30 May
2008
30 May
'08
7:07 a.m.
Tony Duell wrote:
And how exactly are you measuring that? Remember a voltmeter (or in this
case a 'scope) has 2 leads, and measures the difference in votlage
between them, What are you conencing the ground side of the 'scope to?
My guess is the system 0V rail. But the chopper circuit is directly
connected to the maisn, its 'common' is the -ve side of the mains
smoothing capacitors. But of course if you connect the ground lead of the
'scope there, you'll flow fuses if you're lucky, nad do a lot of damage
if you're not, becaue the 'scope is earthed, as is one side of the
incoming mains (effectively). You can't probe around the primary side of
a SMPSU unless either you have a 'scope with differential inputs and
enough common-mode rejection to handle full mains voltage or you run the
PSU off an isolating transformer.
What frequency is this pulsing DC that you're seeing? If it's really the
chopper waveform, it'll be 10's of kHz. My guess is it's nearer 50Hz, and
whay you're seeing is effecively rectified mains due to the fact you've
got the 'scope common connected to ground.
-tony
Hang on a second...
Why do yuo think the transformer is the problem?
This sounds like a switch-mode PSU. And while transformers
can and do fail in those, it's not at all easy to diagnose
the fact. They normally have windings witha very low DC
resistance and will test as a dead short on any nornaml multimeter.
OK, the reason I suspect a transformer; there's power getting as far as the
primary windings of the transformer (can be seen on an oscilloscope); and it
seems to conform to the "chopped" DC that I now know a switcher expects.