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How to Use PWM to Control the Voltage

Pulse-width modulation (PWM) can be used to control the value of a voltage digitally. By switching a voltage source on and off repeatedly, the average current flowing into a connected load resistance can be varied. This will depend upon the ratio of the on-time to the off-time periods, which is called the duty cycle and is expressed as a percentage value. The voltage developed across the load resistor will be a percentage of the value of the voltage source and directly proportional to the duty cycle. The number of on-off cycles accruing per second is the modulating frequency. For our purposes it will be approximately 0.1 cycles per second (cps).

Things You'll Need

Connection leads with clips (available in packs)
Resistors, 1 Meg ohm 0.5 watt
Electrolytic capacitor, 10 Micro Farad 25 Volt
Dry cell battery, 9 volt
Multimeter

Instructions

Assemble the Components
- 1
  Connect one lead of the resistor to the positive lead of the capacitor by twisting them together.
- 2
  Connect the negative lead of the capacitor to the negative terminal of the battery using a connection lead with clips.
- 3
  Connect a connecting lead with clips to the other end of the resistor, leaving one end free.
  
  Set the multimeter to a suitable range, 0- to 20-volt direct current, and measure and record the voltage of the battery. Connect the multimeter across the capacitor.
Complete the Project
- 4
  Attach the free connection clip to the battery positive terminal. Wait until the voltage across the capacitor is equal to half of the battery voltage (9 Volt/2 = 4.5 Volt). Disconnect.
- 5
  Attach the free connection clip to the battery negative terminal and count
  
  "one and two and three and four" then disconnect the clip. Note the voltage reading.
  
  Attach the free connection clip to the battery positive terminal and count:
  
  "one and two and three and four" then disconnect the clip. Note the voltage reading. Repeat this procedure five times. Average the two sets of readings. The mean reading between the two averaged readings should be half of the battery voltage (9 Volt/2 = 4.5 Volt) because the duty cycle is 50 percent.
- 6
  Attach the free connection clip to the battery negative terminal and count:
  
  "one and two" then disconnect the clip. Note the voltage reading.
  
  Attach the free connection clip to the battery positive terminal and count:
  
  "one and two and three and four and five and six" then disconnect the clip. Note the voltage reading. Average the two sets of readings. The mean reading between the two averaged readings should be three quarters of the battery voltage (3 x 9 Volt/4 = 6.75 Volt) because the duty cycle is now 75 percent.
- 7
  Repeat the previous procedure by counting faster. This will increase the modulation frequency. As the modulation frequency increases, the two readings will converge to a limit that is equal to: battery-voltage x duty-cycle.

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