So, when the lockdowns began rolling across the country last year (just as I was set to head down to a collectors' swap meet and spend a nice spring weekend at my friend's acreage in the Ozarks), I ended up dusting off my long-dormant electronics hobby.
I had stumbled across a 'field configuration terminal' on the E place that I could use to play around with some load management devices I'd picked up at some point - it's complete and works, but is missing the power / interface cable (I checked and rechecked the wiring several times before finalizing the construction of the temporary plug I have for it).
I'd planned to work with the original unit as-is, but after reverse engineering the all the boards and recreating the schematics, I realized the internal wiring of the original unit is just plain dangerous (the DC ground has a healthy dose of AC riding on it, even when when the main switch is in its OFF position)
I'm sure my USB logic analyzer took a fatal jolt at one point, as it doesn't seem to work properly since.
So... I decided to see if I could replicate this unit, with three design goals:
1) replace obsolete / inefficient parts with modern equivalents
2) eliminate that $%&?!@#?!! hot-chassis wiring
3) replace the transmit stage with one that works at a lower, SAFER, DC voltage
So, I'll present the boards as they are currently, explaining what I started with and my line of thinking towards my present design.
First up - the all-important power supply.
Pretty straightforward... I started with a transformer-type supply that, as I said, somehow introduces an unhealthy amount of AC to the DC side (how the unit even works like that, I don't know!!!).
The 5V supply used a LM340K-5, and the two 12V supplies (one for the power / display side, the other for the analog side) were nothing more than bulk supplies with discrete bridge rectifier and filter caps.
My sticker shock over how much the LM340K-5 / LM309K commands today led me to look for alternates, and I found some attractively priced DC-DC modules, and which in turn led to the discovery of and the complete replacement of the power supply circuit with three AC/DC SMPS modules (+5, +12, and +24).
The capacitors on the output were chosen based on the data sheet and are near the output terminals, and I did test this board and I have good, steady outputs on all three supplies.
The second jack on the AC side goes to the Analog board for signal coupling, and I'll get to the Analog board a bit later.