldering. Then solder on the wires- could help to tin the wires first and/or clamp them to the resistor while soldering. In theor a mechanical joint is best before soldering, but a good solder joint alone with heatshrink over it is sufficient for a non-stressed connection (no later mechanical force will be applied to it). After resistor is soldered on, slip the heatshrink over the soldered, bare metal area and heat it up to shrink it. The small device is a LinkSys gigabit switch that has a very noisy 40mm fan in it. I have replaced it with a slightly quieter Papst 60mm fan. My intention is to oversize the fan and then reduce the fan speed; then I will be able to maintain the same air flow as the 40mm fan but with less noise. Now, I have the oversized fan mounted inside the gigabit switch. And I can feel that its air flow is definitely stronger than the 40mm fan. This means I can go ahead to reduce its speed without worrying about the possibility of not getting enough air flow. I need to figure out how to reduce its speed. My questions a - I am under the impression that I should use something called an "inline resistor". Is it the right product? Inline simply means it's a serial connection, that you cut the power lead and put each cut end on opposite ends of the resistor. It would be called a "2W" resistor. There are different resistor compositions like metal or carbon and ceramic, etc. They don't matter, just use smallest 2W resistor you can find, OR the cheapest, or whatever's available where you normally purchase parts (to combine with other orders since resistor is tiny fraction of the cost to ship it) or whatever you have handy. - Because the gigabit switch has very limit space inside, the "inline resistor" must be small enough to fit inside. Will it be small enough? Yes, so long as you don't get a giant cermet (cement filled) wire-wound type. They are usually rectangular and white colored, and quite overkill. The typical resistor you might seek is about the diameter of a thin pencil and just under 2 cm long. - The fan is rated as 12V and 0.6 watt with two wires. It's a relatively slow fan already, again I'd try around 100 Ohm resistor first, maybe slightly higher. What type of "inline resistor" should I use? I am under the impression that inline resistor is rated by "ohms". Can someone give me a RadioShack product-number or something like that? Well Radio Shack might have them, I'll check that in a moment but these are common parts, any fully stocked electronics outfit should have something suitable. Seems like Radio Shack brick-n-mortar stores carry fewer misc. parts than they used to, you might not be able to assume what's available online is actually stocked in any particular store- I'd call ahead first to ask as one store might have it but another not. I didn't see any on Radio Shack's website, maybe I overlooked them but I dug up another source, might as well get a good deal on 'em? http://www.goldmine-elec-products.co...m=2&mite m=11 You might call and ask them if they'd just throw 5 (pieces instead of 200) of them in an envelope, if you're trying to get ultra-cheap shipping. Since the fan has such a low RPM already, it's possible that it's not a good candidate for (around) 6V operation, but it might be suitably quiet at higher voltage. You might try a 68 Ohm resistor, or solder two 120 Ohm resistors in parallel (for 60 Ohms), which is just connecting both ends of each resistor together and one end of the cut wire (mentioned previously) to each junction. - Which wire should I solder the inline resistor to the fan? Red-wire or the blue-wire of the fan? Positive, usually red-wire is the typical way, but if you wanted to solder it to the blue wire instead that would work too with no drawbacks since this is only a two (power) lead type fan connection. - Does this matter which way I orient the inline resistor? no |
On Wed, 16 Mar 2005 16:15:30 GMT, "Noozer"
wrote: Just wondering... When trying to slow a fan in a PC, which is better - using 5v and 12v to get 7v to the fan, or adding a resistor inline? The resistor, assuming a reliable connection is made (soldered or good crimp, not just twisted bare wires) and electrically isolated (like heatshrink or similar, as electrical tape can degrade and unravel especially in hotter environments). In practice, either has worked fine for most people trying (either). The resistor is certainly a higher level of control, being able to choose some other voltage... 7V is too slow for some applications and 5V too low for some fans to even reliably spin-up. With today's larger 92-120mm fans becoming more common, in some cases running the fan from 5V (5V rail & ground) could be a good alternative instead. I've got a Media PC that's very quiet except for one case fan. Only issue there is being aware of the resultant temp changes that come from flow reduction. |
"pilgrim" wrote in message ... On Wed, 16 Mar 2005 16:15:30 GMT, "Noozer" wrote: Just wondering... When trying to slow a fan in a PC, which is better - using 5v and 12v to get 7v to the fan, or adding a resistor inline? Troll Look who's talking |
"kony" wrote in message ... On Wed, 16 Mar 2005 16:15:30 GMT, "Noozer" wrote: Just wondering... When trying to slow a fan in a PC, which is better - using 5v and 12v to get 7v to the fan, or adding a resistor inline? The resistor, assuming a reliable connection is made (soldered or good crimp, not just twisted bare wires) and electrically isolated (like heatshrink or similar, as electrical tape can degrade and unravel especially in hotter environments). Thanks. I felt that the resistor was better, but I wasn't sure if there was anything about it that I had overlooked. I've got a Media PC that's very quiet except for one case fan. Only issue there is being aware of the resultant temp changes that come from flow reduction. Machine runs cool. I'd just disconnect the fan completely, but I think that would be a bit drastic as the chassis isn't the best design. |
Noozer wrote:
snip Well, at least you know you don't know anything about electricity. If the figures you give are accurate (which they aren't, they are probably maximums) you want about the same ohmage as what the fan appears to be. This would be about 12 v. / 0.05 A = 240 ohms. All that really does is give you a ball park number. You may want anything from about 100 ohms to that. They should probably be at least 2 watt rated for safety, but have virtually no tolerance requirements. Get a set of 2 watt carbon resistors, worth about 5 to 10 cents each (but expect to pay more unless you can raid someones junk box), say 100, 150, 220, 330 ohms and try them out. No, it doesn't matter which lead they appear in, nor which direction they are connected in. Ohms and watts are an adequate specification, and you don't need anything better than 10 or 20% tolerance. Just wondering... When trying to slow a fan in a PC, which is better - using 5v and 12v to get 7v to the fan, or adding a resistor inline? I've got a Media PC that's very quiet except for one case fan. You can only use the 12-5=7 trick when the 5 V line has enough load on it to sink the 7 V current. That will usually be so. However, that also gives you only one possible operation point, while the resistor allows you to select what you want, at the cost of a watt or two. Please try to get in the habit of preserving attributions for material you quote. -- "If you want to post a followup via groups.google.com, don't use the broken "Reply" link at the bottom of the article. Click on "show options" at the top of the article, then click on the "Reply" at the bottom of the article headers." - Keith Thompson |
Thanks for specifying the range of inline resistors that I can try.
This is especially useful if I need to mail order them. Then, I can order a range of inline resistors instead of just one type. Otherwise, the shipping cost will kill me if I need to order various types multiple times. Anyway, I will look for them in the local RadioShack first. If not, I may have to mail order them. Jay Chan |
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