capacitor on series with transformer primary?
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From: Michael on 27 Apr 2010 15:42 On Apr 27, 12:19 pm, whit3rd <whit...(a)gmail.com> wrote: > On Apr 27, 8:55 am, Michael <mrdarr...(a)gmail.com> wrote: > > > I was taking apart a broken rechargeable razor, and found a 0.15 uF, > > 250V capacitor in series with the primary of the (tiny) transformer. > > The tiny <1 cu. in capacitor was to charge a NiCd battery. > > > What is the function of this capacitor? > > It's a ballast (current-limiting) component; probably > the transformer will burn up if you short across it. > The capacitor doesn't heat up like a resistor, and is > cheaper than a series inductor. > > It's a common trick, seen in lots of plug-in-the-wall emergency > flashlights. Nice! Limiting current without dissipating heat. I'm guessing capacitor polarity is not important, since it's AC? (It wouldn't be an electrolytic, would it?) I'll desolder that puppy too for future experiments. Thanks, Michael
From: Tim Wescott on 27 Apr 2010 16:07 Michael wrote: > On Apr 27, 12:19 pm, whit3rd <whit...(a)gmail.com> wrote: >> On Apr 27, 8:55 am, Michael <mrdarr...(a)gmail.com> wrote: >> >>> I was taking apart a broken rechargeable razor, and found a 0.15 uF, >>> 250V capacitor in series with the primary of the (tiny) transformer. >>> The tiny <1 cu. in capacitor was to charge a NiCd battery. >>> What is the function of this capacitor? >> It's a ballast (current-limiting) component; probably >> the transformer will burn up if you short across it. >> The capacitor doesn't heat up like a resistor, and is >> cheaper than a series inductor. >> >> It's a common trick, seen in lots of plug-in-the-wall emergency >> flashlights. > > > Nice! Limiting current without dissipating heat. > > I'm guessing capacitor polarity is not important, since it's AC? (It > wouldn't be an electrolytic, would it?) > > I'll desolder that puppy too for future experiments. It will _not_ be a 'lytic! Those things are generally poly-something, with generous over-rating to take transients into account. IIRC the dielectric is somewhat self-healing, too, but that memory is coming out of a pretty dusty storeroom, so it may not be accurate. -- Tim Wescott Control system and signal processing consulting www.wescottdesign.com
From: Michael on 27 Apr 2010 16:30 On Apr 27, 1:07 pm, Tim Wescott <t...(a)seemywebsite.now> wrote: > Michael wrote: > > On Apr 27, 12:19 pm, whit3rd <whit...(a)gmail.com> wrote: > >> On Apr 27, 8:55 am, Michael <mrdarr...(a)gmail.com> wrote: > > >>> I was taking apart a broken rechargeable razor, and found a 0.15 uF, > >>> 250V capacitor in series with the primary of the (tiny) transformer. > >>> The tiny <1 cu. in capacitor was to charge a NiCd battery. > >>> What is the function of this capacitor? > >> It's a ballast (current-limiting) component; probably > >> the transformer will burn up if you short across it. > >> The capacitor doesn't heat up like a resistor, and is > >> cheaper than a series inductor. > > >> It's a common trick, seen in lots of plug-in-the-wall emergency > >> flashlights. > > > Nice! Limiting current without dissipating heat. > > > I'm guessing capacitor polarity is not important, since it's AC? (It > > wouldn't be an electrolytic, would it?) > > > I'll desolder that puppy too for future experiments. > > It will _not_ be a 'lytic! > > Those things are generally poly-something, with generous over-rating to > take transients into account. IIRC the dielectric is somewhat > self-healing, too, but that memory is coming out of a pretty dusty > storeroom, so it may not be accurate. > > -- > Tim Wescott > Control system and signal processing consultingwww.wescottdesign.com Oh good... no kablooeys to worry about. Thanks! Michael
From: Phil Allison on 27 Apr 2010 23:09
"Tim Wescott" > > Only very vaguely. That cap may also be there for galvanic isolation from > the mains. ** Nonsense. 150nF is way to big a value for safety isolation and there would also need to be two of them for that job - one in each AC supply line. The shaver is made safe by virtue of having " class 2 " insulation provided between all live and contactable parts. ..... Phil |