Page 235 - 15 Dangerously Mad Projects for the Evil Genius

210 15 Dangerously Mad Projects for the Evil Genius

+V of about 0.6V between the base and emitter when a
transistor is turned on.

Using a 150 base resistor, we could therefore
control a collector current of 40 to 200 times
30mA, or 1.2A to 6A, which is more than enough
for most purposes.

In practice, we would probably use a resistor of
LOAD
1k , or perhaps 270 .
Transistors have a number of maximum
parameter values that should not be exceeded,
otherwise the transistor may be damaged. You can
find these by looking at the datasheet for the
transistor.
c
For example, the datasheet for a BC548 will
b
contain many values. The ones of most interest to
Rb
us are summarized in Table A-3.
e
TABLE A-3 Transistor Datasheet

GND Property Value What It Means
Figure A-4 Basic transistor switch circuit
I c 100mA The maximum current
that can flow through the
most transistors, if the load has zero resistance, the collector without the
current flowing into the collector will be 50 to 200 transistor being
damaged.
times the base current. However, we will be
switching our transistor fully on or fully off, so the h FE 110–800 DC current gain. This is
the ratio of collector
load resistance will always limit the collector
current to base current,
current to the current required by the load.
and as you can see, it
Too much base current will damage the can be anything between
transistor and also rather defeat the objective of 110 and 800 for this
controlling a bigger current with a smaller one. So, transistor.
the base will have a resistor connected to it.
When switching from an Arduino board, the
Diodes
maximum current of an output is 40mA, so we
could choose a resistor that allows about 30mA to In addition to LEDs, there are also “normal”
flow when the output pin is high at 5V. Using diodes. These act a bit like one-way valves, only
Ohm’s law: allowing current to flow through them in one
R V/I direction.
R (5 – 0.6)/30 147 We have used them in a few of the projects in
this book to prevent unwanted currents from
The “– 0.6” is because one characteristic of
damaging components, or when driving inductive
bipolar transistors is that there is always a voltage