Need help

jimkeith

Joined Oct 26, 2011
540
C5 and R5 form a differentiation circuit that turns the positive going square wave edges into short pulses that turn on the flyback transistor for short periods of time. D1 provides a discharge path for C5 when the square wave goes negative.
 

hgmjr

Joined Jan 28, 2005
9,029
The reason for restricting the excursion of the negative-going spike at the base of the transistor as noted by jimkeith is that if the amplitude of the negative-going spike is significant, it can damage the transistor. So the diode is there as a protective measure against damage to the transistor's base-emitter junction.

hgmjr
 
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