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Issue #216 July 2008
Problem 5—Why did R3 and R4 burn up?
Answer 5—Note that each time one of the power transistors turns off, there’s still significant current flowing in L2. The stored energy needs to be dissipated somewhere before current can begin flowing in the other direction. It looks like the only available path is via the base-collector junction of the opposite transistor, through the corresponding base resistor and the winding on transformer B.
It’s possible that the circuit designer didn’t anticipate this, and sized the resistors only for the expected base drive signals coming through the transformer. The overstressed resistors heated up excessively until one of them failed, killing the circuit altogether.
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