??? 06/26/04 06:19 Read: times |
#73185 - Dangerous situation?? Responding to: ???'s previous message |
Hallo Charles,
I don't know your 'Simpson' voltmeter, but if it's working similar to the well known 'Avometers', then the terminals of movement coil are only one diode's voltage drop (diode bridge of voltmeter) away from 'AC common' terminal of voltmeter!! If you now connect the voltmeter in such a way, that 'AC common' terminal gets eroneously connected to 'phase', then a rather dangerous situation can occur, if you connect your board to a PC via RS232 interface (DB9): Pin 2 and pin 3 of U13A get heavy overvoltage, because ground terminal of R17 is now connected to protection earth! Of course, then you would also have direct contact to mains voltage, when touching the movement coil sides of R14 or R16!! So, I would highly recommend you to connect the voltmeter not directly to mains voltage, but via a small saftey class II transformer (safety transformer)! But maybe I'm wrong, and the 'Simpson' is working quite differently... Regards, Kai |