Service manual

STP 11-25R13-SM-TG
P - 4
(4) Transistor Ql could also develop a short or an open. If an open were to occur in either the
emitter or the collector, the current that would normally flow through the load device could no longer flow.
Therefore, the output voltage would drop to zero and the voltage drop across Ql would be at or near the
value of the input voltage.
(5) Now, what if Ql were to develop a short? Once again, the symptoms would depend on the
exact location of the short. For example, if the short were to occur in the emitter-base junction, the
emitter current of Ql would no longer flow through Ql's collector; it would find an easier path through
resistor R2.
(a) As a result, E
R2
would increase, thus increasing the reverse bias of Ql's collector-base junction.
The internal resistance of Ql would increase as a result.
(b) Since Ql would contain more resistance, it would drop more of the input voltage and so the
output voltage would decrease.
(6) Now suppose that the short were to be from the emitter to the collector. With both of its
junctions shorted in this way, Ql would be unable to drop its share of the input voltage. Thus, the load
device would have to drop most or all of the input voltage and this could cause the load device to burn out
if it were unable to dissipate the extra power caused by the increased current flow through it.