On Wed, 18 May 2016 18:44:43 +0100, Ian Field wrote:



"Mr Macaw" wrote in message news
On Tue, 17 May 2016 18:39:44 +0100, Ian Field
wrote:



"Mr Macaw" wrote in message
news On Mon, 16 May 2016 23:19:41 +0100, Wayne Chirnside
wrote:

On Wed, 17 Feb 2016 15:54:29 +0000, Mr Macaw wrote:

On Tue, 16 Feb 2016 23:50:38 -0000, M Philbrook
wrote:

In article , says...

I have a pile of power supplies which used to power CCTV cameras.
They're rated at 12V 1A. They're very light and give out precisely
12V with no load, so they must be regulated switched mode. So why
is
it when I try to power a 0.15A 12V fan (a 120mm Corsair computer
fan),
they fail very quickly? The first one started whining and gave out
only 0.5 volts after only half an hour, and the second one went pop
after half an hour. I've had two of the others powering door
entry RFID coils and the door solenoids and they've been happy for a
few years.

Most likely bad caps , that is most common failure mode for them.

But for two of them? When another two (of the same age from the came
camera set) have worked for a couple of years powering door locks?

I've opened them up, this is what they look like. I can see the power
transistor in the top one (the one that whistled and produced bugger
all
voltage) has been warm enough to discolour and crack the yellow wax
stuff (ringed in green), but the caps look fine. In the one that went
pop, a fuse has exploded (ringed in red).
https://www.dropbox.com/s/q1sc56tx0vtmzuv/PSUs.jpg?dl=0

Back EMF from the fan?

Which should be less than the 12V coming from the PSU, right?

Back emf can be 5 - 8x the applied voltage - its the whole basis of
flyback
EHT in CRT TVs. When designing solid state ign coil drivers for cars; you
have to design for around 200V peak on the LT winding.

Surely that's only on disconnecting the power to the motor?

Back emf happens when you interrupt the current through an inductor. In a
BLDC motor; there's circuitry between the coils and the leads that emerge -
if the circuitry didn't handle the back emf; it wouldn't last long.

Maybe the PSU was really **** then. But I've used 2 of the same model to power a solenoid for years, and they haven't complained.

The fan has a BLDC motor, so any back emf is contained by the internal
circuitry - although PC servicing sites advise against blowing dust and
fluff out of fans with compressed air. The magnetic rotor spinning round
the
coils will develop voltage, but I'd expect any damage to be confined to
the
fan itself.

I blow sharply with my mouth to clear dust (with the PC on), often
accelerating the fans, it's never broken one.

Not quite the same as blasting the fan with a 100 psi air nozzle is it.

That's a narrow jet, I doubt it would speed the fan up any more than blowing.

--
"It is generally inadvisable to eject directly over the area you just bombed." - U.S. Air Force Pilot training manual