Re: Capacitor value for C904 in various 22xx oscilloscopes

Tom Jobe <tomjobe@...>

Thank you Harvey and Ed for your informative replies!
I failed to mention that this C904 capacitor is immediately after the
power switch and immediately before the input to the full wave bridge,
so the prongs of the AC cord should not see the voltage stored in the
capacitor after you turn the scope off.
It was interesting to hear about the subtle things that are upgraded in
a military version of a product like this!
I studied the power supply schematics for scopes that used both values
of C904 capacitors, and they did not add a bleed resistor or change any
other components in the general area of C904 that I could see.
Just for the record, the later 2235A and 2236A both had the value of
C904 raised to .068 uF for C904 (from 2200 pF) as did the military 2235
, the 2232 and other later versions of the smaller 22XX scope series.
Thanks again!
tom jobe...

On 10/27/2017 6:58 PM, [TekScopes] wrote:

As long as it's an X-cap - line-to-line, you can go way bigger, which
helps with EMI. But, as X-caps get bigger, you have to assure a
discharge path to avoid a shock hazard that can occur if the line cord
plug becomes unplugged, which can leave a charge on the cap exposed on
the prongs. If the filter output goes to a transformer winding, for
example, it's no problem, but in front of anything that may
effectively stop drawing current, you need a definite discharge path
to reasonably quickly dump the X-cap's charge. Usually an appropriate
resistor will do.

Let's say your load is an off-line SMPS, and you pull the plug at such
a time that the peak line voltage is left on the cap. The rectifiers
and filter caps behind it will hold up for quite some time, and the
X-cap will eventually discharge through the rectifiers as the load
uses up the stored energy. In the meantime, the prongs of the plug are
hot with the charge on it. It's not too hazardous with a
reasonably-sized cap, but can give a shock, so best to dump it quickly.

Ahh - seems like all the good action in the Series happens right when
I'm yakking here - back to the game.


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