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). When the absorber is in the primary circuit, e.g. a Zener diode (or LED) with voltage Vz connected "backwards" across the primary windings, the current waveshape is a triangle with the time topen determined by the formula Ip = Ipeak,m - Vz×Topen/Lp, here Ipeak,m being the primary current at the time the switch opens. When the absorber is a capacitor the voltage and current waveshapes are a 1/2 cycle sinewave, and if the absorber is a capacitor plus resistor the waveshapes are a 1/2 cycle damped sinewave. When at last the energy discharge is complete, the control circuit becomes "unblocked". Control voltage (or current) to the switch is now free to "flow" into the control input and close the switch. This is easier to see when a capacitor "commutates" the control voltage or current; the ringing oscillation carries the control voltage or current from negative (switch open) through 0 to positive (switch closed). Repetition rate 1/(Tclosed + Topen) 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. When a capacitor and resistor are used to absorb the energy, the repetition rate is dependent on the R-C time-constant, or the L-C time constant when R is small or non-existent (L can be Lp, Ls or Lp,s). Patents US Patent 2211852, filed in 1937, "Blocking oscillator apparatus". (based around a vacuum tube). US Patent 2745012, filed in 1951, "Transistor blocking oscillators". US Patent 2780767, filed in 1955, "Circuit arrangement for converting a low voltage into a high direct voltage". US Patent 2881380, filed in 1956, "Voltage converter". See also Flyback converter Forward converter Joule thief Footnotes References In particular §7-13 The monostable blocking oscillator" p. 203ff and §7-14 The astable blocking oscillator p. 206ff. For a tuned version of the blocking oscillator, i.e. a circuit that will make pretty sinewaves if properly designed, see 17-17 "Resonant-Circuit Oscillators" pp. 530–2. External links An elementary (no mathematics) and informative description of various blocking oscillator circuits, employing BJTs and Triodes. A paper deriving some circuit models in order to predict the switching performance of