Strange DC ripple

[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.

[MichelJasmin]

I'm not sure the regulator will like to have a capacitor at the output and a full ripple at the input. I've learned (30 years ago) you have to put a diode between the in/out pins of linear reg to protect them for any situation where there is a lower voltage at the input vs the output. Also that may explain the waveform you have.

Anyway, take a look a this very interesting circuit:

http://www.dextrel.net/diyzerocrosser.htm

[The Master]

I didn't know that but it makes sense as I've blown regulators up by soldering them in the wrong way round before.

My original thought was that the capacitor was causing the strange waveform so I setup the test circuit which has only 4 diodes (full wave rectifier) and now a resistor for load. There's no capacitor or regulator attached.

Thanks for the link. It's definitely more complicated than what I've got now but it will be really useful as I'm gradually moving towards switching power supplies