24 Dec
2011
24 Dec
'11
1:48 a.m.
What you need is that the impedance of the capacitor
at the mains
frequency is small compared ot the input resistnace of the meter.
Well, at the
putative ripple frequency. [...] 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. 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. A 1uF capacitor should be fine for most cases.
Actually, if you don't have a 'scope, you can get _some_ idea of the
ripple spectrum by trying multiple different cap values, with known
resistors in parallel across your meter if it's particularly high
impedance (like the >10M tony mentioned). You likely will have to
do some arithmetic, though. But if you are serious enough about electroncis 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.
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