People have been replacing failing linear power supplies with switchers for years, at least as far back as the RGVAC days, so this is nothing new. People would also do this to the 6809-based Williams hardware (Stargate, Robotron, etc.) with success but would complain about occasional loss/corruption of CMOS memory (settings and bookkeeping information). Someone eventually figured out that the reason was because of how the old linear supply worked alongside the board's CMOS write protection circuit. The linear had a larger capacitor on its +5v line that was there for more than just regulation - it kept the +5v line up longer than the other voltages. The board's CMOS protection circuit monitored the +12v line, and when it started to go down, it would kick in the CMOS write protection, before the +5v got low enough for the CPU to go nuts.

But when you moved to a switcher, all of the voltages came down at about the same time. This left no time for the CMOS protection circuit to work, and you would get occasional stomping of your CMOS memory. People suggested that adding a large cap to the +5v line should allow the +12v line to drop before the +5v line, allowing the CMOS protection circuit to kick in.

Back in '09 I figured that since a voltage detection circuit was already possible, I wondered if there were any simplified implementations of them floating around. Enter Microprocessor Supervisor circuits like the Maxim MAX820 and MAX792 circuits. Those were a little heavyweight and could actually also implement the CMOS protection circuit that was already on the board, but did not want to have to do any mods to the board. I wanted something that could just drop in.

I never went any further with it because I was as confident in my abilities back then and did not even want to think about PCB fabrication. Recently, when I got back thinking about Crowbar Circuit, I started thinking about other old ideas that might have a place in the Arcade and Pinball repair communities. The LVR Circuit is the result of one of those trains of thought.

I figure a target of $5 for just the PCB? Dunno if I want to be selling assembled units.