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Isolated zero cross detection w/ AC mains
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iroc86:

--- Quote from: Circlotron on October 17, 2019, 07:54:49 am ---To switch off the triac at the peak you would have to force commutate it, and trials are lousy at that. ...

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Gotcha! I had read about forced commutation and it did seem like a bit of work. Going to Ian.M's comment about this...


--- Quote from: Ian.M on October 17, 2019, 12:45:47 pm ---Why would you even want to force  commutate at the peak?

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It was based on the idea of switching on equal half-cycles from a few posts up. I figured that if we switched the transformer on at a peak, we'd want to switch off at a peak n periods into the future. Maybe I was confusing half-cycles with periodic symmetry? What you say about back-EMF and residual magnetization at zero current makes sense, though. This is what I was originally thinking:



So, based on your description of switching above, would the ideal transformer switching waveform look something like the following? This would minimize inrush current at t=0, minimize back-EMF at shutoff, and support the triac's natural tendency to stop conducting at a zero cross. Or am I still confusing things? :P




--- Quote from: schmitt trigger on October 17, 2019, 01:31:06 pm ---if you choose a SSR from a reputable company (for instance Crydom) they will be very robust, and they will already have built in all the isolation and protection that you require.

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Sorry, I should have clarified my question--I actually meant on the zero cross detector itself, not the SSR/SCR/triac. Most of the circuit designs we discussed here just run mains voltage straight through a few high-value resistors into the optocoupler. Would it be necessary to include some input protection on the front end of the detector, or would that be overkill for a 120-240 VAC application? It's not like a multimeter with CAT ratings, but that's just where my mind went since we're dealing with dirty AC and the potential for all sorts of surges and spikes.
Ian.M:
Re: second diagram in reply#55,
Not quite.  Remember the current lags the voltage in an inductor, so depending on the load resistance on the secondary side which 'reflects' to the primary scaled by the square of the turns ratio, the primary resistance and the primary inductance, it will reach zero current and commutate anywhere between the voltage zero crossing after the last firing pulse and the voltage peak of the next half cycle.  If you add a trace for the primary current to your plot,  you'll get a better idea of what's going on.
beduino:

--- Quote from: Ian.M on October 17, 2019, 09:03:28 pm ---Hmmm.  The 2N6504 datasheet says they only have a 50V repetitive blocking voltage rating.   I don't see how that's going to be compatible with a 240V RMS supply.

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Nope, eg. Freescale Semiconductor (2N6509) datasheet says:
"SCRs 25 AMPERES RMS 50 thru 800 VOLTS".
I've 800V SCRs for sure, since no magic smoke so far  ;)
floobydust:
Many firmware examples (i.e. Atmel app note) do not debounce the zero-cross signal. They just make an interrupt based on every zero cross (falling) edge.
This is not good if turning on the transformer makes a glitch on mains (due to inrush) and you get extra zero-cross falsely detected.

The best systems, for 3-phase power phase control have a software PLL where the MCU synchronizes a timer to mains, so mains noise does not cause a retrigger or upset.
After detecting a zero-cross, you would not expect another seen in under a 1/2 cycle time.

Instead of counting AC cycles, you can just use a timer with multiples of mains frequency. Trigger on a zero-cross and make a pulse for n*1/50Hz duration.
TimNJ:

--- Quote from: iroc86 on October 17, 2019, 02:37:31 am ---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...?

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--- 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?

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As of yesterday, I started designing a peak-firing inrush current tester for switch-mode power supplies. When testing inrush current, the worst case is taken at the peak of the line, so this is similar to your thought.

This discussion here has brought up many good ideas. Just a few thoughts that might apply to you, though I'm not completely sure.

1.) Consider keeping the main sensing/firing circuitry non-isolated, while using an isolated human interface. There may be some difficult to predict/correct errors associated with sensing via a step-down transformer. (Phase shift mostly, though maybe it's not that bad.)

2.) My first idea for a peak-firing circuit is the following:


* Rectifiers + high voltage resistor divider chain
* Envelope detector  + filter to create a DC voltage based on the line voltage. The DC level follows the input voltage. If 100VAC, the DC voltage might be 5V. If 230VAC, the DC voltage might be 12V. This way it works irrespective on line voltage.
* With a comparator, compare the stepped-down, half-wave rectified (pulsating DC) signal to the DC level above. When the pulsating DC passes through the DC level, the AC wave is at its peak. If you want to sense both positive and negative peaks, use full wave rectified.
* D-flip flop (I think) to accept user input to stop. Flip-flop only goes low once a peak signal is detected.
* Isolated user interface (trigger, knobs, buttons) can possibly be implemented with optocouplers. Bias current/voltage for the optocouplers can be derived from a low power isolated power supply. Can probably use a tiny isolated 100KHz push-pull converter, maybe something like SN6501.
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