More silent pc without buying new fans.

[peterb]

Just to clear up the mess, as you can then just plug the fan cable stright into the motherboard (via the resistance adaptor which is only an inch or tow long)
So not need to run molex cables about just to power the fan.

Well I ment to give the same effect as adding resistance.
I've never been too happy with the 7v mod because you are feeding the +12v back down the 5v like the 5v was an earth, if you've got an El-cheapo PSU I dread to think of what would happen.
Oh and it can generate heat, quite a bit.
I personally found out something you should NEVER do with 7v mod, that's if the fan adaptor also has a speed monitor cable as well then do NOT plug that into the motherboard.
What happens is the normally earth wire of the monitor cable gets red hot (ie glowing red) but that's after the plastic coating has melted and caught fire.
And it burns out the fan header it was plugged into on the motherboard.
All in under 30 seconds.

No, it isn't the same effect as adding resistance, and you aren't feeding 12 volts down the 5 volt line!

Adding resistance only adds a voltage drop if you are drawing current - so you need to know the running resistance of the fan. The resistance will dissipate the lost energy as heat. A perfectly acceptable way to do it - but be careful where you place the resistor.

No look at using the split rail method.

Lets say the fan draws 120mA at 12 volts. We are looking to loose 5 volts at 100mA, so the fan's dynamic resistance is 100 ohms. We need another series resistance to reduce the current to (using ohm's law) 7/100 ohms or 70mA. The resistor will be 5/.07 or about 70 ohms, and will dissipate 25/70 = .35Watts. My figures are example, the figures for an individual fan will vary. Resistors come in standard values - in this case the nearest values would be 68 ohms or 75 ohms. http://www.logwell.com/tech/componen...or_values.html

The 5 and 12 volt rails are reference to a common earth or ground rail - so it is a potential difference that exists between them.. The only possibly down side is that it might marginally affect the regulation of the 5V rail.

Consider (as an example) a power supply with a common earth and a plus 12 V rail nand minus 12 V rail. The voltage between the two will be 24 volts - and you could power 24 V equipment from it without using the earth point at all. This is exactly the same case - except both rails are 'above' ground potential so the potential difference is only 7V - the difference bertween the two.

I cannot comment on the problem you experienced with the burnt out cable. The only occasion I have seen that happen is with a cheap PSU and a mis-connected floppy header - so perhaps in your case the pug had been displaced or there was a wiring fault in the adaptor. However without looking at the cct diagram of the components and their physical configuration, it would be impossible for me to make an accurate diagnosis.

[peterb]

Haveing thought a little more about this, there is one important proviso, and that is that the current load on the 5V rail is equal to or greater than the current taken by the fan. Under normal conditions (PSU plugged into the mobo) that will be satisfied, but if that is not the case, the regulators on the 5V line will be reverse biased, and there is a very small but finite possibility of causing damage. A good quality, well designed PSU should be fine, and a PSU should not be started unloaded anyway (many won't) but it is something to be aware of.

Under normal operating conditions though, connecting a fan across the 12 and 5 V rails will not cause problems.