Pulse Width Modulation (PWM) LED Dimmer Circuit

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Pulse Width Modulation (PWM) LED Dimmer Circuit

• Using a 555 Timer Chip

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Goals of Experiment

• Demonstrate the operation of a simple PWM circuit
  that can be used to adjust the intensity of a
  green LED by varying the value of one resistor.
• Demonstrate how a square wave generator can be
  built using an operational amplifier.
• Demonstrate how a summing amplifier can be used
  to add a dc offset to a time-varying signal.
• Describe how to test complex circuits.

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Pulse Width Modulation

• An energy efficient technique that is used
  widely in industrial and consumer electronics to
  vary the power dissipated by a load.
• Electric motors
• Electric stove elements
• Light dimmers

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PWM Circuit with 555 Timer
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How a 555 Timer Works

• Astable Multivibrator Mode

http//www.williamson-labs.com/480_555.htm
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http//www.williamson-labs.com/480_555.htm
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Voltage Comparator

• Op Amp circuit without a feedback component.
• Output voltage changes to force the negative
  input voltage to equal the positive input
  voltage.
• Maximum value of the output voltage, Vo, is V if
  the negative input voltage, v1, is less than the
  positive input voltage, v2.
• Minimum value of the output voltage, Vo, is V- if
  the negative input voltage, v1, is greater than
  the positive input voltage, v2.

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http//www.williamson-labs.com/480_555.htm
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Transistor

• As you will learn in ECE 2204, a transistor can
  be designed to act like a switch.
• When a positive voltage is applied to the base of
  the transistor (B), the transistor acts like
  there is a very small resistor is between the
  collector (C) and the emitter (E).
• When ground is applied to the base of the
  transistor (B), the transistor acts like there is
  a an open circuit between the collector (C) and
  the emitter (E).

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Monostable Multivibrator

• The frequency of the output from the 555 timer is
  constant. It is determined by the RC time
  constant of the external components, Ra and C1,
  and the frequency of the voltage source tied to
  the Trigger input pin.
• The width of the 5V pulse within a cycle is
  determined by the amplitude of the signal on the
  Control pin of the 555 Timer chip.

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PWM

• Voltage on Control pin
• Output voltage of the 555 Timer

http//www.hobbyprojects.com/general_theory/pulse_
modulation.html
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Square Wave GeneratorRelaxation Oscillator

• The duty cycle of the output will be 50 when
  R1 R2.
• The frequency of the square wave output is given
  by

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• The delay in obtaining the square wave output is
  a result of
• the initial condition of the capacitor (IC
  0V),
• the amount of electronic noise in the circuit,
• the degree to which your op amp is nonideal, and
  
• the RC time constant of the charging circuit for
  the capacitor.

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Square Wave with Offset Voltage
Vo 0-5V square wave
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Amplitude Reduction and Voltage Offset
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PSpice Simulation using Vpulse
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PSpice Simulation using Digclock
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Light Emitting Diode (LED)

• A nonlinear component that looks like a battery
  when it is on and an open when it is off.

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Discrete LEDs
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Anode vs. Cathode

• Switch your DMM to the diode symbol.
• Place the red probe into the V-W plug and the
  black probe into the COM plug.
• Place your probes across the diode.
• If the result is a very small number, then your
  red probe is contacting the anode and the black
  probe is contacting the cathode of the diode.
• If the result is an overload (overflow)
  condition, then the red probe is contacting the
  cathode and the black probe is contacting the
  anode of the diode.

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Simulating a LED in PSpice

• No LED part in the student PSpice so we use a
  series combination of parts.
• Dbreak (diode breakout part)
• Allows current to flow when the voltage on the
  anode is 0.7V higher than the voltage on the
  cathode.
• Vdc
• Set to the difference in the voltage needed on
  the anode to turn the LED on
• Our red and green LEDs need between 2-2.5V.

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Experimental Procedure

• First, construct and test the square wave
  generator.
• Second, construct and test the summing amplifier
  circuit.
• Only after verifying that the two circuits
  operate correctly, connect them together.
• Third, construct the pulse width modulator
  circuit using the 555 timer.
• If the circuit doesnt work properly, rework the
  circuit as a monostable vibrator and connect the
  Velleman function generator (500 Hz square wave
  with a peak-to-peak voltage of 5V and an offset
  voltage of 2.5V) to the trigger.

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Debugging

• Find reasonably points within circuit to make
  measurements.
• Start at the beginning of the circuit
• Compare measured voltages with expected voltages
  obtained either through circuit analysis or
  simulation.
• This means that you should do your calculations
  and simulations before you construct the circuit.
• If they match reasonably well, move on to the
  next measurement point.

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• If they dont, do the simple things first
• Verify that power is on and connected at the
  right places (particularly V and V- on op amps).
• Check component values.
• Run through wiring to make sure that the wires
  are going where they are supposed to and nothing
  is accidently touching.
• Make sure that your oscilloscope probes are
  connected to the correct points in the circuit.
• Replace Rf on the summing amplifier if the output
  voltage is not close of a 0V-5V square wave.
• The circuit usually works even if the max. of the
  output voltage is 3.5V instead of 5V.
• Only if you are confident that you have
  everything above right, consider replacing the op
  amp or 555 timer chip.
• If the circuit still doesnt function properly,
  see one of the OpEL GTA for assistance.

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Pre-Lab

• This is a longer lab so the answers for the
  questions in the Analysis Section are
• RLED 2.5V/10mA 250 W.
• f 1/2ln(3)(910 W)(1mF) 500Hz
• Vopamp2 -(1k W /3.6k W) Vsqwave -(1k W /3.6k
  W)(-9V
• Vopamp2 -0.278 Vsqwave 2.5V
• You must enter the results from the PSpice
  simulations described in the experimental
  procedure (and on the next slide).

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Pre-Lab Report

• 1. The plot generated from the transient analysis
  of the circuit in Figure 1 using
• a 10 kO resistor instead of the trim pot for Ra
• V2 is a sinusoidal input voltage source with
  amplitude of 2V with a 2V offset voltage and a
  frequency of 60Hz.
• Vpulse should have a magnitude of 5V and a period
  of 200us with a 50 duty cycle.
• V2 and V3, are displayed as well as the output
  voltage. The final time for the transient
  analysis should be 50 ms.
• 2. The plot generated from the transient analysis
  of the circuit in Figure 1 where the value of Ra
  is changed to 1 kO.
• 3. The plot generated from the transient analysis
  of the circuit in Figure 1 where the value of Ra
  is changed to 100 O.
• 4. The plot generated from the transient analysis
  of the circuit in Figure 2 where the value of Ra
  is changed to 1 kO where voltage probes are
  placed atthe output pin of both operational
  amplifiers. The final time for the transient
  analysis should be 40 ms.

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Post-Validation Report

• See the description of items to be include in the
  Instructions for LED Dimmer