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Changing Voltage with duty cycle

 
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mvliege



Joined: 05 Aug 2014
Posts: 7
Location: Turkey

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Changing Voltage with duty cycle
PostPosted: Wed Oct 07, 2015 8:23 am     Reply with quote
Hi,

I have used half bridge to create sine wave.
I would like to change voltage and frequency.
Freq was OK, but the voltage was not changed when I changed duty cycle.
Neither increase nor decrease.

How to increase voltage using duty cycle?

Here is my code:
Code:

unsigned int8 i=94; // for voltage
unsigned int8 x=184;  //for freq

setup_timer_2(T2_DIV_BY_16,x,1);      //6,6 ms overflow, 6,6 ms interrupt

setup_ccp1(CCP_PWM|CCP_PWM_HALF_BRIDGE|CCP_SHUTDOWN_AC_L|CCP_SHUTDOWN_BD_L);
 set_pwm1_duty(i);

// i value VOLTAGE, when I push the button;
      if(input(pin_b1)) 
      {       
          delay_ms(10);
          while(input(pin_b1));
          i+=10;
         
          if(i>=254)
          {
              i=94;
              /*write_eeprom(0,i);
              read_eeprom(0);*/
          }
          set_pwm1_duty(i);         
      }
     
      // x value, FREQ
       if(input(pin_b2))
      {
         delay_ms(20);
          while(input(pin_b2));
          x+=10;
         
          if(x>=254)
          {
              x=184;
             // write_eeprom(1,x);
          }
          setup_timer_2(T2_DIV_BY_16,x,1);
      }
Gabriel



Joined: 03 Aug 2009
Posts: 1067
Location: Panama

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PostPosted: Wed Oct 07, 2015 3:29 pm     Reply with quote
Put a resistor and a capacitor on the output... measure the voltage across the capacitor... vary the duty.

Google PWM.
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Ttelmah



Joined: 11 Mar 2010
Posts: 19545

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PostPosted: Thu Oct 08, 2015 12:52 am     Reply with quote
There seems though to be a big 'gap' somewhere in the understanding.

If he is using a PWM, to synthesise a 'sine wave', then the PWM frequency needs to be a lot above the sine wave he is synthesising, and this needs to be being integrated to generate the sine. The frequency of the PWM, will not directly relate to the frequency of the sine.

<http://solar.smps.us/sinewave-inverters.html>

As you say, his circuit afterwards needs to be performing this integration.
The duty cycle will continuously be changing for each cycle of the sine wave, probably using a look up table, and it'll be the contents of the whole table that needs to change to vary the effective voltage....
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