?

?

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

Jeff, as a guess, your likely insulation breakdown was enough to continue building a carbon track between windings even though the 5v was isolated; and must have eventually destroyed the T-T [turn to turn] insulation on a winding and eventually became a real short. Any T-T short in a transformer is fatal . don



From: [email protected] [mailto:[email protected]] On Behalf Of Jeff AC0C
Sent: Monday, July 1, 2024 5:55 PM
To: [email protected]
Subject: Re: [HallicraftersRadios] HT-37 transformer failure due to rapid STBY-->OFF-->STBY



Two the fellow's earlier comment on not using the 5V line, when
reworking my 32A, that was the first thing I did - move to SS
specifically to idle the 5V line and avoid the risk of the well reported
short involving the 5V line. Toward the end of the process, I was
getting ready to finish up alignment and had let the transmitter run
overnight to ensure it had reached stable temp across the chassis. When
I came down the next morning, the rig was dead. Painfully confirmed
later that some amount of windings on the primary or secondary had
shorted (there were no detectable winding-to-winding shorts at idle -
and I did not have a spare HVPS available at that time to test higher).
However, as the variac was ramped up, the current draw of the
disconnected transformer soared. So the 5V to HV short is apparently
not the only transformer failure mode with this model, unfortunately.

I solved the problem by cannibalizing another HT32. But assuming Murphy
may like to come see me soon, I bought one of those shiny Dahl
transformers and it's here on the bench ready to run to the rescue when
the current transformer eventually dies. Of course, with my luck,
simply having bought the Dahl transformer may keep Muphy away allowing
the current transformer to run the rest of my days. ha ha.

73/jeff/ac0c
alpha-charlie-zero-charlie
www.ac0c.com

ah7i

That thing was probably a Dampp-Chaser (with two P's). They were
used in some receivers and in other equipment to keep moisture out.