20 Jul
2006
20 Jul
'06
4:45 p.m.
O.K. - now on to the power supply.
The +5v channel 1 supply is good, 69 mV peak to peak AC noise (measured on a scope with AC
coupling)
the +-12v channels are ~292 mV of noise
the -5v channel has 500 mV of noise, even with an additional 1000 ?F capacitor across the
output (to hopefully filter this junk out)
(-5V also has periodic fluctuations at a lower frequency)
All of this is high-frequency, excluding the -5v hum.
In short, it doesn't look like adding an external filter cap will help much, unless I
go all the way and throw in a LC filter.
Pulled the P/S apart, and the board with +-12V and -5V has peculiar capacitors mounted on
it - 3 lead Mallory Aerovox.
Are these really the multipart electrolytics that went out of style (I thought) with solid
state?
The wiring is 2 leads connected to one side of the power rail, one lead to the other.
Could twin-lead caps be substituted?
20 Jul
20 Jul
5:02 p.m.
O.K. - now on to the power supply.
The +5v channel 1 supply is good, 69 mV peak to peak AC noise (measured o=
n a scope with AC coupling)
the +-12v channels are ~292 mV of noise
the -5v channel has 500 mV of noise, even with an additional 1000 =B5F ca=
pacitor across the output (to hopefully filter this junk out)
(-5V also has periodic fluctuations at a lower frequency)
All of this is high-frequency, excluding the -5v hum.=20
In short, it doesn't look like adding an external filter cap will help mu=
ch, unless I go all the way and throw in a LC filter.
My first through is that capacitors ahve inducatnce too, and that your
added one might well have significant impedance at the noise frequency as
a result
Pulled the P/S apart, and the board with +-12V and -5V
has peculiar capac=
itors mounted on it - 3 lead Mallory Aerovox.
Are these really the multipart electrolytics that went out of style (I th=
ought) with solid state?
My guess is that the 3 leads (2 on te +ve side?) are there to reduce the
self-inductace, e.g. by connecting to both ends of the rolled-up foil
inside. I think I've seen similar caps in HP stuff.
The wiring is 2 leads connected to one side of the
power rail, one lead t=
o the other. Could twin-lead caps be substituted?
Only if the self-inductance is low enough. Capacitors designed for use in
SMPSUs should be OK, maybe parallel them with a 1uF non-electrolytic for
good measure
-tony
5:34 p.m.
the -5v channel has 500 mV of noise, even with an
additional 1000 ?F
capacitor across the output (to hopefully filter this junk out)
All of this is high-frequency, excluding the -5v hum.
In short, it doesn't look [as though] adding an external filter cap
will help much, unless I go all the way and throw in a LC filter.
If the noise is really high frequency, that 1000?F cap may be too
large. Large-value caps tend to have a comparatively high effective
series inductance. If you want HF noise filtering, you should not only
use a large cap like that but also small caps, like .01-.1 ?F, in a
technology like cermaic disc that doesn't have high effective series
inductance. Preferably, scatter the filtering around through the
circuit, to soak up noise near its source.
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6:02 p.m.
On Thursday 20 July 2006 07:45 pm, Scott Quinn wrote:
O.K. - now on to the power supply.
The +5v channel 1 supply is good, 69 mV peak to peak AC noise (measured on
a scope with AC coupling) the +-12v channels are ~292 mV of noise
the -5v channel has 500 mV of noise, even with an additional 1000 ?F
capacitor across the output (to hopefully filter this junk out) (-5V also
has periodic fluctuations at a lower frequency)
All of this is high-frequency, excluding the -5v hum.
A 1000uF cap isn't going to help you at all with high frequency noise, at
which point it probably looks more like an inductor than a capacitor. When I
want a really quiet power supply I generally put a 1 - 10 uF tantalum cap
across the big electrolytics, _and_ a smaller ceramic disc cap of somewhere
say up to 100nF but no higher, which tends to give you better coverage.
In short, it doesn't look like adding an external
filter cap will help
much, unless I go all the way and throw in a LC filter.
If what I suggest in the above paragraph doesn't help then maybe a pi-filter
is perhaps the next logical step.
Pulled the P/S apart, and the board with +-12V and -5V
has peculiar
capacitors mounted on it - 3 lead Mallory Aerovox. Are these really the
multipart electrolytics that went out of style (I thought) with solid state?
Probably not.
The wiring is 2 leads connected to one side of the
power rail, one lead to
the other. Could twin-lead caps be substituted?
Maybe.
I haven't seen these but I'd guess that they're something special with that
configuration. I remember years back (1974!) Zenith used a special 4-lead
cap in their TVs on the output side of the flyback transformer. And some cap
maker messed up, changed what the materials were or something, and ended up
hurting bigtime because they failed left and right and you had CRTs getting
ruptured from 40KV+ voltages and similar nonsense. I heard that Zenith
covered all of that and that they got it all covered by the cap maker...
Have you done any searching for those parts?
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