I recently purchased a used 10A Carroll & Meynell Variac from eBay for use in the lab, however the variac often caused the 32A B-curve MCB in the consumer unit to trip due to the high inrush current of the variac core. To prevent this from happening and to add a few additional features I created the soft start circuit outlined on this page.
The idea of the soft start circuit is to limit the inrush current whilst the variac core is first magnetising. There are many ways to achieve this and most involve adding some form of resistive element in series with the transformer to prevent the transformer appearing as a very low impedance to the mains AC supply.
The method used in this project was to use a high power resistor in series with the transformer to limit the current. Once the current has settled, the resistor is shorted out by a relay to then allow the full load current of the variac to be drawn from the mains.
The schematic is attached below:
The electronics adds a few additional features such as an emergency stop button and remote control of the variac, however, it’s worth noting that this is not really a suitable ‘Emergency’ stop button which should really be a fully hardware implemented solution – NOT involving software running on a microcontroller!
The contactors used in this project are a little bit on the physically large side, partly due to the plug-in form factor:
Squeezing everything into the box was a little bit of a challenge, so the PCB is tucked in near the variac core:
The electronics also needed their own power supply. If the overall current draw of the electronics and contactor coils was less than about 50mA, I would have been tempted to just use a simple non-isolated capacitive dropper circuit from the mains, however I chose to use a small linear power supply with a toroidal transformer:
When assembling the device together, it was important to make sure there was enough clearance around the variac core and the wiper assembly – the whole wiper becomes energised when the variac is powered up!
Here is the completed photo of the internals of the variac:
And finally with the covers on: