Blocking oscillator

Basis of solid-state Blocking Oscillator (using NPN transistor)

A blocking oscillator is the minimal configuration of discrete electronic components which can produce a free-running signal, requiring only a capacitor, transformer, and one amplifying component. The name is derived from the fact that the transistor (or tube) is cut-off or "blocked" for most of the duty-cycle, producing periodic pulses. The non-sinusoidal output is not suitable for use as a radio-frequency local oscillator, but it can serve to flash lights or LEDs, and the simple tones are sufficient for applications such as alarms or a morse-code practice device. Some cameras use a blocking oscillator to strobe the flash prior to a shot to reduce the red-eye effect.

Due to the simplicity of the circuit, it forms the basis for many of the learning projects in commercial electronic kits. A secondary winding of the transformer can be fed to a speaker, a lamp, or the windings of a relay. A potentiometer placed in parallel with the timing capacitor permits the frequency to be adjusted, but at low resistances the transistor will be overdriven, and possibly damaged. The output signal will jump in amplitude and be greatly distorted. The frequency of the oscillator is also affected by the supply voltage.

External links

• The blocking oscillator, web page by James B. Calvert. An elementary (no mathematics) and informative description of various blocking oscillator circuits, employing BJTs and Triodes.
• Circuit models to predict switching performance of nanosecond blocking oscillators, J. McDonald, IEEE Transactions on Circuits and Systems, 1964, Volume 11, Issue 4, 442- 448. A paper deriving some circuit models in order to predict the switching performance of BJT blocking oscillators.