Power supply design question 1 / 2 - shock hazard - Page 1
Forum section: electronics / projects

author: metertech58761
author_rank: Frequent Contributor
timestamp: August 04, 2024, 04:35:04 pm
content: So I reverse engineered a load management terminal and have two separate questions, so I intend to have two separate threads as the two don't relate to each other much.Anyway, this board set is used in three different pieces of equipment - a load management unit (base unit), distribution control unit (base unit + different firmware + daughterboard - no change to the power side), and field configuration unit (modified unit to make it 'portable').The schematic below is how the power side of the unit would be connected for the load management / distribution control modes. The incoming 240V 3-wire power would be connected at E22 (Hot 1), E21 (neutral / center tap), and E24 (Hot 2).Panel B carries the transmitter circuitry, with the drive and relay control signals coming from the logic circuitry. Panel D is the main power supply, analog interface, and incoming signal pickup (this was part of a system that communicated over the power lines via modulated carrier wave injected into the 60Hz mains).I just want to be sure. This should be the ONLY part of the circuitry that has mains voltage +/- 170V DC, right? There shouldn't be any kind of shock hazard on the primary of T1 or the secondaries of T3 and T4, right?The Drive signal is a TTL-level digital signal, Xmt_Rly is a TTL-level signal fed through a ULN2003A buffer, T3 is the main power transformer with two separate +12V supplies and +5V supply, and Signal is a low-level analog signal fed to the front-end receive amplifier circuitry.I'll get into why I asked in my next post on this thread where I show the modifications that were done to make this a 'portable' unit, and which made it impossible for me to analyze that unit.

author: metertech58761
author_rank: Frequent Contributor
timestamp: August 04, 2024, 07:25:24 pm
content: I came across one of the 'field configuration' units on Ebay some time back, and was surprised to realize that it had one of the 'load management' unit board sets inside with three additional boards.Two boards were an interface board (which connected between J402 and the logic board) and a display board (which connected to the interface board).The third board was a power distribution board, which appears in the lower half of the schematic below, to the right of P404 / P405.The +12VA connection had a wire running to C47+ on Panel D, +12V running to C53+, and from C47- to GND on the power board.There were also connections running from VBUS to C10+ (!) and from C10- to GND (?!!!).I didn't realize the implications of that connection until I went to retrieve the EPROMs while the unit was powered down but still plugged in.I HATE getting stung by 120V!!! (and I do mean stung, as I don't see much difference from a wasp sting)Supposedly that power distribution board is to allow the unit to be powered from vehicle 12VDC.I ended up rigging up a 12V SMPS to feed power to the unit through the 12VDC power input (J401 pins 26, 27) to try and take a few readings.However, getting zapped like that burned the interest out of me (other than picking apart the code), and I ended up passing the unit along to a friend who plans to open a museum if he can find a suitable space.So, am I right in assuming that the connections to C10 created a dangerous shock hazard inside the unit? I tried opening the negative lead to C10, but the shock hazard persisted.Weren't D405 / D406 supposed to block the +170V?Thoughts?