This page is devoted to the discussion of all
semiconductor devices with just a single P-N
Diodes are components that allow current
to flow in only one direction (up to a point, but more on
that later), due in part to the fact that they only have
a single P-N
Like many components, diodes have a positive side or
leg (a.k.a, their anode),
and a negative side (cathode).
When the voltage on the anode
is higher than on the cathode
flows through the diode (the resistance
is very low). When the voltage is lower on the anode
than on the cathode
then the current
does not flow (the resistance
is very high).
An easy way to remember this is to look at the symbol
for a diode -- the "arrow" in the diode symbol points the
direction in which it allows current
flow) to flow.
of a diode is generally marked with a line next to it (on
the diode body). You can see a similar line in the
schematic symbols, above.
Diodes are also some times marked with an identifying
color code (similar, but not identical, to that used for
a good explanation is given here.
Note that when current
is flowing through a diode, the voltage on the positive
leg is higher than on the negative leg (this is often
referred to as the diode's
drop"). The magnitude of the voltage drop is a
function of (among other things) the semiconductor
material that the diode is made from.
Silicon diodes (for
example, the 1N914
the most common and cheapest, and have a forward voltage
drop of about 0.65 volts.
(for example, the 1N34A)
have a forward voltage drop of about 0.1 volt. Germanium
diodes, though, are typically much more expensive than
silicon diodes; luckily, they're salvageable
from lots of circuit boards. To test whether or not a
diode is Germanium, get out your multimeter, put it on
diode test, and measure the diode's forward voltage drop
directly. A Germanium diode will read less than 0.3, a
Silicon will read above 0.5.
You can avoid voltage drop entirely in some cases by
using a transistor instead of a diode -- see Wilf
Rigter's article here.
The Zener diode is designed to have a specific
reverse breakdown voltage (i.e., conduction voltage when
Because of this, Zener diodes can be used by themselves
as voltage-sensitive switches, or in series
with a current-limiting
to provide voltage regulation.
junctions are light sensitive; photodiodes are just
junctions that are designed to optimize this effect.
Photodiodes can be used two ways -- in a photovoltaic
(here it becomes a current source when illuminated -- see
solar cell), or photoconductive
To use a photodiode in its photoconductive mode, the
photodiode is reverse-biased;
the photodiode will then allow a current to flow when it
has an interesting site on the technology behind
All diodes emit some light when forward-biased.
LEDs are made from a special semiconductor (like gallium
arsenide phosphide) which optimizes this light output.
Unlike light bulbs, LEDs rarely burn out unless their
current limit is passed. A current of 0.02 Amps (20 mA)
to 0.04 Amps (40 mA) is a good range for LEDs (never go
past 50 mA). LEDs have a forward voltage drop of about
LEDs have a cathode
and an anode
just like regular diodes. To determine an LED's polarity,
you can do one of three things:
- Look for a line in the metal inside the LED (it
may be difficult to see). This line is closest to the
of the LED.
- Find a flat spot on the edge of the LED -- this
flat spot is on the cathode.
- The anode
of an LED is generally longer (at least, when it's a
When current is flowing through an LED the voltage on
the positive leg is about 1.4 volts higher than the
voltage on the negative side (this varies with LED type
-- infrared LEDs have a lower forward voltage
requirement, others may need up to 1.8 V). Remember that
there is very little resistance to limit the current, so
a resistor must be used in series
with the LED to avoid destroying it (note, though, that
some panel-mount LEDs come from the factory with a
current-limiting resistor soldered to them).
Also note that LEDs can be used as photodiodes
(tho' their sensitivity is relatively low, so they're
only useable this way in very bright conditions).
Flashing LEDs (FLEDs)
A flashing LED is just an LED with a built-in
microcircuit to cause it to flash periodically. Since the
FLED draws current
when it flashes, we can use FLEDs to drive a number of
timing-dependent circuits (via the fact that it
periodically becomes conductive). In particular, see the
discussion of the FLED
design. Like other LEDs, FLEDs
are light-sensitive, and so flash faster in brighter
light. Note that some FLEDs
need 3 V minimum to work in, but FLEDs
don't in general require current-limiting resistors (at
least, I've never seen one that does).
I have a writeup on diode usage in BEAMbots here.