221308 Engineering Electronics I

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# 221308 Engineering Electronics I - PowerPoint PPT Presentation

221308 Engineering Electronics I . 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 . Analog Integrated Circuits (Integrated Circuit Timer).

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## 221308 Engineering Electronics I

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### 221308 Engineering Electronics I

Asst. Prof. Dr. Montree SiripruchyanunMatheepot PattanasakDept. of Teacher Training in Electrical EngineeringFaculty of Technical EducationKing Mongkut’s Institiute of Technology North Bangkok

(Integrated Circuit Timer)

Asst. Prof. Dr. Montree Siripruchyanun

The Capacitor
• The capacitor is a device that stores electric charge
• Rislove’s Rules of Capacitors (DC Circuits)
• 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

Asst. Prof. Dr. Montree Siripruchyanun

Charging and Discharging
• Voltage across a capacitor increases as it charges
• Large resistor + large capacitor = longer charging time

Asst. Prof. Dr. Montree Siripruchyanun

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

Asst. Prof. Dr. Montree Siripruchyanun

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

Asst. Prof. Dr. Montree Siripruchyanun

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)

Asst. Prof. Dr. Montree Siripruchyanun

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

Asst. Prof. Dr. Montree Siripruchyanun

Monostable Multivibrators
• Principal use: to adjust the duty cycle of a pulse train

Asst. Prof. Dr. Montree Siripruchyanun

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

Asst. Prof. Dr. Montree Siripruchyanun

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

Asst. Prof. Dr. Montree Siripruchyanun

Timing Diagram

Asst. Prof. Dr. Montree Siripruchyanun

555 Timer

Asst. Prof. Dr. Montree Siripruchyanun

The 555 Timing IC

Note: A comparator outputs a HIGH when the + input is at a higher voltage than the - input

Asst. Prof. Dr. Montree Siripruchyanun

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

Asst. Prof. Dr. Montree Siripruchyanun

Timing Diagram
• tLO = 0.693RBC
• tHI = 0.693(RA+RB)C
• The output frequency is f = 1/(tLO+tHI)

Asst. Prof. Dr. Montree Siripruchyanun

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)

Asst. Prof. Dr. Montree Siripruchyanun

Monostable Multivibrator

Asst. Prof. Dr. Montree Siripruchyanun

Monostable Multivibrator

Asst. Prof. Dr. Montree Siripruchyanun