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Old May 19th 16, 08:51 PM posted to alt.electronics,alt.binaries.chatter
Mr Macaw Mr Macaw is offline
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Default Flimsy power supply won't drive a little fan!

On Thu, 19 May 2016 20:07:49 +0100, Ian Field wrote:

"Mr Macaw" wrote in message news
On Thu, 19 May 2016 18:15:23 +0100, Ian Field

"Mr Macaw" wrote in message
news On Wed, 18 May 2016 18:44:43 +0100, Ian Field

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

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

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

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

In article , says...

I have a pile of power supplies which used to power CCTV
They're rated at 12V 1A. They're very light and give out
12V with no load, so they must be regulated switched mode. So
it when I try to power a 0.15A 12V fan (a 120mm Corsair computer
they fail very quickly? The first one started whining and gave
only 0.5 volts after only half an hour, and the second one went
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
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
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
transistor in the top one (the one that whistled and produced
voltage) has been warm enough to discolour and crack the yellow
stuff (ringed in green), but the caps look fine. In the one that
pop, a fuse has exploded (ringed in red).

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
EHT in CRT TVs. When designing solid state ign coil drivers for cars;
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
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
power a solenoid for years, and they haven't complained.

What type of PSU?

If its multi rail, probably only one rail is sampled to control PWM. If
don't draw current from that rail; all the others collapse.

Single 12V rail. PSUs that came with a set of CCTV cameras (one PSU per 4
cameras I think).

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

I've spun fans as fast as an angle grinder disk with the jet from a
air nozzle.

Don't you tend to just do a momentary squirt to remove the dust?

The fan blades chopped the jet of air and made a noise like an air raid

But the fan doesn't speed up all that much.

A bleached blonde and a natural blonde were on top of the Empire State Building.
How do you tell them apart?
The bleached blonde would never throw bread to the helicopters.