Just For Starters
Only a brief pulse is necessary to
turn on an SCR — and it stays on.
There are two ways to turn it off.
The first is by applying a negative
pulse to the gate and the second is
by disengaging the output load. This
second method is popular as a reset
in alarm circuits (see Figure 4).
One thing that is necessary
when working with SCRs is a
constant output load. Most well
designed circuits will provide a
steady load to maintain the on condition of the SCR. Experimentation
will determine whether or not the
circuit will sustain an on condition in
an SCR controlled situation.
Figure 5. Diode isolated voltage sequencer
LEDs
by heavy, slow, expensive mechanical relays can be executed by an SCR. Loading problems are another disadvantage
of relays that disqualifies them from low current circuits,
especially since they need a constant DC input to stay on.
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Light Emitting Diodes (LEDs)
have been around for quite a while now and almost all electronic devices have one or more. The LED is made by
adding metals — such as aluminum, gallium, and indium
— to arsenic antimony and phosphors. By changing the
ratio of elements, it is possible to vary the color, the
amount of infrared radiation, and the brightness greatly
from one LED to another.
Wherever you need a visual indication of electrical
activity, the LED is the answer. Seven segment displays
enable light emitting diodes to indicate letters and
numbers and are used in clocks and calculators. Arrays —
consisting of several LEDs in a single case — are used to
indicate sound levels in recording devices. New products
are being developed all the time and amazing devices are
finding their ways to the shelves of your suppliers.
Diodes as Isolators
No circuit demonstrates the ability of diodes to pass
Figure 6. Bar graph sound level indicator
16
Circle #150 on the Reader Service Card.
SEPTEMBER 2004
