andy pugh wrote: > I have made a servo PSU. It simply rectifies mains voltage to give me 325V DC. > https://picasaweb.google.com/lh/photo/HdNJXBD5wPqDUi9E8C46HQ?feat=directlink > https://picasaweb.google.com/lh/photo/dTcU2i7GYh53xSrqjh--BQ?feat=directlink > There is a bit of extra componentry in the box to discharge the caps > on power-off and to switch in a resistor for soft start, but basically > it is just a rectifier and some caps. > > I am a bit puzzled about the status of my -V. > I guess that my 0V will be a few volts away from neutral, which is a > few volts from earth. Should there be an explicit connection between > my DC- and earth, machine 0V, mains neutral, none of the above? > > No, if you rectify the mains, the -V terminal will NOT be anywhere near neutral. It will essentially be floating near whichever mains terminal is currently more negative than the other. So, it will be swinging 120 times a second between neutral and -170 V from neutral, assuming this is on a US 240 V single-phase feed. > I think perhaps I should have a 1:1 transformer to float the DC, which > would then allow such a connection. > Obviously you have full galvanic isolation between the power transistors and the driving circuits. If anything in that isolation fails, you will have a HUGE explosion, with red-hot parts flying through the air, and your computer and servo interface will be blitzed. So, you should make sure that the isolation barrier has no "weak spots" that could be bridged by metal flecks, condensation, corrosion, etc. Also, make sure the motor connections are very well-made, and that movement of the axes could not cause a short to ground when the wires fatigue.
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