Transcript Practical Electronics III

221308 Engineering Electronics I
Analog Integrated Circuits
(Integrated Circuit Timer)
Asst. Prof. Dr. Montree Siripruchyanun
Matheepot Pattanasak
Dept. of Teacher Training in Electrical Engineering
Faculty of Technical Education
King Mongkut’s Institiute of Technology North Bangkok
7/16/2015
Asst. Prof. Dr. Montree
Siripruchyanun
1
The Capacitor


The capacitor is a device that stores electric charge
Rislove’s Rules of Capacitors (DC Circuits)
1)
2)
3)
4)
5)
When voltage is applied, capacitors will briefly allow current to flow as
they charge
Voltage across a capacitor increases as it charges
Once charged, capacitors no longer allow current to flow and the
voltage across the capacitor is constant
Capacitors with large capacitance (measured in Farads) take longer to
charge
Capacitors can be discharged by short-circuiting them
7/16/2015
Asst. Prof. Dr. Montree
Siripruchyanun
2
Charging and Discharging
 Voltage across a
capacitor
increases as it
charges
 Large resistor +
large capacitor =
longer charging
time
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Asst. Prof. Dr. Montree
Siripruchyanun
3
The RC Time Constant
 The time it takes for a capacitor to charge or
discharge depends on two things:
• How much current is flowing (limited by the resistor
R)
• How much charge the capacitor can store (limited by
the capacitance C)
 R x C is called the time constant and determines
the charging time
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Asst. Prof. Dr. Montree
Siripruchyanun
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Capacitors as Backup Power
 Very large capacitors (a few Farads capacitance)
can be connected between VCC and Ground near
the power supply
 If the power fails, the capacitor can supply current
to the circuits for up to a few hours at a time
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Asst. Prof. Dr. Montree
Siripruchyanun
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Capacitors in Digital Circuits
 99% of capacitors in digital circuits connect VCC
to ground near a chip in order to cut down on
noise
 The rest do one of the following:
• Provide backup power
• Provide external capacitors for the clock circuit
(known as an astable multivibrator)
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Asst. Prof. Dr. Montree
Siripruchyanun
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Mutivibrators
 A multivibrator is a circuit that changes its output
between two states continuously or on demand
 Three types:
• Bistable multivibrators (i.e. flip-flops)
• Monostable multivibrators produce a single pulse once triggered
by an input signal
• Astable multivibrators produce an oscillating train of ones and
zeroes spontaneously
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Asst. Prof. Dr. Montree
Siripruchyanun
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Monostable Multivibrators
 Principal use: to
adjust the duty
cycle of a pulse
train
 Feel free to read
more in your
text—enough
said
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Asst. Prof. Dr. Montree
Siripruchyanun
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Astable Multivibrators
 An astable multivibrator is inherently unstable,
switching constantly between HIGH and LOW
 Astable multivibrators are used principally to
generate clock signals in digital circuits
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Asst. Prof. Dr. Montree
Siripruchyanun
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A Simple Astable Multivibrator
 The input (and output)
constantly switches from
HIGH to LOW as the
capacitor is charged and
discharged
 Uses feedback to make
the circuit unstable
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Asst. Prof. Dr. Montree
Siripruchyanun
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Timing Diagram
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Asst. Prof. Dr. Montree
Siripruchyanun
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555 Timer
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Asst. Prof. Dr. Montree
Siripruchyanun
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The 555 Timing IC
Note: A
comparator
outputs a
HIGH when
the + input
is at a
higher
voltage
than the input
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Asst. Prof. Dr. Montree
Siripruchyanun
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The 555 as an Astable Multivibrator
 The discharging time
constant is RBC
 The charging time
constant is (RA+RB)C
 In this case, the circuit
takes longer to charge
than to discharge
A small (~0.01 uF) capacitor
should be placed on pin 5
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Asst. Prof. Dr. Montree
Siripruchyanun
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Timing Diagram
 tLO = 0.693RBC
 tHI = 0.693(RA+RB)C
 The output frequency
is f = 1/(tLO+tHI)
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Asst. Prof. Dr. Montree
Siripruchyanun
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A 50% Duty Cycle Clock
 For a 50% duty cycle
clock, the charge and
discharge RC time
constants must be the
same
 The circuit charges
through RA and
discharges through RB =
RA
The output frequency is f =
1/(2x0.693xRAC)
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Asst. Prof. Dr. Montree
Siripruchyanun
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Monostable Multivibrator
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Asst. Prof. Dr. Montree
Siripruchyanun
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Monostable Multivibrator
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Siripruchyanun
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