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Thanks, all, for the comments and lessons!?
It seems we're converging on using zero-crossing transformers that are capable of switching whatever current the transformer would draw if only the winding's DC resistance limits the current.? I measured the HT-37 primary to be 0.616 ohms (4-wire, Keithley 2100 DMM).? A #18 AWG power cord 1.5 meters long would add 63 mohms to this and a 4A SB fuse about 31 mohms more.? The wiring from the AC socket to the breaker panel might add another 100 m ohms.? This means the maximum surge current could be as high as 150A if there is really no back emf.? The estimate from the Opto22 document suggests 10 times nominal which is much less than this.
The HT-37 transformer is probably close to 250 VA, so the [M]P120D4 is the OPTO22 recommendation.? 4A continuous and 85A for a single cycle, 66A for 3 cycles.? This seems ample capacity if using Opto22's "10 times" estimate.? If a turn-on drives the transformer into saturation, then the additional current that flows doesn't actually cause any more flux and thus no voltage surge at the HV secondary.? OTOH, a relay that turns on immediately could cause the flux to rise more quickly, to saturation or not, causing the voltage spike.? Did I get this right?
In many cases, I the zero-crossing SSR in use actually does have the capacity to handle the surge current even though the person installing it wasn't paying attention to this issue.? Thus, many people using ZC-SSRs haven't had problems.?
The part I don't understand is:
"Large inrush currents can occur during the first half cycle of line voltage if a zero-voltage SSR
happens to turn on during the positive half cycle of voltage when the core is already in positive saturation." (Opto22 document cited earlier in this thread)
How could a transformer already be in positive saturation if it has been unenergized for a while prior to turn-on?
Halden VE7UTS