I think the essence of it is: the pcb "rings" the transformer with a sub-20V AC signal, while putting a CRC snubber across the winding under test (2 parallel nodes - a C and an RC). The first C is 0.01uF, and the RC is a 0.15uF in series with a 1K trimmer resistor. You use two scope probes: one to trigger on the falling edge of the ringer signal, and the second on the winding to look at the ringing. You adjust the scope to see the oscillations, and then dial the trimmer to snubber same. Then you pop out the trimmer and measure the resistance, and that plus the two C values gives you your snubber.
The much longer and detailed (and accurate) version is attached.
It seemed to work fine on the BeePre, but I was having problems getting it dialed in on the Kaiju - so I suspected I was shorting the PT incorrectly. But I now think that at least part of the problem is the trimmer that came with the parts kit - the BOM calls for a Murata or a Bourns 3296W (about $2 each), but my kit came with a "Bochen". It's gone all screwy on me. It's supposed to be 25 turn, but with less than a quarter turn the measured resistance can shoot from say 10R to 2000R and then drop to below 1R. I looked up "Bochen 3296W" and found it for sale from a major Chinese online retailer @ 100 pieces for $12! As best as I can tell, no US/Canada retailer sells them. I am going to pick a Bourns tomorrow morning from a local electronics store.
But I'd still like to know if I'm shorting the primary correctly.
cheers and many thanks, Derek
