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| How to set reference for a sine wave with dc supply |
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| anishkgt:
--- Quote ---Hod did you measure the voltage? Before diode it is AC and normally measured with RMS voltage. After diode it is half-wave and I wouldn't measure it as as AC. Rather, I'd looked at peak value. Concerning the circuit, I don't understand the whole upper block. Doesn't look right to me. Like, why it uses thyristor and not triac. Or why fuse on live_out and not on live, etc. Why using C3? I think it shouldn't be there, as it rectifies voltage, that's not what you want for peak detector. But even if it was needed, it should have been placed after resistor divider. This way it can be much smaller, smaller voltage rating, and less current through diode. --- End quote --- The RMS voltage was what is measured at 33vac. Earlier i did not measure the peak it was from a DMM. But now checked it with a scope and it reads about 76vac max. Talking about the upper block, It is quite common to use Thyristors in an industrial environment as they handle more current in a back-to-back configuration. A triac would also work but i decided to go ahead with thyristors. There are two fuse to the whole circuit one at the input and the second at the output (LIVE_OUT). The C3 was used to get a DC voltage for the comparator to compare against the reference voltage of 2.6v. Secondly, at the put, there is 240v as well. The reason being the once the opamp output goes high the microcontroller would switch on the thyristors for welding. Attached two screen shots from scope. Transformer_OUT is the voltage from the spot welder transformer. Since i did not have way of isolating the mains voltage to use a scope. Second, After_DIODE is the voltage (referenced to GND) after the diode into the cap C3. The voltage 33vac is applied via a microcontroller with a phase shift after the zero cross is detected, a common way to control power on ac. as explained here http://www.bristolwatch.com/arduino/arduino_power_control.htm |
| exe:
Ah, now I noticed the second SCR. They are drown a little bit apart, so didn't notice at first glance, sorry. Did you fix the lower circuit? E.g., removed the capacitor? Without fixing it first taking waveforms doesn't make much sense. I'd also removed upper circuit until the lower part proven to be working. Labels are also sort of confusing to me. There is live and neutral on the schematic (making me thinking this is important), but it doesn't really matter. I'd just call it AC1 and AC2 or something. I know you think I'm nit-picking, but what I suggest really helps reading the circuit for a random reader. |
| anishkgt:
Thank you for the time. I've removed C3 and attached the wave form. I can't remove the upper circuit which is where the voltage is coming through as they are driven from microcontroller. I'don't have a bench power supply or a wave form generator. The LIVE and Neutral are actually how the wiring goes. The neutral the one connected the one of the primary of the transformer and the LIVE goes through the thyristors which is why it is LIVE_OUT. Basically and switch in series with the transformer. Do let me know if there is anything more i would need to help you understand better. |
| Zero999:
What's the purpose of the project? If you want to control a load, with zero crossing, then an opto-coupler can be used with built-in zero crossing, such as the MOC3041. If it's an undervoltage protection circuit, then use a low pass filter, before the comparator circuit, to get the average voltage of the rectified waveform. |
| exe:
To me the resistor divider and diode work: I see truncated sine wave. Not sure where 4V offset comes from. But! Do you have opamp's ground connected to neutral? They must be connected for this to work. **But this also makes this circuit unsafe.** |
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