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Isolated zero cross detection w/ AC mains

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Circlotron:
To switch off the triac at the peak you would have to force commutate it, and trials are lousy at that. You can do it with an SCR but it is a bit of work. If you are interrupting current in an inductive circuit you need to have some sort of snubbing to deal with the energy in the inductance. Possibly not worth the effort.

Ian.M:
Why would you even want to force  commutate at the peak?

TRIACs naturally commutate when the current through them falls to zero, which is when you would want to switch off an inductive load anyway to avoid a massive back-EMF 'kick' from stored energy.   Assuming an iron core inductive load, switched off at zero current, that leaves it with the residual magnetization from the previous half-cycle. 

N.B. immediately after switched off, it will *NOT* have zero volts across it as for an inductive load the current lags the voltage, so it wont reach zero current till somewhere in the half-cycle after the one you last triggered the TRIAC in.

It would be a *BAD* *THING* to switch it again at a voltage zero crossing, because with no current flowing initially it has a whole cycle to build up, to what would be more than double its normal max. flux.  This of course results in saturation and a massive surge current at switch on. Ignoring remnant magnetization, switch on at peak voltage, when the current would be zero for a perfect inductor and the flux buildup in the first half cycle is halved.

However to make sure the flux in the first half cycle OPPOSES the residual magnetization, switch on during the OPPOSITE phase half cycle to the last whole half cycle it was on for.  This can be critically important if your transformer is running right on the ragged edge of saturation, e.g. microwave oven transformers with the original primary, and if the application permits it, is good practice.  Otherwise if the application doesn't permit a delay to let it choose the half cycle, it may be advisable to move the switch-on to a little after the voltage peak.   

Edit: corrected choice of half cycle to switch on

schmitt trigger:
Iroc;
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.
Most times, all you require is an external fuse and/or a thermo-magnetic breaker. Just follow the datasheet recommendations.


The Crydom SSR example linked below has an output rating of 24 to 280 volts. If you are switching 120 volts then you have nothing to worry about.

https://www.digikey.com/product-detail/en/sensata-crydom/EZ240D5R/CC2235-ND/752094

beduino:

--- Quote from: schmitt trigger on October 16, 2019, 11:58:41 pm ---how are you galvanically isolating the gate driver from the SCR itself?

--- End quote ---
In my spot welder with bulky quite big transformer, I use two 2N6504 SCRs in inverse parallel with AC mosfets switch instead of optotriac MOC3020 in similar circuit shown below, so I can use eg. TLP351 optoisolated mosfet gate driver to turn AC  mosfet switch on/off powered from galvanic insulated power supply eg. small transformer.


Ian.M:
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.
Mouser have ST branded 40A 600V TRIACs in stock at under $5 USD e.g. T4050-6PF, which is in an isolated TO3 package so can directly bolt to a grounded heatsink.  Its supposedly snubberless, so with a suitable snubber should commutate at least as well as the paired SCRs.  To maintain the 600V blocking voltage rating, I would suggest a MOC3052 random phase OptoTRIAC to trigger it.

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