You can design (or buy) an SSR with a 120VAC control input and zero-voltage switching output. Anything more than a few amperes will need consideration as to how to get the heat out. Probably run you $20-$30 for a safety-agency approved unit from a good maker capable of a few amperes (like the dimmer you mention).
Designing it could be accomplished with a zero-voltage switching triac output optocoupler with a mostly capacitive dropper and bridge rectifier at the input. The optocoupler would drive the main triac. Nothing would be isolated from the line in such a design, so proper care would have to be taken or it could easily be unsafe.
Such an installation (including the use of a UL/CSA approved SSR) may well not conform to code requirements and you'd be well advised to find out what your local regulations are- both for safety and to ensure your insurance coverage is not compromised.
Am I correct in calculating the observed V.us as 190
No, you're not, I think.
For simplicity, let's consider 'worst case' with 50% duty cycle (in fact it is not worst, all duty cycle are equivalent, but let me not go to such details at the moment).
First, transformer coil (because of C4 Cap) gets only AC component, so it magnetizes from -V.us to +V.us which effectively doubles available span.
Second, at 50% duty cycle C4 charged to the half of the driver supply (i.e. to 5V DC). So, primary winding gets only 5V amplitude).
Third, saturation should be prevented only while half of the clock period, because while the other half transformer magnetizes to the opposite polarity.
Hence transformer should have 0.5*5V*10us=25 (not 200) V.us product to avoid saturation. Datasheet states for Minimum V.us, so concrete sample may have value, greater than 15.
So, those are reasons, which in my opinion may explain why you find it works well at 53kHz.
Best Answer
The arguments against zero crossing switching transformers applies to primary of secondary. The claim (largely borne out in practice) is that the core saturates when subject to the rising voltage while unmagnetised. This is generally not an intuitive outcome. It makes sense when you consider that in an inductor under steady state conditions voltage and current are 90 degrees out of phase. By switching the voltage on at the peak of the voltage waveform you have maximum voltage and zero current so the 90 degree relationship is automatically established as the initial condition. You still need to build the "magnetising current field" but you are at the best starting point.
Nicely summed up in the final reference below:
The current starts out already settled.
At a minimum what happens is that the current waveform is offset relative to zero so that a current sinusoid DC offset by Ipeak/2 initially occurs. This leads to about double expected peak currents if the core does not saturate - and some sources suggest even higher initial surges, due to saturation effects.
The subject is covered reasonably well in Rod Elliot's inrush current -
and especially section 4. "Inductive & Transformer Inrush".
Useful paper here - less severe claims
Effect of Switching angle on Magnetizing flux and Inrush current of a Transformer
(ie saturation effects not dealt with)
Open electrical wiki - transformer inrush
Some practical experiences reported here To Zero Cross or Not To Zero Cross.
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