seven_segment
Member level 2
Pulse width modulation
I'm breadboarding a sine wave inverter using variable PWM and adjusting the mark/space ratio. I have a PIC micro runing @ 20Mhz with a quarter wave sine lookup table with 10 values per quadrant. I'm trying to achieve an output frequency of 50Hz, PWM input to the transformer is currently at 2Khz. The transformer is a standard 50Hz chassis mount, I had hoped that it would filter the input and give a relatively clean sine wave, but that doesn't seem to be the case.
The driver circuit consists of two sets of BC108/2N3055 darlington pairs. The transformer has dual secondary coils, the centre tap is wired to positive and the outer taps are grounded at set intervals during each half cycle using timing values from the lookup table. Note that only one transistor is involved in switching during each half cycle, the other one is left in the off state.
The transformer is rated at 20VA. From the primary I'm driving another smaller (3VA) transformer to give (along with a resistor voltage divider) a suitable voltage to input to my sound card so I can record the waveform. I have attached an image showing the output them I'm getting from this thing. It vaguely resembles a sine wave, although wasn't what I was hoping for. Can anyone tell me where I'm going wrong?
Cheers,
7 segment
I'm breadboarding a sine wave inverter using variable PWM and adjusting the mark/space ratio. I have a PIC micro runing @ 20Mhz with a quarter wave sine lookup table with 10 values per quadrant. I'm trying to achieve an output frequency of 50Hz, PWM input to the transformer is currently at 2Khz. The transformer is a standard 50Hz chassis mount, I had hoped that it would filter the input and give a relatively clean sine wave, but that doesn't seem to be the case.
The driver circuit consists of two sets of BC108/2N3055 darlington pairs. The transformer has dual secondary coils, the centre tap is wired to positive and the outer taps are grounded at set intervals during each half cycle using timing values from the lookup table. Note that only one transistor is involved in switching during each half cycle, the other one is left in the off state.
The transformer is rated at 20VA. From the primary I'm driving another smaller (3VA) transformer to give (along with a resistor voltage divider) a suitable voltage to input to my sound card so I can record the waveform. I have attached an image showing the output them I'm getting from this thing. It vaguely resembles a sine wave, although wasn't what I was hoping for. Can anyone tell me where I'm going wrong?
Cheers,
7 segment