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.
I assume I should install these? I take it they provide some kind of filtering, but I'm not clear.
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
Safety Capacitors First: Class-X and Class-Y Capacitors - Technical Articles
The value is not so critical, often up to 50nF has been used in some equipment.
Whatever value they recommend will certainly be ok.
Whatever value they recommend will certainly be ok.
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.🙂
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.🙂
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?
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.
Class Y is only for between the AC line and ground.
Both can reduce noise, the X more for switching noise.
Last edited:
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...
It is important to remember what the cap does. It reflects the noise but does not dissipate it.🙂
Can I use a Y for line-to-line? I read that Y can be safely used for either but X can't be used to ground.
Some safety capacitors are rated for both classes (under differing conditions), but I would use
each single-rated type only for its intended purpose. There are also sub-classes under each type.
Yes, a true snubber requires both a resistor and a capacitor. The resistor dissipates the energy.
each single-rated type only for its intended purpose. There are also sub-classes under each type.
Yes, a true snubber requires both a resistor and a capacitor. The resistor dissipates the energy.
Last edited:
I used a 0.1 uF/400V and a 470E resistor in series as an arc quencher in parallel with a 275/20 MOV.
Worked.
Worked.
Y types have more stringent specs and can be used for X duties, but regular Y's for consumer applications rarely go beyond 4.7nF, maybe 10nF but not more because of the leakage current limit (special types for professional applications like CISPR networks have no limit, but they are not widely available)
I searched for a value of 3300pF because that is what was specified on the First Watt general power supply. The only radial-legged (non-SMD) safety capacitors on Mouser with this value were Y-type (Y5R/U/V). I take it there's no reason to go out of my way to find an X-type?
> I take it there's no reason to go out of my way to find an X-type?
No, as long as you stick with safety caps, X or Y doesn't matter for X duties.
For that reason, X-only types <4.7nF are rarely available for inventory rationalization reasons.
The tiny cost gain for a low-value X cap is not worth the trouble of creating another category
No, as long as you stick with safety caps, X or Y doesn't matter for X duties.
For that reason, X-only types <4.7nF are rarely available for inventory rationalization reasons.
The tiny cost gain for a low-value X cap is not worth the trouble of creating another category
Reminds me of a story about a UK vacuum cleaner company who designed a new model.
Made thousands of them and they all went out.
Next thing they know lots of people complaining about getting shocks from the plugs !
They had a cap across live and neutral which depending on where mains cycle was at turn off could be charged to up to mains voltage !
I think they fixed it with 1 meg across the cap.
Made thousands of them and they all went out.
Next thing they know lots of people complaining about getting shocks from the plugs !
They had a cap across live and neutral which depending on where mains cycle was at turn off could be charged to up to mains voltage !
I think they fixed it with 1 meg across the cap.
You mean like the dirty trick of plugging a 3300pF/3KV cap directly into the wall (being very careful where you hold it), and then tossing it to someone unsuspecting? Here, catch!
