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



"Mr Macaw" wrote in message
news
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.

What type of PSU?

If its multi rail, probably only one rail is sampled to control PWM. If
you
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).

Its rare - but some PSUs won't work with too little load.

Try adding a 4W dashboard bulb to the terminals. If nothing
else...........you can see if the output collapses.