Just For Starters
help achieve a critical threshold
point. Oscillators, pulse generators, and RF energy generation
are the areas where the tunnel
diode is most applicable.
Voltage regulation is an
important function in modern
electronics and the zener diode is
designed to do just that. It achieves this
in the reverse bias mode. You can see in
the diagram (Figure 2) that the zener is
wired across a power supply output
with the anode lead connected to
ground and the cathode lead connected
to the supply output.
Like regular diodes, zener devices
resist the flow of voltage when reverse
biased. The zener, however, is designed
to resist the flow of voltage only to a
given breakdown point. Anything over
the breakdown point will be passed to
ground. Acting as a reference source,
the zener diode is capable of providing
very accurate, constant current loads.
Popular cutoff points are 5, 9, and 12
volts to accommodate most power
Figure 3. Diode types
diode. The varactor acts as a voltage
sensitive capacitor in series with a
resistance. Its use is basically
confined to microwave equipment.
in Nuts & Volts.
An SCR (Silicone Controlled
Rectifier) is like a diode with an extra
lead. The device will not pass current
until a pulse is applied to the third lead,
which is called a gate. Many switching
functions that were formerly controlled
Figure 4. SCR operation
One of the most unusual devices
is the varactor or variable-reactance
As you can see in the illustration,
diodes come in a wide variety of
shapes and sizes. The size of a diode
will most likely relate to its power
handling capacity. Leads are identified as anode and cathode and —
on most diodes — polarity is identified by a line on the cathode side,
as shown in the diode type illustration (Figure 3).
Your local supplier should have
most types in stock. Low level glass
diodes, such as the 1N914, are
inexpensive (about 10 for $1.00)
and widely available. Harder to find
diodes can be ordered from supplier catalogs or from the advertisers
Circle #121 on the Reader Service Card.