Question about reversing fans by switching wires...

A LOT of fans (like 95%+) will NOT work with the polarity reversed. In any case, if yours does, theres a chance that not only will the leds not light, they could burn out.

So literally you're more bothered about which way the LEDs face than the efficiency of the fan?

Have you considered just putting in cold cathodes, since a computer is just for looking at?

LEDs won't burn out. Don't worry, just try...

Better yet ...



He cares about pretty lights on the back of the case.

BTW: LED's can burn out very easily, over volt them.

You are 100% wrong. Why did you post something you have no experience doing? Brushless DC fans are not going to run reversed with reversed polarity, NEVER.

Please refrain from guessing or whatever false logic caused you to arrive at your prior post, as providing accurate information is the WHOLE POINT of these forums.

The bigger problem IF it would even work, wouldn't be the LEDs. Who gives a !@#$ about LEDs, the problem (if it could work) would've been that fan blades are engineered to turn in one direction and not the other, will have drastically worse performance spinning the other way except for a very few extremely crudely made blades that werent engineered for either direction, which would not only be very bad in the original direction but a sign the entire fan is an extremely crude product not worthy of use at all.

The answer to the problem is simple- if these fans must be used, forget about the integrated LEDs and add your own. A typical ultrabright LED bought from a 3rd party and mounted wherever you like will do fine if aimed as desired and mounted permanently, ideally with a mostly transparent solution like clear epoxy (have the LED powered and aim it to direct the light while setting it in the epoxy).

An ultrabright LED running off 12V as a fan uses, will need roughly 470 Ohm 1/8W resistor in series, or if you want lower voltage fan or series resistors, use an LED calculator as found with google search, etc.

That seems a little off.
Assuming a typical hi-brightness BLUE led with a forward voltage of abt 3.4V, and a 12V power source, you would need a (< or =) 820 Ohm 1/2Watt resistor for a standard 12ma current. The resistor could be lowered to the next value down for a higher brightness but the resisotr has to be rated at 1/2W.

1/8W reissotr is way too low, it would burn out quickly, 1/4W could be used, but it would heat up a lot and very quickly. Use 1/2W to be safe.

Blue - use above ratings
Red - 560Ohm 1/2W
Green - 560Ohm 1/2W
Uber Ultra Bright/Very expensive Nichia type leds - same as Blue

That seems a little off.
Assuming a typical hi-brightness BLUE led with a forward voltage of abt 3.4V, and a 12V power source, you would need a (< or =) 820 Ohm 1/2Watt resistor for a standard 12ma current. The resistor could be lowered to the next value down for a higher brightness but the resisotr has to be rated at 1/2W.

1/8W reissotr is way too low, it would burn out quickly, 1/4W could be used, but it would heat up a lot and very quickly. Use 1/2W to be safe.

Blue - use above ratings
Red - 560Ohm 1/2W
Green - 560Ohm 1/2W
Uber Ultra Bright/Very expensive Nichia type leds - same as Blue

Check you math, the majority of hardware uses surface mount 1/8W resistors, including motherboards, routers, modems, etc, etc, etc.

1/2W is unnecessary. Sure, you could use one at higher wattage w//o problems, but that doesn't make it necessary. Part of how significant the wattage rating is, is how well 'sunk the resistor is, worse case scenarios in isolation are more limiting than taking same resistor and soldering into this type of circuit with (larger 'sinking) wire leads.

Further, there is no such thing as "standard 12mA current". The max for a typical 5mm encased LED is 100mW. The forward drop will change what current is allowed, but it will ALWAYS be substantially higher than 12mA. Typical conservative current is 20mA. Standard max current with reasonable lead-sinking is 30mA.

That seems a little off.
Assuming a typical hi-brightness BLUE led with a forward voltage of abt 3.4V, and a 12V power source, you would need a (< or =) 820 Ohm 1/2Watt resistor for a standard 12ma current. The resistor could be lowered to the next value down for a higher brightness but the resisotr has to be rated at 1/2W.

1/8W reissotr is way too low, it would burn out quickly, 1/4W could be used, but it would heat up a lot and very quickly. Use 1/2W to be safe.

Blue - use above ratings
Red - 560Ohm 1/2W
Green - 560Ohm 1/2W
Uber Ultra Bright/Very expensive Nichia type leds - same as Blue

Check you math, the majority of hardware uses surface mount 1/8W resistors, including motherboards, routers, modems, etc, etc, etc.

1/2W is unnecessary. Sure, you could use one at higher wattage w//o problems, but that doesn't make it necessary. Part of how significant the wattage rating is, is how well 'sunk the resistor is, worse case scenarios in isolation are more limiting than taking same resistor and soldering into this type of circuit with (larger 'sinking) wire leads.

Further, there is no such thing as "standard 12mA current". The max for a typical 5mm encased LED is 100mW. The forward drop will change what current is allowed, but it will ALWAYS be substantially higher than 12mA. Typical conservative current is 20mA. Standard max current with reasonable lead-sinking is 30mA.

General rule of thumb is approx ~20ma current for a standard LED. 12ma is a the safe value for high brightness LEDs. Sure, you can push more current through itm but dont expect them to last more than 20000-30000 hours, compared to their normal 100 000 hours lifespan.

the bearings will be loaded differently, when the direction of rotation is switched.

i suggest not starting the system & leaving for the weekend, for example. you should be there to keep an eye on it, for a period of time. maybe the first 16 hours of an informal "burn in" period.

Could someone post some links about switching wires around to switch the direction of rotation. I never heard of that happening before. In my experience switching the wires around will give you either 7v or 5v, making the fan spin in the same direction it was, only slower. It's a trick for reducing noise.

switching wires is literally that - reversing the positive and negative wires. Normally, with DC brushless fans, THIS DOES NOT WORK and the fan simply does not spin. Whoever it was that posted that is incorrect in saying it will work.

In the case is does work, the airflow would be poor as the blades are angled in a forward direction. Spinning them backwards would only cause turbulence and do more harm that good.

[quote="technology-sponge"]

Define "high brightness". The typical ultrabright LED (which is what these fans use, albeit low grade/binned LEDs as they're cheaper) does not have any such "safe value" 12mA, it is just a number someone made up.

You are not doing some extreme "push more current" kind of scenario by giving them 20mA, in fact it is absolutely manditory to get remotely close to their MCD rating, and is the figure at which lifespans are generally given. Some as low as 15mA but nothing these days is rating for 12mA, you are substantially UNDERdriving them without reason at such a low current unless the particular use has a goal of limiting brightness deliberately (as is often the case on consumer electronics such as amplifier/etc power indicator lights, where there was no goal to have light shining out on other things).

Lifespan will of course go down, but trying to reach 100K hours isn't so important. If a system ran 12 hours a day that'd be 23 years, not until they failed but merely dropped below the spec'd brightness threshold, but lower brightness was exactly what would be caused by the lower current too.

In short, for this application there is no good reason to shoot for under 20mA, for all practical purposes it would only make the fan dimmer.

That wont work, period.
Either they fry, or they stand still. Worst case they'll short circuit your PSU.

Just physically turn the fan over to have it blow the other direction, that's why they're fitted with eight holes, duh.

[quote="I"]

Define "high brightness". The typical ultrabright LED (which is what these fans use, albeit low grade/binned LEDs as they're cheaper) does not have any such "safe value" 12mA, it is just a number someone made up.

You are not doing some extreme "push more current" kind of scenario by giving them 20mA, in fact it is absolutely manditory to get remotely close to their MCD rating, and is the figure at which lifespans are generally given. Some as low as 15mA but nothing these days is rating for 12mA, you are substantially UNDERdriving them without reason at such a low current unless the particular use has a goal of limiting brightness deliberately (as is often the case on consumer electronics such as amplifier/etc power indicator lights, where there was no goal to have light shining out on other things).

Lifespan will of course go down, but trying to reach 100K hours isn't so important. If a system ran 12 hours a day that'd be 23 years, not until they failed but merely dropped below the spec'd brightness threshold, but lower brightness was exactly what would be caused by the lower current too.

In short, for this application there is no good reason to shoot for under 20mA, for all practical purposes it would only make the fan dimmer.

20ma is in NO way extreme, extreme would be forcing maybe 50ma through an led.

maybe this might be of service:

First you need to know the LED voltage drop. It is safe enough to assume 1.7 volts for non-high-brightness red, 1.9 volts for high-brightness, high-efficiency and low-current red, and 2 volts for orange and yellow, and 2.1 volts for green. Assume 3.4 volts for bright white, bright non-yellowish green, and most blue types. Assume 4.6 volts for 430 nM bright blue types such as Everbright and Radio Shack. Design for 12 milliamps for the 3.4 volt types and 10 milliamps for the 430 NM blue.

seeing as this is a pc fan, typical colors would be blue/white, so in this case, 12ma would be perfectly acceptable.

assuming for a moment that i was delusional and that 12ma is underpowering, why would you need eye glaring blue light shafting out of a BACK of a PC for??? to get a good view of the build up of dead cockroaches???

LEDS have a maximum lifespan of 100K hours under perfect operating conditions ie

25C ambient, clean filtered power source will low ripple, heatsinked leads, which is basically never. allow for manufacturing run variances and 'binend low grade leds' and you'll be lucky to make it past 80K hours at all.

-------------------------------------------------------------------------
in the case that (guy who orignally posted hsu name i forgot) does want the leds brighter, it is possible to increase current to 18ma:

"You can design for higher current if you are adventurous or you know you will have a good lack of heat buildup. In such a case, design for 25 ma for the types with voltage near 2 volts, 18 ma for the 3.4 volt types, and 15 ma for the 430 NM blue."

Seems like a lot of trouble just because you don't like the way the LED's are facing. Chances are, even if the LED's are between the fan and the case, given that you've got clear plastic fans, they will still illuminate the entire fan and surrounding areas. LED's are brighter than you might think. What case do you have? Does it have a side window?

No, just no. This thread is ridiculous. We're talking about a $5 part here, just replace it if it doesn't meet your needs. If you need to do a mod, then put a window in your case. Then you can see all your pretty internal lights :roll:

SPOILSPORT!!!   :twisted:

where do you live? $5 dollar fans!!!! ther lyk 20-25 bucks here in oz, cheapest iv ever seen is 12 at flea markets oem, with no packaging and proprietary plug that had to be replaced with a molex!

NO he can't, because it's probably a BRUSHLESS fan. They don't run backwards.

I thought we'd already discussed this?

Edit: Replied emotionally before reading the remaining posts...

The OP does. Unfortunate, given your tyrade....as usual. :roll:

You could just snap the blades off and reattach them to the hub at the opposite angle  

Try reversing the polarity... most fans do a little jiggle-dance...

*prepares to be struck down in a hail of flame by I*

You're joking, right?

You're joking, right?

next time i go to the states, im buying computer fans! 3-5 bux for a led fan, dats around 4.20-6 oz dollars approx

A big thanks to all that replied. Just wanted to post a few things:

1) Yeah, I care about the look of my computer. Thats the whole point for having LEDs! I work with my computers for about 9-10 hours a day, and thus not only does the functioning of it matter to me, but also how it looks. Not only do I feel good, but it also looks good to my clients. I can't get the point why some people are freaking out that having a good looking computer is a bad thing. If you want a computer for nothing but its performance, why even buy a case? Just leave the motherboard out in the open and attach all the parts to it. Its probably going to get the same dust if it was inside the case, dust seems to get everywhere!

2) I bought 4 of these fans, just received the remaining 3 in the mail yesterday, and 2 of them run well, but the LEDs don't light up. So I guess I will be using those 2 at the back, so pretty much I don't need to reverse the fans anymore. But this topic sure got me some useful knowledge into how fans work. Especially LEDs.

3) Err...fans cost $5.00-$10.00 (with LEDs). Of course you get ones that are $20.00 as well, but those are probably so powerful that when you attach 2 at the bottom, the case lifts-off!

Thanks to all once again!

Yes they do cost 5-10 bux, in america, but here down under in oz, the prices are extravagant. its 25-30 for a high airflow, quiet led fan