| Electronics > Projects, Designs, and Technical Stuff |
| Isolated zero cross detection w/ AC mains |
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| dmendesf:
This is a zero cross circuit recommended at Time Nuts as being robust to noise... I've used it myself and works very well: https://web.archive.org/web/20180209111314/http://www.dextrel.net/diyzerocrosser.htm |
| langwadt:
there's also the crazy one, two optos, LEDs anti parallel, the two transistors arranged as push-pull |
| beduino:
--- Quote from: floobydust on October 05, 2019, 01:18:06 am ---AC mains is always dirty, so you need transient protection and low pass filtering. --- End quote --- My home mains 230VAC 50Hz is quite clean and antiparallel EL817 shown in my circuit a few posts above after ATTiny85 8MHz (no crystal during this test only calibrated OSCAL) processing close to zero crossing signals from those optos gives mains period 0.020078 s, which is only ~0.4% faster than theoretical 20ms and as we can see below, generated zero crossing waveform (green) but as pulse train is quite close to optos outputs: positive mains waveform (red) and negative mains waveform (blue) 8) It will be interesting to see some results with 8Mhz external crystal, however probably it will be better run MPU @ 5V 16MHz external crystal, since experiments showed that, this code below which creates those zero crossing (green) waveform takes due to division by 32bit period ~90us, so looking now for a way to create zero crossing pulse train in a more efficient way ::) |
| Circlotron:
--- Quote from: iroc86 on October 05, 2019, 10:42:48 pm --- One of my goals with the new design is to improve the weld repeatability. I wonder if the variation was due to de-energizing the welder transformer in mid-cycle. --- End quote --- If the load is switched with a triac or pair of SCRs it will remain energised until the current zero crossing despite the gate drive to the semis being removed. If you needed to, you could detect this by the sudden rise in voltage across the semis as they unlatch. |
| floobydust:
Try OR'ing the optos outputs and using a high value pullup for more gain. Then compare to the actual mains sine wave. There is a phase-shift due to the opto's actual turn on point. EL817 CTR is all over the place 5-600% so they won't be matched and there is always a deadband. My fuzzy math, 18Vpk is 3° for 240VRMS, so a 6° deadband or 0.34msec at 50Hz. For welding batteries, double-pulse is used where a ~1/8 time pulse clears any oxidation or plating, dirt as a weak weld- then a pause then the main welding pulse. Your weld inconsistency can be caused by transformer residual magentization, and you don't want to zero-cross switch a transformer. You switch on during a peak for inductive loads. |
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