Re: Capacitor value for C904 in various 22xx oscilloscopes

Tom Jobe <tomjobe@...>

Thanks Ed,
That's an angle I never thought of... when the power switch is still
turned on and then the electrical cord gets pulled from the wall... Ouch!
Whatever there is to bleed off the charge, it is common to all of the
versions of these small 22xx's that I'm talking about, whether they
originally came with 2200 pF or the .068 uF X2 capacitors
There is a schematic on the IEC inlet filters (the problematic
'Schaffner' brand unit I took out and the new 'Qualtek' (?) brand
replacement I got from Digikey) and neither show a bleed resistor to ground.
The X capacitors in the IEC inlet filters are are 0.1 uF and 0.047 uF
I appreciate all of these 'real world' things we can learn on Tekscopes
from people who actually know something!
tom jobe...

On 10/28/2017 11:47 AM, [TekScopes] wrote:

Tom, I wasn't talking about turning off the unit, then pulling the
plug - it's about the worst-case condition that can happen from
pulling it at any arbitrary time, running or not.

In your 2200 series scopes you will likely find that there is a
discharge resistor for the X-cap - it's often included inside of
component-type line filters. It may or may not show up on the scope
schematics, but sometimes the line filter's equivalent circuit is
printed right on it. You can easily measure the built-in R (maybe
around 47K to 1 meg) across the line input pins - after it's been
unplugged for a while to be sure it's fully discharged. If the line
filter doesn't have an R, then it's internal X-cap must be fairly
small, so be aware that if you add lots of C, it's best to put a
discharge R in too.

I believe that this sort of thing is (or should be) addressed in
various safety standards like from UL or TUV. There should be
something about energy storage limits and discharge rates, etc.


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