I have built a zero crossing detector that acts as a square wave generator according to the circuit diagram shown. 

It is connected to a potential transformer. 

The output from the potential transformer is measured to be 6V at 50 Hz. 

However, the output from the ZCD is measured to be 100 Hz. How has this frequency doubled?

BTW, when I connect a current transformer to the same ZCD set up, the output from the current transformer is 50 Hz and the output from the ZCD is also 50 Hz.

The IC used for the Op Amp is LM358 and the diodes are 1N4148.

![Circuit diagram of ZCD](https://i.stack.imgur.com/3DoNg.jpg)