Safely box-in transformer so that I can play with it without risking shock

[king.oslo]

Diver, why would i want a thermistor and not a resistor in series with the transformer?

And it is rated at 1.4W. Surely that isn't enough?

Thanks for your time!

Kind regards,
Marius

 

[Diver300]

When you first turn on a transformer, the core often saturates and there is a huge current for a short time. Even a small resistor will reduce the current to levels that don't cause problems like blowing fuses or tripping circuit breakers. In this case, 5 ohms keeps the current below 50 A which the supply can easily manage for the 1/4 cycle or so that the inrush lasts for.

However, at full load, there is 2 A flowing all the time. That would generate 20 W of heat in the resistor, so a large resistor would be needed and 5 % of the power would be lost, as well as the fact that the voltage feeding the transformer would be 5% down.

So once the transformer is turned on, the resistor isn't needed. A complicated solution, which is often used on larger transformers, is to short out the resistor after 1 second or so. However that needs a timer and a switch, as well as the resistor.

The thermistor does the job of the resistor, timer and switch all in one, and it is cheap. It starts at 5 ohms, and limits the current just like a resistor. However, when the transformer draws current, the thermistor heats up, and being an NTC thermistor, its resistance falls, so that it never gets too hot (although it would burn your fingers) and the voltage drop is small.

The thermistors I linked to are specifically designed for that inrush limiting.