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praveen_palaparthi
Joined: 15 Feb 2008
Posts: 112
Helped: 1
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 13 Nov 2008 10:16 pic control output voltage |
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| hi friends i m doing push pull inverter using PIC 16F72 i have done in open loop control.i want to control output voltage to 230 volts by giving feedback.My problem is how to control output voltage using pic controller .
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darla1
Joined: 21 May 2008
Posts: 149
Helped: 11
Location: India
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13 Nov 2008 12:08 Re: help me how to control duty cycle |
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measure the out put voltage as feedback...based on feedback need to change the duty cycle...
darla
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praveen_palaparthi
Joined: 15 Feb 2008
Posts: 112
Helped: 1
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14 Nov 2008 5:36 Re: help me how to control duty cycle |
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| can u explain me in detail i have taken feedback of output voltage and now how to control the duty cycle if output voltage is increased or decreased. if output voltage is increased i have to decrease the duty cycle and vice versa i know this but how to do it.
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AR15
Joined: 29 Mar 2007
Posts: 9
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14 Nov 2008 14:54 Re: help me how to control duty cycle |
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You should probably only use the PIC to create a reference signal for an analog PWM controller. The PIC is too slow to control the feedback, unless the load will remain constant. An analog PWM controller usually consists of an op amp circuit that is fed a reference voltage (from PIC) on the + pin, and output feedback voltage (through a feedback circuit - voltage divider, op amp, etc.) on the - pin (negative feedback). The op amp then does what it can to drive the difference (error) to zero. The op amp output is then fed into a comparator where it is compared to an oscillating wave (ususally a triangle/sawtooth) whose frequency determines the switching frequency (usually >20kHz). The output of the comparator is PWM.
You can write code to generate a PWM reference signal (a digital representation of a sine wave) on a PIC I/O pin. Then use a low pass filter to remove the switching frequency. This should give you a clean sine wave to feed into the op amp (error amp).
Use another feedback circuit to step the output voltage down to a level that is compatible with the PIC. A peak detector circuit could be used to produce a DC representation of the maximum amplitude of the sine wave, then read with the PIC's A/D. If the amplitude is off adjust the reference. Relatively speaking, adjusting the reference does not have to be done very often. Output regulation should be done by the analog PWM controller.
This is just one idea, I'm sure there are many other ways to do it.
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praveen_palaparthi
Joined: 15 Feb 2008
Posts: 112
Helped: 1
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15 Nov 2008 7:52 Re: help me how to control duty cycle |
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thank you for giving me good idea but i dont want to use any other circuits like op amp.
In my hardware simple feedback circuit is present .what i m doing is i m stepping down the output voltage and rectifying using full bridge rectifier and zenar diode all those circuits are there to take care not to exceed 5 v as input to microcontroller ADC channel.Now after taking the adc count tell me what next i have to do to control or stablize the output voltage at 230 volts irrecptive of load varations....
help me guys struggling for past 1 week to slove this without using any other extra circuits..........
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