Blocking oscillator

A blocking oscillator (sometimes called a pulse oscillator) is a simple configuration of discrete electronic components which can produce a free-running signal, requiring only a resistor, a transformer, and one amplifying element such as a transistor or vacuum tube.

The name is derived from the fact that the amplifying element is cut-off or "blocked" for most of the duty cycle, producing periodic pulses on the principle of a relaxation oscillator.

The non-sinusoidal output is not suitable for use as a radio-frequency local oscillator, but it can serve as a timing generator, to power lights, LEDs, EL wire, or small neon indicators.

If the output is used as an audio signal, the simple tones are also 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 circuit's simplicity, it forms the basis for many of the learning projects in commercial electronic kits.

Instead of a resistor, a potentiometer placed in parallel with the timing capacitor permits the frequency to be adjusted freely, but at low resistances the transistor can be overdriven, and possibly damaged.

Alternately the switch may get some of its control voltage or current directly from Vb and the rest from the induced Vs.

A negative control voltage will maintain the switch (e.g. NPN bipolar transistor or N-channel FET) open, and this situation will persist until the energy of the collapsing flux has been absorbed (by something).

In the simplest case, the duration of the total cycle (Tclosed + Topen), and hence its repetition rate (the reciprocal of the cycle duration), is almost wholly dependent on the transformer's magnetizing inductance Lp, the supply voltage, and the load voltage Vz.