capacitor on series with transformer primary?

M

Michael

Guest
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?
 
Michael wrote:
On Apr 27, 11:56 am, George Herold <ggher...@gmail.com> wrote:
On Apr 27, 11:55 am, Michael <mrdarr...@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?
Click to expand...
Only very vaguely. That cap may also be there for galvanic isolation
from the mains.

Seems odd they would choose a capacitor...?
Click to expand...
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...
Click to expand...
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
 
Michael wrote:
On Apr 27, 12:19 pm, whit3rd <whit...@gmail.com> wrote:
On Apr 27, 8:55 am, Michael <mrdarr...@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.
Click to expand...
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
 
On Apr 27, 11:55 am, Michael <mrdarr...@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?
Click to expand...
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.
 
On Apr 27, 11:56 am, George Herold <ggher...@gmail.com> wrote:
On Apr 27, 11:55 am, Michael <mrdarr...@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.
Click to expand...

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
 
On Apr 27, 8:55 am, Michael <mrdarr...@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?
Click to expand...
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.
 
On Apr 27, 12:19 pm, whit3rd <whit...@gmail.com> wrote:
On Apr 27, 8:55 am, Michael <mrdarr...@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.
Click to expand...

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
 
On Apr 27, 1:07 pm, Tim Wescott <t...@seemywebsite.now> wrote:
Michael wrote:
On Apr 27, 12:19 pm, whit3rd <whit...@gmail.com> wrote:
On Apr 27, 8:55 am, Michael <mrdarr...@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
Click to expand...

Oh good... no kablooeys to worry about.

Thanks!

Michael
 
"Tim Wescott"

Only very vaguely. That cap may also be there for galvanic isolation from
the mains.
Click to expand...

** 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
 
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