Capacitors
Ben
make sense to me (sadly enough)
How is it that a capacitor rated at 300 V, such as that in a
disposable camera, gets charged to that voltage even though it only
has like a 1.5 V AA battery charging it?
Does it just take longer? o.O
K S
Try sci.electronics
[ Off Topic response: There has to be some circuit between the 1.5V
battery and the capacitor such that the capactor's leads
"see" (Thevenin voltage) the voltage of whatever it gets charged to.
The voltage rating of a cap. is only the max. voltage it's rated to
withstand without failure. ]
Rob Johnson
"Ben" <benjam...@gmail.com> wrote:
>How is it that a capacitor rated at 300 V, such as that in a
>disposable camera, gets charged to that voltage even though it only
>has like a 1.5 V AA battery charging it?
>
>Does it just take longer? o.O
The voltage rating is the maximum voltage that can be put across the
capacitor. What a capacitor does is store charge proportional to
the voltage measured across it. The ratio is the capacitance measured
in farads (1 farad = 1 coulomb/1 volt). So a 3 microfarad capacitor
with 1.5 volts across it will hold 4.5 microcoulombs. Putting
capacitors in parallel will add their capacitance.
The amount of time it takes to charge up is dependent on how much
current flows across the capacitor. 1 amp x 1 second = 1 coulomb.
Rob Johnson <r...@trash.whim.org>
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"Ben" <benjam...@gmail.com> wrote in message
news:1170298771....@j27g2000cwj.googlegroups.com...
there is a small oscillator and small transformer that takes the 1.5 volt up
to about 150 volts needed for the flash unit.
done all the time in Flash units. Flash unit has a high trigger voltage too.
con...@email.rahul.net
See: http://en.wikipedia.org/wiki/Charge_pump
John
--
John Conover, con...@email.rahul.net, http://www.johncon.com/
Bob Kolker
The unit of capacitance is the Farad (named after Micheal Fariday), not
the volt. Volt is (potential) Energy per Charge. Capacitance is Charge
per Volt.
In matters of physics, units are everything. There are very few pure
numbers in physics.
Bob Kolker
John Bailey
The one answer above that came close nonetheless described the case
where multiple capacitors are used. US Patent 4,942,340 (now easily
found on Google) is one of many patents which show how to charge a
single capacitor to a higher voltage than the source. The trick is to
generate a high frequency by electronically interrupting the DC from
the source. This gives a high frequency signal with a voltage near
that of the source. A transformer can now boost this voltage to a
much higher voltage. Now using a diode (a one way current device) the
circuit then adds charge to the capacitor at the peaks of the high
voltage ac.
Listen carefully as your flash charges. Mine has a faint whine as it
charges.
Michael Stemper
>Ben wrote:
>> How is it that a capacitor rated at 300 V, such as that in a
>> disposable camera, gets charged to that voltage even though it only
>> has like a 1.5 V AA battery charging it?
>
>The unit of capacitance is the Farad (named after Micheal Fariday), not
>the volt. Volt is (potential) Energy per Charge. Capacitance is Charge
>per Volt.
Very good. And capacitors have voltage ratings, based upon the strength
of their dielectrics. Using a capacitor with a 300V rating in a hand-held
electronic device is perfectly safe. Using a capacitor with a 300V rating
to provide VARs on a 23kV feeder wouldn't work too well.
--
Michael F. Stemper
#include <Standard_Disclaimer>
A preposition is something that you should never end a sentence with.
fred....@gmail.com
Usually the high voltage is obtained with a boost swirched-mode power
supply. This scheme uses an inductor to provide the voltage increase.
The voltage across an inductor (coil) is the inductance L (in Henries)
multiplied by the rate of change (d/dt) of current i. if the current
flowing in an inductor is changed in a very short period of time, the
inductor will create a high voltage when the stored magnetic field
collapses. A good example of this mechanism is the spark/ignition
system on a car. The capacitor(s) are usually used to smooth the high
voltage (to keep it constant) once it has been produced by the
inductor circuit. Generally, capacitors oppose changes in voltage (and
can produce large currents as a result) and inductors oppose changes
in current (and produce large voltages as a result). For more
information, see "switched mode power supplies".
Fred.
Robert Mabee
> there is a small oscillator and small transformer that takes the 1.5 volt up
> to about 150 volts needed for the flash unit.
There was a first generation circa 1960 that only charged the capacitor
to the battery voltage, using a many-cell battery (over 20 volts). The
capacitor then discharged through a disposable flash bulb rated for a
much lower voltage. The overvoltage made the bulb ignition very quick
for better synchronization with a fast shutter. The capacitor was
needed because the battery by itself would not supply a high current;
multi-cell batteries dry out quickly, developing high resistance, but
will continue to provide a trickle of power for many years.