An electrical transformer is an impedance matching device. This is a well-known fact. Here is an interesting application taking advantage of that fact.

If you load the secondary of a transformer with zero (0) ohms (short circuit), the impedance looking into the primary is also zero (0) ohms. Conversely, if the load on the secondary of the transformer is infinite ohms (open circuit), the impedance looking into the primary is also infinite ohms.

Now, if you insert the transformer primary in series with any load across a voltage source, you can switch the voltage to the load on and off. If you short the secondary, the voltage source is applied directly to the load since the transformer primary impedance is zero (0) ohms and all the voltage is fully dropped across the load. And, if you open-circuit the secondary, the source voltage is now fully dropped across the infinite primary impedance and zero (0) volts reaches the load.

Unfortunately electrical transformers are not perfect switches because of inefficiencies. The primary and secondary windings have some impedance which is present even when the secondary is short-circuited. Further, for a transformer to function, there is an excitation impedance present which prevents the primary impedance from reaching infinity, even when the secondary is open-circuited.

But, when a designer knows the load impedance, and the actual open-circuit and short-circuit impedance’s of the transformer, they can very often use this application to perform as effective switch. A good application for such a switch would be where the actual switching is done in a high-voltage circuit but the recognition of the switching action is required in a low-voltage control circuit. The transformer insulation system will isolate the high-voltage switch from the low-voltage control.