Lesson
13 PWM method
What is PWM? (Pulse Width
Modulation)
PWM method is widely used in the industrial control of motor speed, lamp
brightness, heater temperature….
W=V * I (Power = Voltage *
Current)
According the equation, we
may vary voltage or current to control the power. But these ways of control are
not efficient. Also it is not easy to vary the voltage or current
automatically.
PWM method control the time
of the on and off of power. It is easy for electronic circuit to control the current
on or off. Also it is easy for digital electronic circuit or microcontroller to
control the rate of ON time.
We will
explain the PWM method with a 555 IC in the following simple experiments.
Experiment 1 LED on and off periodically (C take 100μF)
T1--This is the time that current flow through R1 and R2 to charge Capacitor
C until the capacitor is full.
T2---This is the time that current current flow out from C through R2 to
pin 7.
You may use 100μF for C at first, use 10KΩ or 20KΩ for R1 and R2 at first.
You should try different R1,R2 , and observe the On and Off of the LED.
The current decrease as R1 or R2 increase. So the capacitor need more time
to charge.
If we use lager capacitor, the capacitor will need more time to charge.
So T1 and T2 increase as R1,R2 or C increase. and the LED on,off,on,off…
slowly.
Experiment 2 Adjust the brightness of LED with PWM
method
(C take 0.00000003μF)
We use 103
capacitor instead of 100μF capacitor here.
103 maens 10
add 3 zeros equals 10000
. the unit is P
p means 10(‾12) that is
0.000000000001.
The periodic time will be much shorter. The LED on,off,on,off… so quickly
that we can not distinguish the on and off. What we see is a dim LED.
Try adjusting the
potentiometer, you will see that the brightness of LED vary. In fact we only
change the time of on and off.
Likewise, if we
control heater with PWM method, we do not change the flow rate of the electronic
current of heater, but we control the time rate of on and off.
R1+R2=constant, because these 2 resistors
belong to the same potentiometer. So the Time T always be constant no matter
how you adjust the potentiometer. We adjust the ratio of on and off. In another
words, we modulate the width of pulse.
If a potentiometer is 20KΩ, that means that from A through B is 20kΩ
R1+R2=20kΩ
Experiment 3 Adjust the speed of DC motor with PWM
method
(C use 103 = 0.00000003μF)
Try adjusting
the potentiometer, You will see the speed of DC motor changes.
There will be
a large current flow when the DC motor is off .,Because the magnetic field of
the coil terminate suddenly and generate a large reverse electronic field
simultaneously and cause a large current flow through transistor. So we connect
a diode to bypass the current and protect the transistor.
Because the DC
motor need more current than LED, so we use 2 transistors to amplify the
current. This is called
