connecting a scope probe to a resonant LC circuiit

Thread Starter

Scott McKie

Joined May 26, 2013
I previously have been successful in connecting a scope probe to an LC circuit using a toroidal core, in order to view it's maximum waveform at resonance. The probe was connected to the ground input and it's ground clip connected to the V+ connection to the LC circuit.
I added a second winding, wound at 90 degress (physically in quadrature) to the first winding, and again was succesful in connecting the scope probe with no visable increase in tank current, which was around 40 -- 50 mA / 12 VDC. But suddenly I am no longer able to connect the probe/w ground clip without the circuit's input current increasing to over 2 Amps and the V+ voltage dropping to 0 VDC.
However, when I connect the scope probe to the second winding, the input current now returns to just above 200 mA (still not where it used to be, but workable) and a sinewave voltage waveform is present on the second winding. The measured "L" value is now 20-30% higher than originally measured, and thus the calculated resonant frequency is 20-30% lower than originally measured. However: when the scope probe is connected across the second set of windings; when it's sinusoidal waveform is increased to it's maximum -- the operating frequency has increased back to the original resonant frequency of the single winding LC circuit.
What I need is an explanation as to what changed within the L values of the LC circuit itself so that I can again connect the probe across the LC circuit itself and thus get back to the original input current values. Thanks.


Joined Feb 3, 2014
What changed? You added a winding right? Coils build up current and release it in the opposite way of capacitors. It doesn't take a lot to change things.

Got a schematic, pictures?


Joined Dec 13, 2013
when checking the resonance of my tesla coils, I use a large value resistor to cancel out the capasitance of the probe. also I use a similar large value resistor to isolate a signal generator when injecting signals. 100k to 1 meg is usually suficient.