2014-04-13 Joule Thief As Armstrong Oscillator

In the wiki for Joule Thief on Wikipedia, someone claimed that the Joule Thief is an Armstrong Oscillator.  I dispute that claim and prove it’s wrong in the following discussion.

An oscillator is capacitance and inductance ( j.omega )that forms a resonant circuit, and according to Barkhausen criterion, an amplifier that is a linear circuit and excites this resonant circuit by maintaining its oscillations by replenishing the losses during oscillation. The resonant frequency is the point of minimum loss, so the circuit operates at this frequency.

The Joule Thief does not have a tuned circuit because there is no capacitor. It also is not a linear circuit – it operates as an on/off switch. Some claim that the capacitance is the parasitic capacitance of the circuit, most of which is the collector capacitance of the transistor. Typical transistors used in this circuit have about 3.5 picofarads capacitance.

Using a typical coil, the inductance of the collector winding is about 100 microhenrys. With 3.5 pF capacitance across the coil, the resonant frequency would be more than 8 MHz. The typical Joule Thief doesn’t operate at 8 MHz, it operates at 100 times lower, or about 80 kHz.

The Armstrong Oscillator puts out a sine wave at about the resonant frequency of the capacitor – inductor combination. But the Joule Thief operates 100 times lower, or about 80 kHz, which is much lower than the resonant frequency. Therefore, for the above reasons, the Joule Thief is not operating as an Armstrong Oscillator. The Joule Thief is operating as a relaxation oscillator, using the inductance and resistance of the circuit as a flyback voltage booster. Thus the Joule Thief is simply not an Armstrong Oscillator.

Check references other than Wikipedia. It can’t be any simpler.

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