24 Dec
2011
24 Dec
'11
11:35 a.m.
On 12/24/2011 01:40 PM, Tony Duell wrote:
So would I - hence the "broken" possibility.
But I also know I don't If an SMPSU ithat should be running at several
10's of kHz is actually
running aobut 1000 times slower, you would know about it. By the chopper
transisotrs hittlign the ceiling for a start. The transformer would
certianly saturate, the drive currents would go sky-high.
If it's lower than the mains it's running in shutdown mode where it
tries to start
and then shuts down. As a result it sounds like a very slow click.
The likely reason for that is NO LOAD, most older switchers have to be
loaded
to about 10% of total load to start and run correctly. The other fault
could be
overload where the load is a short of other fault. If those things are
ok then
the issue is likely SMPSU internall issue with startup, regulation or
other failure.
FYIL DEC H780 SMPSU and those of that ear had the most annoying mid audio
range (6-12khz) scream. They didn't switch at much more than 10khz.
I think it was something that, like a boat or airplane,
is not part of
the large-scale power grid but which does have enough of a power
distribution system for it to be fair to call it "mains". Aircraft
supplies are often 400Hz since that leads to smaller/ligher
power trnasformers running direectly off the supply, as was done before
the common use of SMPSUs.
Not always, many are 14V or 28V DC and run linear or even switch mode
power supplies.
The older tube radios used vibrators and dynamotors for the nigh
voltages neeeded.
Most large aircraft have DC buses as well as 400hz (where present).
Allison
Sure. What I would do in
my workshop (where I have decent test gear) is
differnet to what I'd do at a remote site (given only the instrueltns I'd
brought with me). Whether I would build an active filte would also depend
on waht components I had available (which depends on whether there would
be time to do an order to Farnell, say, whether there's a local
electroncis shop (or company stores) that at least has common R's and C's
and op-amps, or whether I have to manage with what I've got or could
'borrow' fro mother parts of the device).
However, I think the OP was planning on doing this repair at home, and
that time weas ntot too critical. In which case I think my comment
applies that while you can produce workarounds (and while it's certainly
good to understnad enough to think up said methods), a 'scope would
probalhy be a sensible thing to obtain if you anre going to take
electronics that seriously.
-tony
SMPSU [...]
the switching frequency (and if that's not well above
mains frequency you've got a broken, or at least seriously weird,
SMPSU...
I would be _very_ suprised if any SMPU was deisgned to switch at a
frequency lower than mains. know
everything, and there just might be some SMPSU out there that
switches at a frequency lower than mains for some bizarre reason.
OK, you might be
right. It's _possible_ there's one (I certainly don't
know everythign, and it's entirely possible somebody di do this for a
special reason that none of us have thought of).
However, 1 uF capcitror has reactance of about 160k Ohms at 1Hz
(Xc = 1/(2*\pi*f*C). Use that as a blocking capacitor for a 10M Ohm
input impedance DVM, and you would still detect the ripple at close ot
the correct level. and if the ripple freqeuncy isd slower than 1Hz, you
could see the needle flickering on an analogue voltmeter ;-).
(Maybe it does FM instead of PWM to control the
output voltage, and
under low-to-zero load the switching frequency ratchets way back?)
I don;t think
any supply would get a slow as that. And in any case, we've
put a dummy load on it, right?
or something unusual for mains power; ISTR something
somewhere that
had a mains power frequency in the kHz, or at least hundreds of
Hz....)
IIIRC, power line losses increas with frequency, so this would be
unusual to say the least. But if you are serious enough about electronics to
want to do
soemthing like that, you are also serious enough to want to get a
'scope I think.
In general, I would agree. The scenario I had in mind was
less the
"let's build the tools I'm going to use routinely" one and more the
"okay, I'm stuck here with minimal equipment and parts, what can I
improvise to find out the information I want?" one.