Dear Kai,
Dear Kai,
Thanks for your reply,



I compute RMS using following method.

- Hardware I2C is enabled in intertupt mode.
- ADC does the conversion and sends using i2c bus.
- When the byte is ready micro gets an interrupt.
- ISR squares the sample and adds it to square sum.
- When 2ms are past a cycle complete the RMS is calculated by first finding mean of this sum and then taking square root.

- A separate signal of magnitude 0-6V AC coming from transformer is used for zero cross detection using LM311 based circuit.
- Zero cross detection is using separate PCA interrupt to micro.
- high to low transition from Zero cross detector marks end of a cycle and start of another one.
- 2ms Past the end of cycle RMS is computed.

Since the welding transformers are inductive loads. The current is still flowing even after next zero cross. Thus I keep on taking samples upto 1.5ms after zero cross.

Next cycle firing is always done 2.5ms past zero cross never before that.



Yes, Kai I do use mains zero crossing for RMS computation.

Thanks & Regards,
Prahlad Purohit