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March 10, 2012


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Edwin Pettis

Some years back, I was the project engineer on a large component burn-in system. The system was supplied off of a 3-phase power transformer so that the loads could be as balanced as possible. I had designed TRIAC controllers to ramp up the line voltage to minimize the line surge with +90 degree phase shifting. Each power supply was a very large, 50lb, power transformer with multiple secondaries. To tell a long story in short, the 'chief' engineer insisted on replacing the three small power transformers I had specified with the one big monster and he also insisted on using zero cross over phase control to minimize the current suges despite my arguing for the other method. To him, zero voltage across the primary meant zero current....

The PC boards were finished per the Chief's changes and a few power supplies were built. When they were switched on, there was a snap and flash of light in each one of them, a PCB trace was vaporized due to the huge magnetizing current resulting from the zero cross over phase control. Since the boards had already been built, modifying them was out of the question, the only thing I could do was to place a large ballast resistor in the primary to limit the current surge. The Chief refused to accept the premise that it was his zero cross over and monster power transformer causing the problem.

Andreas Fechter

Thank you for helping me avoiding a classic, but big mistake!
I was looking for a good idea to switch over (on the fly) two power supplies: one coming from a photovoltaic driven line inverter (24V DC -> 230V AC),the other one is the common line supply, both of each 230 V AC. And naturally, as "open source" supply one can never exclude the inductive components - up to the
USV driven PC station....

tx again!

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