[sdiy] The life and death of a Phonic power conditioner

Richie Burnett rburnett at richieburnett.co.uk
Sat Jan 28 13:04:25 CET 2017


> However... I thought, let's check if the recto before it is working 
> correctly. I thought I'd be able to see this with my scope on the 
> waveshape easily, and connected the scope's ground terminal to the 
> circuits local ground (the negative side of C4) and boom! Big explosion! 
> Fried part of my scope's ground clamp...

Capacitor dropper power supplies are dangerous to work on.  Always use a 
mains isolation transformer when doing this type of repair work.  Never be 
tempted to float the ground connection to the scope, that's how fatalities 
happen.

> Also... I was expecting this circuit to behave such that the local ground 
> would adapt to any other ground applied to it.

Yep, don't clip scope probe grounds onto anything that might be mains live. 
If in doubt probe it with a DVM first.

> And finally.... why would a big (film, not elco!) cap get fried way below 
> it's operating voltage of 400V...

You most likely exceeded it's peak current rating when you shorted one side 
of it to ground, resulting in vaporisation of the thin metal foil, or 
failure of a weak point where the ends of the foil plates are connected to 
the component lead.

> Or is that a DC voltage rating?

Will be the max DC voltage rating.

> And finally, why is my flu still not gone! (perhaps that explains it 
> all...wait before repairs until you're recovered?)

I think lots of people who've been ill over Christmas/New year are still 
puzzling over that one.  (Seriously though, work on something low-voltage, 
or write some code, but don't probe around live mains powered equipment when 
you're not at your most alert!  Fortunately you didn't get bitten, or get 
anything in your eye from the unexpected component explosion.  Damaged 
scopes and power bars can be replaced.)

-Richie, 




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