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| Isolated zero cross detection w/ AC mains |
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| schmitt trigger:
Since SCRs like to be driven hard to ensure fast turn on, more so if you are driving an inductive load, a good practice is to provide what is called "picket fence" triggering. Just as its name implies, it is a burst of narrow, high current pulses, that last perhaps a quarter cycle from start to finish. Picket fence is a challenge to produce with discrete components, but with a microcontroller is a straigntforward task. Another benefit of picket fence triggering is that you can employ a transformer as the gate driver, which ensures isolation without the hassles of an auxiliary power supply. |
| Unixon:
I would recommend to try the circuit from figure #11 of ONS app.note AND9282, works perfectly. :-+ |
| beduino:
--- Quote from: schmitt trigger on October 16, 2019, 01:10:42 pm ---Since SCRs like to be driven hard to ensure fast turn on, more so if you are driving an inductive load, a good practice is to provide what is called "picket fence" triggering. --- End quote --- Probably something like this I've already implemented in my spot welder software, mentioned also in ABB Gate-drive Recommendations for Phase Control Thyristors https://www.5scomponents.com/Pdf/5SYA2034-01.pdf First fire pulse is 100us at given offset (0us in example above for full spot welder power) from mains peak voltage, than followed by 10kHz (100us period) pulses with 50% on/off up to 1ms before next predicted ZC. |
| schmitt trigger:
You got it! :-+ :-+ BTW: how are you galvanically isolating the gate driver from the SCR itself? |
| iroc86:
OP here. Lots of good info in this thread, but much of the discussion has been related to beduino's project since he popped in (which is great to have an active circuit to discuss). I'd like to pull back a little and see if anyone can help answer some of my questions above. We've touched on these concepts, but not directly. Any thoughts? ;) --- Quote from: iroc86 on October 09, 2019, 01:42:48 am ---I'd like to introduce another topic: safety and suppression. How might some of these designs better accommodate unexpected AC spikes or the wrong input voltage? MOVs, fuses...? --- End quote --- --- Quote from: iroc86 on October 15, 2019, 02:49:28 am ---...to balance the positive and negative AC cycles, how might we switch off the triac at a peak? --- End quote --- |
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