Hi, I have a very simple circuit setup to detect AC zero crossing. There's a mains AC transformer (230 to 12V) which connects into a full wave rectifier. There are no other components.

When I measure the raw AC out of the transformer with a scope I see a nice sine wave as expected. When I measure after the rectifier I see what almost looks like DC ripple but with every 2nd trough cut off. (screenshot attached)

Has anyone see this before and does anyone know what's causing it?

Name: DC Ripple.png Views: 1722 Size: 26.9 KB