3 Phase 32A 400VAC Soft Start Power Supply


Construction of a 3 phase 32A 400VAC Soft Start Power Supply with safety features and a data monitor. All parts like enclosure, current transducers, voltage step down, contactors and wiring is reused from my 3 phase 13 A Lübcke variac build, that unfortunately had one of the auto transformers burn up.

Published: 26th August 2023.


WARNING!: Working with electricity is dangerous, all information found on my site is for educational purpose and I accept no responsibility for others actions using the information found on this site.

Read this document about safety!http://www.pupman.com/safety.htm


Firstly I wanted a simple power supply unit that could do basic voltage and current measurements. Secondly, be safe enough to not energize by itself when power is plugged in. Thirdly, have soft start feature for when powering large capacitive loads, that the initial inrush current does not trip my breakers.

I wanted to reuse as many old components and parts, from my storage, as possible. Despite that the PMA Data Monitor KS4010 is quite old and slow, I figured it would still be good enough with its 6 analogue inputs and 125 ms sample rate. The only downside to that instrument, is the rather big size. Mostly due to the fact that it is hosting a 3½ floppy disk drive for saving measurements.

32A 400VAC Soft Start Power Supply Schematic

Mains voltage schematic with start and emergency mains switch. Step down transformers for voltage measurement and current transformers are also shown.

Energizing the unit will light up the red lamp, indicating that it is not ready or is in soft start mode. For safety reasons, there is not a status light that can be trusted, other that all states indicate a powered and dangerous state. Using lamps to indicate powered and “dangerous” state, can lead to accidents, if the lamp is not lighting up when it should. Pressing the start button activates contactor Q3, that self-holds and turn on Q1. This also starts the count down timer that switches on Q2, which bridges out the soft start resistors.

 Construction details

21st July 2023

I spent a day taking the old enclosure apart, sawing up the frame and plates to be a smaller cabinet. I made the schematic and prepared all components and wires, so it was all ready for assembly in one go.

29th July 2023

For the voltage measurements I used 400 VAC to 20 VAC step down transformers acquired on ebay and through a single diode and a resistor voltage divider getting it to a level that the data monitor can work with.

Back in 2015, I found the green and affordable AC RMS current transducers. They output a 0-5 VDC signal, which is within the range of the data monitor inputs.


Lockable mains switch, soft start, self-holding start circuit that prevents output if power has been lost and data collected on the monitor makes this a very safe and versatile power supply.


Introduction, construction and test / demonstration video. For the test load I used a 9 kW electric heater, which at nominal voltage will draw 13 A.. For the soft start test I switched on mains to the DC bus capacitance of my DRSSTC 1 Tesla coil.

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