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Filter capacitor across AC plug?
Filter capacitor across AC plug?
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Old 26th September 2021, 09:59 PM   #1
SoaDMTGguy is offline SoaDMTGguy  United States
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Default Filter capacitor across AC plug?

Looking at the First Watt power supply schematics, they feature a 0.0033uF (I think those are the right units? It's unlabeled) capacitor across the AC input wires. The BA power supply has one before the switch and another after, while the general power supply only has one after the switch.

I assume I should install these? I take it they provide some kind of filtering, but I'm not clear.
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Old 26th September 2021, 10:15 PM   #2
rayma is offline rayma  United States
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Noise filters, and if used they must be an X rated safety capacitor. No, not that kind of X rating.

Safety Capacitors First: Class-X and Class-Y Capacitors - Technical Articles
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Old 26th September 2021, 10:25 PM   #3
SoaDMTGguy is offline SoaDMTGguy  United States
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Thank you, that's great info. Was I right about the capacitance?
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Old 26th September 2021, 11:25 PM   #4
rayma is offline rayma  United States
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The value is not so critical, often up to 50nF has been used in some equipment.
Whatever value they recommend will certainly be ok.
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Old 26th September 2021, 11:48 PM   #5
jhstewart9 is offline jhstewart9  Canada
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Default Mallory Disc Ceramic Caps, UL Recognized

For across the line applications as follows-
UN102M 1000 pF
UN472M 4700 pF
UN103M 0.1 microF


I've used the UN103M in some projects. And in fluorescent lighting fixtures to reduce conducted emissions.
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Old 26th September 2021, 11:57 PM   #6
SoaDMTGguy is offline SoaDMTGguy  United States
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Oh cool, thanks for that list! What is the effect/tradeoff of the different values?
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Old 27th September 2021, 02:53 PM   #7
WntrMute2 is offline WntrMute2  United States
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I'm curious as to which capacitor X or Y is better for filtering and reducing the pop that sometimes occurs when switching some equipment?
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Old 27th September 2021, 03:06 PM   #8
rayma is offline rayma  United States
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Class X is only for across the AC line.
Class Y is only for between the AC line and ground.
Both can reduce noise, the X more for switching noise.

Last edited by rayma; 27th September 2021 at 03:09 PM.
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Old 27th September 2021, 03:43 PM   #9
jjasniew is offline jjasniew  United States
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Filter capacitor across AC plug?
Quote:
Originally Posted by WntrMute2 View Post
I'm curious as to which capacitor X or Y is better for filtering and reducing the pop that sometimes occurs when switching some equipment?
I assume that's switching on. Depends on what's making the pop - I assume a current surge disturbing the AC line, or creating an EMI pulse that getting into other stuff.

According to a former colleague, putting a cap across the AC line reduces the impedance. Therefore the current surge will be higher. However, if you can contain it within the offending equipment - instead of imparting it onto that equipment's AC cord (and everywhere else) that just might work.

Try putting the caps inside the box of the offending piece. I'd start with X...
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Old 27th September 2021, 04:07 PM   #10
jhstewart9 is offline jhstewart9  Canada
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It is important to remember what the cap does. It reflects the noise but does not dissipate it.
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