The NTE2355 and NTE2356 transistors are designed for use as digital
switches. They have built-in biasing resistors and a typical maximum frequency
of 250 MHz. Their emitter-to-base breakdown voltage is higher than most, at
10V.
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Some low-power circuits that connect to an RS-232 port don??™t need an external
power supply. Instead, they draw all the power they need from the interface
itself.
Figure 4-3: This 5V-only interface will work on many short links.
Chapter 4
56
The power comes from otherwise unused outputs. To comply with the standard,
an RS-232 driver??™s output must be at least 5V with a 3k load. Each output
can thus source at least 1.6 mA at 5V. In practice, most RS-232 outputs can
source more than this minimum current, but staying within the standard
ensures that a circuit will work on any port.
.
!
Figure 4-5 shows ways of using RS-232 outputs as a power source. When
asserted, RTS and DTR are between +5 and +15V. Figure 4-5A shows an
unregulated output. To double the output current, tie two lines together as
shown, with a 1N5819 Schottky diode in each line. The diodes prevent current
from feeding back into the interface if the voltages differ. You can use any rectifier
diodes, but Schottkys have a lower forward voltage than other silicon
diodes. You can even use the TX line as a power source by setting the Break signal,
but of course this means you can??™t use the line for data.
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