A few months ago we did a blog post about our variac. It was tripping the circuit breakers on our distribution board and we made some mods to the variac to reduce the inrush current. Our story generated a lot of interest and was picked up by Hackaday. We used a 2 terminal NTC inrush current limiter from Ametherm.
Following our story, the good folks at Ametherm and Rhopoint Components (our local distributor) were in touch with us. They gave us some great technical advice and did some calculations for us. They have a whole range of limiters and it turns out that we had actually chosen quite a good model for our application. They kindly sent us some samples.
There are many ways to soft start a load. We originally thought about adding a series power resistor and a shorting relay. We also had some suggestions around Triac and IGBT solutions. Each of these methods have their merits and can allow fine grained control of switch-on. Unfortunately there was very little room inside the variac case and we didn’t want some kind of external box. So we looked at these relatively simple 2 terminal limiters.
Carefully chosen, these components are an elegant solution. They can probably last the life of the application with no maintenance. Placed in series with the load, they have a relatively high resistance at room temperature which limits the initial inrush current. When current flows through them they self-heat and the resistance falls very quickly, so they have little effect on the normal operating current.