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#31
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#32
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On Fri, 18 Mar 2005 21:49:53 -0800, ric
wrote: wrote: I was quite surprised that I could only find one place (eBay) that has more than one type of 2-watt resistors that are in the range that I am looking for. Many places don't sell 2-watt any resistor. Those place that sell 2-watt resistors only sell 2-watt resistors in one ohm rating (but I need resistors in a range of ohm ratings). Why 2-watt? Even at a 5 volt drop, current would have to be 0.4a to equal 2 watts. At 7v (a 5 volt drop), most 12v fans draw in the 0.05-0.1a range. I've used 1w carbon resistors (much smaller) and they hardly get warm. True, either 1 or 2W could work for this specific application, but the 2W seems potentially less problematic. At the time I didn't realize OP would have problems finding them or go to ebay for these. Why not 2W? Typically one does not try to match up watt per watt on power resistors, rather using what the space and budget allow, overengineering them. It's not a PCB space issue and cost is maybe a few cents, or maybe even cheaper per piece since this is spot-pricing on items not often (percentagewise) sold 1 at a time. Yes, 1W would usually work, but IMO, 2W is as good or better choice if someone has nothing (yet). Also, "barely" warm is a relative term. They can and do get warm, even 2W, and some components in a PC/router/etc shouldn't be placed in contact with a "warm" power resistor. Granted in this one instance it shouldn't get as warm as others. The installer could isolate it, but then has to consider that method and typical nylon wire-ties aren't so great if the resistor did get more than warm. The router itself will already be warm inside, hence the need for the fan. Metal, well, sure if challenged we could all think up some way to do it but ultimately choosing a larger resistor also results in lower thermal density, it's not going to be as hot against (whatever it's touching if it were touching anything). |
#33
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kony wrote:
True, either 1 or 2W could work for this specific application, but the 2W seems potentially less problematic. At the time I didn't realize OP would have problems finding them or go to ebay for these. Why not 2W? Typically one does not try to match up watt per watt on power resistors, rather using what the space and budget allow, overengineering them. It's not a PCB space issue and cost is maybe a few cents, or maybe even cheaper per piece since this is spot-pricing on items not often (percentagewise) sold 1 at a time. Yes, 1W would usually work, but IMO, 2W is as good or better choice if someone has nothing (yet). Only because I found a good assortment of 1W carbon resistors at my local Radio Shack. 2W? No such luck. 1W is plenty. |
#34
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On Sat, 19 Mar 2005 12:12:07 -0800, ric
wrote: kony wrote: True, either 1 or 2W could work for this specific application, but the 2W seems potentially less problematic. At the time I didn't realize OP would have problems finding them or go to ebay for these. Why not 2W? Typically one does not try to match up watt per watt on power resistors, rather using what the space and budget allow, overengineering them. It's not a PCB space issue and cost is maybe a few cents, or maybe even cheaper per piece since this is spot-pricing on items not often (percentagewise) sold 1 at a time. Yes, 1W would usually work, but IMO, 2W is as good or better choice if someone has nothing (yet). Only because I found a good assortment of 1W carbon resistors at my local Radio Shack. 2W? No such luck. 1W is plenty. Do they have one of those packs with several assorted values inside or is one forced to buy the individual smaller packs? I suppose it could depend on the store too, those I've been to in the past couple years seem to have scaled back their stock of these types of components. The individual packs from Radio Shack always seem wildly overpriced to me, though it could be a good idea for only a few values. |
#35
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kony wrote:
Only because I found a good assortment of 1W carbon resistors at my local Radio Shack. 2W? No such luck. 1W is plenty. Do they have one of those packs with several assorted values inside or is one forced to buy the individual smaller packs? I suppose it could depend on the store too, those I've been to in the past couple years seem to have scaled back their stock of these types of components. The individual packs from Radio Shack always seem wildly overpriced to me, though it could be a good idea for only a few values. Man, this was years ago. Before I started using potentiometers. As I remember, each package had about 5 resistors of the same value. That store isn't even there any more. Progress! |
#36
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Well I've bought stuff from them before... plus I've heard
of others calling, asking them to send very small items in an envelope for cheapest shipping/total-cost. In another post I listed a few more full-featured electronics companies with online stores... generally such parts would come from electronics-oriented companies rather than computer focused. I finally use the following power resistor from eBay to reduce the speed to the point that the noisy gigabit switch is now very quiet: 100 ohm 2W 5% carbon composite I have a feeling that I still have more air flow from the new 60mm fan with reduced speed than what the old noisy 40mm fan can produce in full speed. This means I could have reduced the speed further. Obviusly, I am not going to mess with something that is working so well (plus the fact that this is so difficult to find that type of power resistors). But just for the purpose of curiosity, which rating of power resistor would you recommend if someone in my situation wanted to further reduce the fan speed slightly. Thanks for your help and any other info that you may provide in the future. Jay Chan |
#37
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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 Seem like what you have suggested previously probably is a better choice than what I end up buying. Yours is 120 ohm, and what I get is 100 ohm. Your probably can reduce the fan speed slightly better than what I get can do. Moreover, yours is _much_ cheaper than what I have paid for ($20). Oh well... Jay Chan |
#38
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#39
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kony wrote:
I dont' know how much further that particular fan will tolerate a voltage drop so I can't help much. Past a certain point a larger resistor will prevent it from spinning up which obviously should be avoided. Placing a cap in parallel with the resistor seems to cure startup problems. |
#40
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On Tue, 05 Apr 2005 23:04:54 -0700, ric
wrote: kony wrote: I dont' know how much further that particular fan will tolerate a voltage drop so I can't help much. Past a certain point a larger resistor will prevent it from spinning up which obviously should be avoided. Placing a cap in parallel with the resistor seems to cure startup problems. "Cure"? To what extent? I mean, does it actually allow a lower working RPM then or just offset the overly high resistance? This is an interesting idea, do you have an example of what gain there will be for any particular fan? |
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