pcbldrNinetyEight wrote:
> Paul <nospam@needed.com> wrote in news:fpracp$qph$1@aioe.org:
>
>> Mike wrote:
>>> The fans were getting real noisy so I decided to replace the two 80mm
>>> 12V fans. I bought two 12V fans and replaced the old ones making sure
>>> I hooked up the right colored wires together, black to black and red
>>> to red. For some reason neither fans worked when I turned on the
>>> computer. The computer turned on without any problems. I tested the
>>> new fans with a 9 volt battery and they worked. I even tried
>>> reversing the wiring (black to red, black to red) without any luck.
>>> Anybody know how I can get these fans to work?
>>>
>>> thanks
>>>
>> Did you check the old fan with the 9V battery ?
>> Since you say the old fan was still working, but noisy, it
>> should pass the 9V test as well. Always be careful with the
>> polarity when using the battery method.
>>
>> By reversing the wiring on the new fan (black to red), you may have
>> damaged it.
>>
>> I don't see a particular reason a power supply would care, but fans
>> come in several types. If you buy a "case fan", it typically has two
>> wires. Red and black go to +12V and GND (assuming it is a 12V fan and
>> not some other voltage). At least the black color should be the
>> ground, leaving the other color as the more positive connection.
>>
>> The next type, is the three wire fan. The third wire is for tachometer
>> readout. But a PSU doesn't particularly care about RPMs. I have one
>> power supply here, where they include a "sensor" cable, so the RPM
>> signal from the fan can be connected to the motherboard fan header
>> (I can read out the power supply RPMs as a result.) But the power
>> supply itself, is probably not monitoring RPMs (it costs too much
>> to bother).
>>
>> Another kind of three wire fan, includes a different signal on the
>> third wire. Instead of a constantly pulsing signal, they put a
>> logic signal called "locked rotor". If the fan detects that it is
>> below a certain number of RPMs, the signal changes state. So that
>> flavor of fan, has a signal that is "go/no-go" for the connected
>> equipment. Such fans are included on "classy" equipment, such as
>> lab equipment perhaps. Locked rotor would normally be a non
>> pulsing signal, and stays in one state most of the time.
>>
>> So you have some steps to carry out, as part of your repair work.
>>
>> One is, to put the original fan back, and test that the power
>> feeding the fan is still present.
>>
>> You also need to retest the replacement fans, to see if they're
>> damaged by the reverse polarity. The company making the fans,
>> doesn't usually go out of their way, to include a polarity
>> reversal protection diode. (I haven't tested my fans, by
>> reversing the polarity, so I can't tell you whether they'd
>> be damaged or not. There can be either a chip inside the
>> fan hub, or at least a couple transistors, so in fact not
>> all fans have exactly the same design or components inside
>> the hub.)
>>
>> A power supply can be "bench tested", before being placed back
>> in the computer. For example, get a copy of the ATX standards
>> for power supplies. Connect "PS_ON#" to COM or ground. When
>> the power supply is plugged in and the switch on the back
>> of the supply is "ON", the missing step is to connect a
>> paper clip or jumper wire, from PS_ON# to COM.
>>
>> Having the cover in place on the power supply, protects you
>> in the event that the main cap exploded. (Not that this
>> is likely to happen, but the cap does hold a lot of
>> energy. It is also charged to a lethal voltage when
>> operating.)
>>
>> In a previous post I made on the subject, someone suggested
>> that it is wise to always have a slight load on the supply.
>> If you have a couple old hard drives you are no longer using,
>> you might consider connecting them to the 1x4 Molex hard
>> drive connectors. That would provide a very small amount of
>> loading, about 5V @ 1A and 12V @ 0.6A or less. A couple of
>> drives would double that load, to 2A and 1.2A. Some supplies
>> actually have a minimum loading spec, printed on the label,
>> and if your "dummy load" is not meeting the minimum stated
>> value, then you'd look for another way to meet the
>> minimum load.
>>
>> I have a load box for testing power supplies, that I built
>> myself. I bought about $50+ of power resistors from the only
>> good electronics store in town. They also had Mini-Fit Jr.
>> crimp pins, so I could make a connector assembly. I included
>> a fan in my load box as well, to blow over the power
>> resistors. The box might draw about 100W or so, which is pretty
>> small as loads go. The load box has a switch, which connects
>> PS_ON# to COM.
>>
>> Using a box like that, I can test a new supply, before using it
>> on a new build. I can connect my multimeter and verify the
>> voltages under that 100W load. At least I can determine whether
>> the supply is way off, on one of the output rails, before
>> connecting an expensive motherboard.
>>
>> I've only ever replaced one fan on a supply, and the only reason
>> for doing it, was the supply was from an old Macintosh
>> computer. Like yours, the fan was noisy. Fortunately, the fan
>> had the standard three pin connector on the end, so removing
>> the old and inserting the new, was easy. The three pin header inside
>> the supply, had the standard keying scheme, but I also verified that
>> the wire color scheme on the old fan, the voltage etc, also matched,
>> before doing it.
>>
>> For PC supplies, I'd just buy a new one. But for that Mac, finding
>> something with the right power connector pinout, would have been
>> a hassle. Probably nothing but pulls would have been available.
>>
>> Some power supply specs can be found here. The oldest spec is for a
>> 20 pin supply which still has -5V. The second is for 20 pin supply,
>> where the -5V has been removed. The third, is for a 24 pin supply,
>> where the extra pins provide more current flow for some of the
>> rails. You can find the color and location of PS_ON# in these
>> docs.
>>
>> http://web.archive.org/web/200304240...factors.org/de
>> veloper/specs/atx/ATX_ATX12V_PS_1_1.pdf
>>
>> http://www.formfactors.org/developer...X12V_1_3dg.pdf
>>
>> http://www.formfactors.org/developer...2_public_br2.p
>> df
>>
>> Working inside a power supply, should only be attempted if you know
>> what all the potential risks are. In my case, since the old fan
>> could be unplugged, I didn't need to take the supply apart, and
>> expose any of the high voltage areas. Otherwise, I might have just
>> bolted the whole thing back together, and lived with the fan noise.
>>
>> Did I mention I don't really like high voltage ? Or getting shocks ?
>>
>> :-)
>>
>> The purpose of giving warnings, to stay out of power supplies, is
>> some day, we're going to read in the newspaper, where someone
>> was killed, because of advice given. And we definitely don't want
>> that. It is not like you'd be posting back, if you were killed.
>>
>> Another reason the replacement fan might not have worked, is the
>> PSU can use variable voltage to feed the fan. When the PSU is
>> cool, at startup, it might deliver 7V to the fan wires. As the
>> internal component temperature rises, the voltage is increased, to
>> make the fan spin faster. It could be, that the new fan and old fan
>> have a difference in the minimum starting voltage. For example,
>> the other day, I saw a fan spec, where 10.8V was listed as
>> the minimum starting voltage. There are some other 12V fans,
>> that will start with as little as 5V present.
>>
>> Fans also have different air moving capacities, measured in CFM.
>> Roughly speaking, fans comes in low, medium, high, and ultra
>> ratings. When replacing a fan, you want to try to match the
>> characteristics of the original fan.
>
> Useful information about pinouts, power supplies and building a load box
> too. Thanks Paul. I was unaware that PC fans were sensitive to polarity but
> on reflection it makes sense since I assume they use a brushless motor in
> order to avoid arching.
>
> To the OP: A Digital Multimeter will indicate true negative in it's display
> regardless of probe orientation so there's no guessing about wire color in
> relation to D.C. polarity.
>
> As an alternative to building the load box Paul describes you could use a
> PSU tester:
> http://www.newegg.com/Product/Produc...82E16899705003
> Such a tester doesn't provide provide a full load test but will provide a
> quick and convient test for functionality.
>

I provided some info here, to help someone build a load box. The hardest part
is finding the Mini-fit Jr. pins, and I was surprised my electronics store
had them. I don't think they carry them any more. Digikey and Mouser are
possible sources on the net.

http://groups.google.ca/group/alt.co...b?dmode=source

Good power resistors can be hard to find, and the best ones are typically
surplus. An example here. I've used this style before. But if you need
a handful of these for a project, the price adds up fast. I think I've even
got some of these from surplus sources (when I was a kid). There are other
form factors for about half the price.

http://www.newark.com/01F9750/passiv...H-25-5R000-1.0
http://www.newark.com/28K6272/passiv...TICOMP-MC14658

Paul