How to blow up a fan!

[scaryjim]

I'm confuzzled today.

Last night I was doing some updates to one of our work computers, and as the user had complained about how noisy the fans were I thought I'd have a look-see if there was any temperature headroom to slow the fans down a little. Well, it turned out that there probably was, so I plugged in a 7V mod cable to one of the fans and turned the PC back on.

This is where things began to go wrong. Speedfan suddenly registered temps of ~ 84C, so I figured that 7V had pulled the fan speed back too far and I'd better tell them to live with it (while I tried to talk the boss into a couple of Zalman fanmates ). So I popped the fan connector back on the motherboard header, stuck the lid back on and fired the computer up. Temps still in the mid-80s. Pop the lid off and the fan isn't moving at all. Bugger. Check the header with another fan: no problems. Check the fan on another mobo: no movement.

So, somehow, by running 7V through a 12V fan, I appear to have blown it. How is that possible? Was I actually just really unlucky and the fan was on its way out already? Has sitting on top of an 80C+ heatsink for 2 minutes caused some kind of heat damage to the fan? Is there some electro-magnetic problem that I haven't thought of that would cause this to happen? Please help me be less confused by this!

[Pob255]

By 7V mod cable do you mean one of the little resistor based jobbies or do you mean a molex mod?

[oolon]

I think it was not enough voltage for the fan to spin, the coils in the fan normally are not on constantly these then burnt out them out, so now it does not rotate.

[scaryjim]

By 7V mod cable do you mean one of the little resistor based jobbies or do you mean a molex mod?

Molex jobby (short the 12V to the 5V - glad I did A-Level physics or I'd never believe that would work ). Actually, I keep wondering if I could do something similar with the +12V and -12V rails of a PSU to make a 24V rail, but given how low the max current on -12V usually is I'm not sure that's a good idea

I think it was not enough voltage for the fan to spin, the coils in the fan normally are not on constantly these then burnt out them out, so now it does not rotate.

I wondered about that (something burning out because the fan had a higher starting voltage than 7V), but tbh I'm not entirely "up" on how brushless fans / motors work. The power draw of an undervolted fan should be minimal, but I guess it could be enough to burn the fan if it wasn't turning...

[oolon]

Shorting the 12V to the 5V rail.. On paper that should work not sure if a switched mode PSU would object though. Its not like some coil with multiple take offs, in a switch mode psu there are lots of components that do not work backwards. If the fan drew enough current to reverse the electron flow on the rail it could be quite interesting. If you had two PSU one which did 5V and one which did 12V and you connected the grounds together think that would be safe? not sure I would, so thinking it would be ok, if in the same PSU when we don't know how the 5V is generated is a problem.

[scaryjim]

Shorting the 12V to the 5V rail.. On paper that should work ...

It's a fairly common method of doing this, although I guess it would be better if the 5V line was connected back to earth via a load of some description to complete the circuit. Perhaps I just need to make sure I plug the pass-through cable into a CD drive next time

EDIT: thinking about it, as long as the 5V is a single rail and *something* is pulling a 5V load to earth it will be fine - it's just like using a voltage divider. You'd only have problems if the 5V rail was completely floating...

EDIT2: thinking about it more, I can see how it might be an issue if the "grounds" for the 5V and 12V lines were completely separated: but I'm struggling to think of a circumstance where that could happen given that a PSU only has one genuine "ground" (from the ring main) to return to....

[Pob255]

No psu's use a single common ground.
however it could be a shonky psu, any decent psu can cope with the 7v mod trick (my front fan is currently happly running at 7v via this simple mod)

You didn't do something silly like get the pins mixed up and rewire the 5v to the 12v and the 12v to earth? That'll up a fan
Most fans cut off point is around 5v, which is why most fan controllers don't go below 6v (ok many can do off but they almost always have 6v as the lowest voltage point)

One thing to remember when doing the 7v mod is if the adaptor has an extra fan plug so you can plug it in to still monitor the speed, then DON'T because this lead will still have an earth, which is now a +5v.
I speak from experience here, what will happen it that the wire going to speed monitor plug will heat up, to the point that the insulation will melt and catch light while the wire glows orange hot.
If you're as lucky as I was it'll just kill the fan header (two resistors on the motherboard burnt out) and I dread to think of the damage I could of done.

[scaryjim]

I bought the adapter since it was going in a work machine (£3 from Maplin, I know I could make them for pence but for the ease I was happy to shell out ). I'm starting to think it was just a fan already on its way out, as one of the other fans in that case has started making a very odd sound today. Looks like I'll be putting in a bigger component order than I planned!

Although I'm also willing to accept that I'm cursed at the minute, given what happened tonight when I got home (new thread about to be started for that little mishap ). I'm not touching Project CP for at least a week