GE 4PE350A3 Instructions Manual page 47

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EBI-6351
transistor
leads
at the point where they
enter, or attach to, the header. Some leads
bent during testing or handling may easily
break later since the bending life
of the
lead has already been spent. Plated leads
subjected to excessive bending and twisting
can generate cracks at the header; such
cracks offer openings for moisture to enter
and contaminate the device, thereby causing
gradual degradation
of
gain and voltage
characteristics. To insure against this, al-
ways allow for a clearance of at least 1/16
to 1/8 inch between the header and the start
of the lead bend.
3. Overheating
If, during soldering, the maximum spec-
ified junction temperature of a device is
exceeded, the device can be destroyed. Heat
transmitted over connecting leads and printed
circuit board leads to the header can also
be destructive. Junctions can be shorted.
Lead connections may open. Unequal expan-
sion between the header and the package
may break the hermetic seal. Safety pre-
cautions include the removal of the tran-
sistor from the immediate socket to which
heat is being applied, keeping in mind that
the heat can quickly travel along connecting
wires to neighboring sockets, the use of
heat shunts (clips, pliers,
etc.) connected
between the heat source and the device, and
the use of a soldering iron of heat delivery
adequate for the job to be done. Most small
-
signal transistor circuit work can be ac-
complished by use of a 20- to 50 -watt iron.
Larger irons can be used, of course, but with
increased chance of damage to the device.
At any rate,
it
is always best to
"heat -
shunt" to insure against damage and to solder
cleanly and quickly.
ELECTRICAL ABUSES
1. Excessive Voltage
Do not ever exceed the absolute maxi-
mum voltage (usually specified at 25 C)
given by the manufacturer. In signal am-
plifier circuits this means that peak -to -peak
voltage swings should not exceed the inter -
element absolute maximum voltages of the
transistor. A good rule is to use a supply
voltage equal to half the maximum voltage
rating. Maximum inter -element voltages can
also be exceeded by voltage transients (in-
ductive
and capacitive kicks,
etc.) when
connecting a transistor into a hot circuit.
BEFORE REMOVING OR REPLACING A
TRANSISTOR IN A CIRCUIT, ALWAYS TURN
THE POWER OFF. Transistor testing by
use of an ohmmeter can also cause damage
by application of excessive voltage. Since
the emitter -base reverse breakdown voltage
for most transistors is from one to five
volts, the transistor can easily be damaged
when subjected to the high -voltage ranges
of an ohmmeter (many use 222- to 30 -volt
batteries). When measuring breakdown volt-
age, always use a current limiting resistor.
Voltage spikes can cause a build-up of im-
purities concentrated at a point in th, col-
lector or emitter junctions and can result
in punch -through (internal short from col-
lector to emitter) across the base region.
2. Excessive Power
Exceeding the maximum junction tem-
perature of a transistor can permanently
change the gain and breakdown voltage, and
cause opens and shorts. To guard against
such damage when testing for gain at exces-
sive power dissipation levels, use a pro-
tective heat sink or test with a low duty
cycle pulse.
3. Miscellaneous
When a transistor is used in the com-
mon emitter configuration, opening the base
while voltages are still applied can result
in junction heating, thermal -runaway, and
eventual burn-up of the transistor. Even the
right conditions of applied voltage, current
gain, and reverse leakage can be destructive,
particularly to germanium transistors, where
leakage currents may be a thousand or more
times greater than in silicon transistors.
With the base disconnected, the collector -
to -emitter leakage (Ice°, base open) equals
the collector -to -base leakage (Icbo, emitter
open) magnified by the forward current gain
(beta) of the transistor. High values of Ice°
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