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Reaction Scanning



The horizontal output stage is the work horse of TVs and monitors. The horizontal output stage consists of the flyback transformer and the horizontal output transistor (H.O.T.). It is the source of high deflection current, and its byproducts are high voltage for the CRT andlower DC supplies for signal processing, video drive, CRT (screen) and in some cases, G2 focus. Because of high voltage and high current the flyback transformer and the H.O.T. is very prone to failure.

Reaction scanning, developed by Nicoli Tesla in the early forties, makes use of energy stored in yoke and flyback transformer at the end of the H trace. This provides retrace (flyback) and generates the pulse that the flyback boosts to drive the high voltage rectifier.

Deflection starts at zero yoke current (electron beam at horizontal (H) center). At this time, the H.O.T. switches on connecting the primary supply across the yoke. This causes current to rise in a linear fashion. It may seem odd that current rises linearly, but at the high switch rate, the yoke behaves as an almost pure inductor. Think of a linear ramp of current causing an expanding magnetic field at a constant rate of change in the coils. The result would be a fixed induced voltage. Conversely a fixed voltage results in a linear ramp of current.

When the point, the H.O.T. cuts off. Current is at its peak and the CRT beam is at far right. The field built up around the coil now collapses rapidly inducing a high voltage. The coil is now a source and polarities reverse. Energy has no place to go but to charge the shunt (stray) capacitance. The circuit in fact "rings" at the natural resonant frequency, about 70 kHz. In a quarter cycle current swings through zero, and voltage peaks (point Ÿ). The capacitance starts its discharge and current reverses building to a peak in the next quarter cycle (deflection far left). At this point , the field again collapses and the coil becomes a source. The ring would continue in sine wave fashion, but now the polarity of the coil source has again reversed and the damper diode comes into play. The coil discharges through the low impedance of the B supply. Current thus drops towards zero as a linear ramp. Now the H.O.T. turns on again and the cycle repeats.

Due to losses in the circuit energy contributions by the H.O.T. and the damper are not equal, and the conduction interval for the H.O.T. is longer than 50% of the scan time. The high flyback pulse that appears at the collector of the H.O.T. is boosted by the flyback transformer to drive the high-voltage rectifier.