Download Capacitors: Reservoirs for Electricity

Capacitors: Reservoirs for
Electricity
Quick Recap:
• Capacitors are used to store electrical charge
• Think of a bucket storing water, current is fed into the
capacitor just like a hosepipe fills a bucket. The more current
that flows into the capacitor, the higher the voltage gets.
•So, if a large bucket can hold a large amount of water, a
large capacitor can store a big charge. The larger the value
of the capacitor the longer it will take to charge for a given
flow of current. Likewise, a bigger bucket takes longer to fill
with a given flow of water
QUICK RECAP:
• Capacitors values are expressed in
submultiples of Farads(F), ranging from
picoFarads(pF) to microfarads(uF)
• Hence: 1,000,000,000,000pf = 1F
1,000,000,000nf = 1F
1,000,000uF
= 1F
QUICK RECAP
• So, where are capacitors used?
• Many circuits today, including your proposed
project will run on extremely low currents!
• If a capacitor has a large value and the
current drain is small, it is possible to use
capacitor as a backup power system for
devices such as memory & time keeping
chips inside electronic equipment.
QUICK RECAP:
• WORKING VOLTAGE: This is the
highest voltage that a capacitor can
withstand before becoming damaged.
The dynamic duo of resistors
and capacitors
• Why do they work so well together?
• The larger the value of the resistor, the less
current flows through it for a given voltage,
which means?…….it takes more time to fill a
capacitor. By picking the combination of
capacitors and resistors, you and ONLY you!!
can determine your project’s charge or
discharge time.
Charging & Discharging Capacitors
First the Theory
• A capacitor stores electrical current, and a
resistor controls the flow of electrical current,
put these two together, and you can control
how fast you want a capacitor to fill(I.e charge)
and how fast you want the electrical current to
empty out (discharge) from the capacitor
NOW THE DEMONSTARTION!! (USING A
CAPACITOR WITHIN A TV REMOTE CONTROLLER)
Hence, by selecting the combination of capacitors and
resistors, YOU! Can determine your project’s charge or
discharge time!
HOW?
The diagram below shows how
+V
100K
+
10uF
R
C
In this circuit, C=10uF and R = 100k
(10uf 0.000001F)
0V
The Maths
• TIME CONSTANT = C x R
= 10uF x 100K
= 10 x (100x1000)/1000000
= 1 second
• When a capacitor charges, it takes a certain
amount of time to be fully charged and equal
the power supply voltage V. The rate at which
the capacitor charges will depend upon its size
and the size of the limiting resistor
• TIME CONSTANT
This is the rate at which a capacitor charges
through a resistor.
Time Constant (seconds) = C(Farads) x R (ohms)
Hence: The smaller the Time Constant the
quicker the capacitor charges (or discharges)
NOTE WELL!
• After one time constant the capacitor
would only be charged to about 0.6 of
its full charge, and would require a
further four time constants to be fully
charged, hence in the example it would
take 1 second x 5 = 5 seconds to be
fully charged
CHARGING &
DISCHARGING
• COPY GRAPH TABLES IN TO WORK
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