[vcf-midatlantic] Safety capacitors, was Re: The RIFA Cap Says: PFFFFFT!

[Systems Glitch]

Dave,

Thanks for the notes! I'll see if I can find any information on how they're supposed to fail. It looks like replacement X2 rated components from Mouser are mostly polyester and dipped instead of potted, so I suspect they're less likely to absorb moisture and short out.

Thanks,
Jonathan

On Mon, 14 Dec 2015 13:26:14 -0500
Dave McGuire via vcf-midatlantic <vcf-midatlantic at lists.vintagecomputerfederation.org> wrote:

> On 12/14/2015 01:07 PM, Systems Glitch via vcf-midatlantic wrote:
> > Just had one blow in an old ECL monitor, releasing tons of stinky
> > capacitor smoke. Had to take the monitor outside to let it finish
> > smoking.
> 
>   Yuck.
> 
> > I know these are "X2 safety rated" devices, but what does that really
> > mean? "Keep two handy cos they fail a lot?" Seems to me that not
> > blowing up would be a great safety feature, something you wouldn't
> > relegate to cheap rice paper in epoxy.
> 
>   These are called "safety capacitors".  The important part is that
> they're designed to fail only in very specific failure modes.  An X2
> capacitor is designed to be placed across the line ("X") and is impulse
> rated to 2.5kV in most cases.  Here is my entire set of notes on safety
> capacitor types and ratings:
> 
> --------------------------------------------------------
> Type X: across the line (X1, X2 and X3)
> Type Y: line to ground (Y1, Y2, Y3 and Y4)
> 
>      impulse test V   peak service V
> ------------------------------------
> X1:     4.0kV[1]      2.5kV - 4.0kV
> X2:     2.5kV[2]      <= 2.5kV
> X3:     -             <= 1.2kV
> 
> Notes:
> [1]: If C<=1.0uF.  For C>1.0uF, V(kV)=4/sqrt(C).
> [2]: If C<=1.0uF.  For C>1.0uF, V(kV)=2.5/sqrt(C).
> 
> 
>      impulse test V   rated V
> ----------------------------------
> Y1:     8.0kV          <= 500V
> Y2:     5.0kV          150V - 300V
> Y3:     -              <= 250V
> Y4:     2.5kV          <= 150V
> --------------------------------------------------------
> 
>                 -Dave
> 
> -- 
> Dave McGuire, AK4HZ
> New Kensington, PA