I left a few comments on this Youtube Video of a Joule Thief that behaves oddly. The waveform on his o’scope is the usual JT waveform, but it ‘gallops’ – the tops of the waveform have a periodic change up and down, which seems to change with the scope timebase setting. He gives a schematic later in the comments. Essentially it’s just a simple conventional Joule Thief with the 1k resistor replaced with a 50k potentiometer. He calls them harmonics, but I would call them subharmonics because harmonics are multiples of the fundamental frequency, but these are lower than the fundamental frequency.
He investigates this further with a followup video where most of the clip leads have been removed, leaving the circuit much more visible and uncluttered. The elimination of the clip leads means less stray capacitance which might be affecting the circuit. As can be seen, the circuit is simply a Joule Thief.
Now that I see what the circuit looks like, I can replicate the circuit exactly as shown. I’m using a DSO-2250 digital sampling oscilloscope with the software running under XP. I connect the scope probe to the collector and the scope ground to negative. The waveform I see does not have any of the galloping or subharmonics that his circuit has. I have scoped dozens of JT circuits with my scope and also with an analog o’scope, and they just don’t have any of this odd behavior.
He says that he has changed coils and transistors and the odd behavior remains. I don’t remember if he said he had changed the LED. The only other parts are the battery and pot, and they are so simple they should not be contributing to the odd behavior.
My only conclusion is that the mini digital sampling oscilloscope he is using must be causing the odd behavior. I am guessing that as it is sampling it is internally generating enough of a signal that the signal is beating with the JT input, which mixes and causes the subharmonic. I don’t think that the power line could be causing it, because the JT circuit and scope are not connected to the power line – both are battery operated. To verify this, it would be easy to put the JT and scope on a tray and take them outside and away from any power lines such as the inside wiring of a dwelling.
Back to pondering…