Considering building this. Not really sure it is necessary but it is something to do. Found this circuit intended for 220/12V 50Hz and planning on using a Triad F-25X 120/12/6V 60Hz transformer in a small project box. Added fuse and input on/off switch. The circuit was for 12V secondary and I added selector switch to utilize the Triad's secondary center tap for 6V. I also see some circuits using for R2 a 10K pot to attenuate current? Is it needed? Comments and advice welcomed.

 

Hello Sam,

Techlib has a nice page on the octopus circuit:
http://www.techlib.com/electronics/curvetrace.html
Bertus

 

@bertus Yes, I looked at the TechLab but was a bit unsure with ganged multi-selector switched resistors. The reason I am somewhat ambivalent is the cheap chinese M-Test does a very good job. Also looking into the using a function generator and scope together method. I have some BNC tees but had to order a male to male BNC cable to do what I think I need it to. http://literature.cdn.keysight.com/litweb/pdf/5990-8366EN.pdf

Thx, Sam

 

Hello Sam,

It looks like the keysight document is only looking at the voltage behaviour.
A octopus circuit also looks to the relation of the voltage versus current behaviour.

Bertus

 

As R2 is the current sense shunt resistor, its accuracy is or is not important, depending on your requirements. If you want to be able to look at the beam deviation on the scope graticule and determine the current peak-to-peak value, then a fixed, precision resistor (1% tolerance or better) is needed, preferrably with a non-inductive construction. If all you need is an easy way to scale the display so it is not running off the edges of the screen, then a pot is fine.

ak