I was looking at the references in the Joule Thief Wiki at Wikipedia, especially the first reference, [1] after “Armstrong” in the first line of the text. This refers to an article which describes this as an Armstrong or Meissner oscillator. This article then points back to Wikipedia as a reference. What we have is a case of one article supporting the other, with no factual support from any ‘foundation’ article, hence no actual factual support. I don’t know what Wikipedia’s policy is on this, but to me it’s unethical and a worthless reference.
Also, I don’t believe that this is an Armstrong oscillator. In the Wikipedia article for Armstrong Oscillator, it says “inductance and capacitance” (my emphasis). The Joule thief does not have any capacitance, therefore it is not an Armstrong oscillator.
One might claim that it has stray capacitance, but at the very low switching rate – less than 100 kHz – at which the Joule Thief switches, the few pF of stray capacitance is insignificant; it plays no part in the circuit’s operation.
I’m going to write this up in the discussion section of this wiki.
Hello,
I find it interesting, because most people, me too, doesn’t know about the Joule-thief working principle. In the German Wikipedia it stand that the Joule-thief working as a unregulated boost-converter which has characteristics of a blocking oscillator.
Ps. I find your site interesting 🙂
Petr
The situation is much like the blind men and the elephant: everyone thinks he knows what it is, but everyone has a different idea. Worse, no one can explain it simply and clearly.
Any progress on determining if the circuit should be considered in the class of armstrong oscillators or not? I’ve noticed some references which say the basic circuit can be modified to be made Armstrong.
Maybe someone should write a section on Wikipedia that talks about its relationship to the Armstrong oscillator.
IMHO an armstrong oscillator has a tuned circuit consisting of an inductor and capacitor. Since a Joule Thief does not have a capacitor, it does not have a tuned circuit and thus cannot be an Armstrong oscillator. It’s simply a matter of definition. If a capacitor is added to make a tuned circuit, then yes, it can be.
I heard back from a query to the authors of the paper that is cited on Wikipedia. Here’s the meat of the response:
“Wording is always difficult in electronics. People with a more mathematical background often use different definitions than people with a more circuit experience based background.”
“Defining an electronic oscillator by its type of feedback, an oscillator with magnetic coupling (transformer) is an Armstrong/Meißner oscillator. The inductance of the LC-circuit in the joule thief is part of the transformer and the capacitance is hidden in the parasitic effects of the components, mainly the transformer and the pn-junctions of the transistor contribute to the capacitance. The joule thief runs in a relaxation oscillator mode, as a “blocking oscillator”. In contrast to an harmonic oscillator, where the amplifier is in its linear region all the time, the relaxation oscillator “jumps” between “maximum” output and “minimum” output. You can tune your Joule Thief to an harmonic oscillator by reducing the feedback gain, e.g. reduce the coupling of the inductances or add a resistance between the base and transformer. The light output of the LED will be reduced, but the output voltage will look more like a sine.”
As you say, this definitely seems to come down to definitions. I don’t really have a strong opinion on this one, but I thought I would pass this along.
After reading that reply i almost hit the delete key (not because of you, though). So if it’s not in the “Armstrong mode”, but it’s in the ” relaxation mode” then it is a relaxation oscillator. If they admit it’s a relaxation oscillator, then why do they call it something other than a relaxation osc? DUH!
I posted a question on Electrical Engineeeing Stack Exchange to try to change this: https://electronics.stackexchange.com/q/363218/ and it worked. One of the leading members did this. I wanted to get agreement from the EE community before changing this myself. I’m glad that I did. Merry Christmas, Rusty Bolt!