Octopus questions

[DPotts245]

I cobbled together a simple octopus circuit.  I have questions and odd results.

I'm using an external 6.3Vac power supply.  The probes are not connected to anything. I should be getting a horizontal line on my oscilloscope but I'm getting a reading that more resembles a capacitor.   

What do the 3 ground wires connect to?

Are the lack of ground circuits the cause of the odd readings?

[janoc]

Floating inputs - you are likely picking up mains hum. Which is roughly sinusoidal, so it looks like a circle on the screen. Two perfect sinewaves would create a circle in XY mode. However, since there is noise and distortion on the mains hum, you get that distorted shape.

Basically you have stuck two antennas into the inputs of your scope.

If you want to build this curve tracer, you need to build it into a shielded enclosure and ground it, this cannot be built "free-form" and with no grounding at all. The three ground connections connect together, to the shielded (metal) enclosure - and the ground of your scope through the outer conductor of the coaxes you use to connect this to the scope inputs.

[DPotts245]

Thanks for the reply. 

Previously I had an octopus built on a small circuit board and installed into a plastic housing.   See this post. https://www.eevblog.com/forum/beginners/stupid-octopus-question/#msg5330774

I'm still at a loss as to how I had the whole thing grounded before but your explanation of the ragged circle makes sense.

[DPotts245]

I sorted it out.   With all 3 grounds tied together I'm getting the expected results from the circuit.

Apparently during the move I'd disturbed the grounds in my original setup.   

[janoc]

Glad to hear it is working again.