capacitor on series with transformer primary?
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From: Michael on 27 Apr 2010 11:55 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? Can the transformer safely function without the capacitor in series?
From: George Herold on 27 Apr 2010 14:56 On Apr 27, 11:55 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? > > Can the transformer safely function without the capacitor in series? Sounds like it was there to limit the current... at 60 Hz 0.15uF looks to be a bit more than 10k ohm....(Assuming I'm ready the chart correctly.. I'm too lazy to punch in the numbers.) something like 100mA of current. George H.
From: Michael on 27 Apr 2010 14:59 On Apr 27, 11:56 am, George Herold <ggher...(a)gmail.com> wrote: > On Apr 27, 11:55 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? > > > Can the transformer safely function without the capacitor in series? > > Sounds like it was there to limit the current... at 60 Hz 0.15uF looks > to be a bit more than 10k ohm....(Assuming I'm ready the chart > correctly.. I'm too lazy to punch in the numbers.) something like > 100mA of current. > > George H. Ah, so would a 10k resistor in series with the primary do something similar? Seems odd they would choose a capacitor...? Also I realized I made a mistake: the TRANSFORMER was less than 1 cu. in., not the capacitor. It's a really cute transformer; I was planning to desolder it for use elsewhere, but wondered what would happen if say a couple of 1.2V NiCd batts want to suck too much current from the secondary... Thanks, Michael
From: whit3rd on 27 Apr 2010 15:19 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.
From: Tim Wescott on 27 Apr 2010 15:22
Michael wrote: > On Apr 27, 11:56 am, George Herold <ggher...(a)gmail.com> wrote: >> On Apr 27, 11:55 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? >>> Can the transformer safely function without the capacitor in series? >> Sounds like it was there to limit the current... at 60 Hz 0.15uF looks >> to be a bit more than 10k ohm....(Assuming I'm ready the chart >> correctly.. I'm too lazy to punch in the numbers.) something like >> 100mA of current. >> >> George H. > > > Ah, so would a 10k resistor in series with the primary do something > similar? Only very vaguely. That cap may also be there for galvanic isolation from the mains. > > Seems odd they would choose a capacitor...? The cap limits current without dissipating heat. A 10k resistor with 120V across it will dissipate 1.5W (120V^2 / 10k). Assuming that most of the voltage drop is in the cap, you'd throw away more energy to heat than you would spend charging the battery. > Also I realized I made a mistake: the TRANSFORMER was less than 1 cu. > in., not the capacitor. > > It's a really cute transformer; I was planning to desolder it for use > elsewhere, but wondered what would happen if say a couple of 1.2V NiCd > batts want to suck too much current from the secondary... They wouldn't, because the capacitor would limit the current -- if you left it in there. -- Tim Wescott Control system and signal processing consulting www.wescottdesign.com |