Strange DC ripple

[The Master]

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?

[MichelJasmin]

Do you have some kind of load?

[The Master]

Nope. There's no load. Could that be the problem?

[MichelJasmin]

Yes I think. Try with something like 1k ohm. Probably the capacitance of the junction or some other esoteric phenomenon.

[richard]

I have seen that before , I was due to the ground reference for the scope not being the same as for the rectified waveform

[The Master]

I've tried connecting a resistor as the load but it didn't have any effect.

I've actually got 2 circuits. The one described in the initial post is a test circuit to try to isolate a problem I was seeing in the full circuit. The scope ground is connected to the negative side of the rectifier output in both circuits.

The full circuit has 2 resistors used as a voltage divider between the rectifier output and a PIC chip. It also uses 6 diodes for the rectifier to ensure the rest of the circuit doesn't affect the zero crossing detection.
The PIC chip is also only seeing half of the zero crossings (which is what prompted testing with a scope).
I've attached a rough schematic with the relevant parts of the full circuit.