LED CONSTRUCTION::
ELECTRONICLES:~
Electronics/Electrical FAQs::
In recent years LEDs and
LED lighting has emerged
as the leading
candidates for
next-generation lighting
due to their low energy
consumption, high
efficiency, and long
life.
The following are 25
topics and keywords
covering the
comparatively complex
field of LEDs.
Power Supplies
Topic 17: Switching power supplies
Power supplies are used in virtually all electronic devices
A switching power supply outputs a stable DC voltage based on rectification and control via a semiconductor switch that performs ON/OFF operation at high frequency. It is the preferred choice in a variety of devices due to its relatively small size, light weight, and high efficiency. Additional features are often included, such as noise filters, activation/rectifier circuits, and protection functions for better performance.
Topic 18: AC/DC and DC/DC converters
Essential for voltage rectification
• An AC/DC converter is a switching power supply that converts AC voltage to DC.
• A DC/DC converter is a switching regulator that converts one DC voltage (i.e. from a battery) to another. Normally features excellent switching efficiency.
Topic 19: Fixed voltage drive
Simple, cost-effective LED lighting method
This type of driving method provides the required current by supplying constant voltage to the LED(s). This is the simplest method to implement and is cost-effective. However, one major drawback is that the current will vary based on the different Vf of each LED and the heat generated by the chip itself. This will result in fluctuating brightness. Heat loss at the limiting resistor must also be considered.
Topic 20: Fixed current drive
Efficient lighting method
This type of drive provides constant current, improving efficiency while reducing heat generation. Although more complex and costly than constant voltage circuits, brightness fluctuations are minimized.
Topic 21: Duty control method
Required for adjusting the brightness without changing the color
This method
controls the
brightness by
switching the
LEDs ON and OFF
at high speeds
with constant
forward current.
The duty ratio
is expressed as
the fraction of
the ON/OFF cycle
the LED is lit
up (or ON).
Brightness is
determined based
on the value of
the forward
current (If).
This type of
circuit is used
in circuits
where the
brightness
fluctuates
greatly with
just a small
change in
current or
applications
where the color
tone varies with
brightness.
Topic 22: PWM control
Brightness based on duty factor
The PWM (Pulse Width Modulation) method controls LED brightness by varying the pulse width. The ratio the LED is ON is called the duty cycle. LEDs are turned ON and OFF at high speed and the brightness is adjusted by varying the ON time. If switching is early, residual imaging may occur.
Topic 23: Phase control
Brightness changes based on the amount of power
Phase control
involves
controlling the
power supplied
to the load
using power
control elements
such as
thyristors.
Current flows
only when a
signal is
supplied to the
Gate terminal of
the thyristor,
even if voltage
is provided.
Current (and
therefore
brightness) is
adjusted by
controlling the
conduction time.
Advantages
include
continuous
adjustment due
to the wide
output range, a
small, low-cost
control circuit,
and reduced
power loss.
Topic 24: Wireless communication
Signal transmission via electromagnetic waves
This method transmits information without wires. Various standards are used, such as infrared, Bluetooth, Zigbee, wireless LAN, and radio broadcast. Differentiating factors include transmission rate, communication distance, and power consumption.
Topic 25: Volt, Watt, Amps and C
A: That's not a
question, but okay.
Volts are electrical
potential, amps are
electrical current,
watts are total power
equal to volts*amps, and
C is electrical current
as a function of battery
capacity. Think of volts
as the width of a pipe:
In general, a wider pipe
has more punch than a
narrower one. Think of
amps as the water
flowing through a pipe:
Some pipes can only
handle little trickles
of water, and others can
handle lots of water
pushing through with
great force. Think of
watts as a combination
of volts (pipe width)
and amps (flow of
water): A large pipe
with water flowing
through really slowly
has the same output as a
small pipe with water
blasting through it.
This is why high-voltage
applications are
preferred over
high-current
applications, as a
stream of water zooming
at 200mph through a
1"-diameter pipe is much
more dangerous and
difficult to maintain
than a calm, 3mph flow
of water through a
4'-diameter pipe. As for
C rates, that's just a
function of current draw
and battery capacity.
Any power source
discharged at a 1C rate
will be depleted in 1
hour, any power source
discharged at a .25C (or
C/4) rate will be
depleted in 4 hours, and
so on. As an example, a
1.8Ah AA NiMH capable of
an excellent 10C
discharge rate can
manage 1.8*10=18 amps.
Topic 26: Parallel or
Series
Series connections
have a device's positive
terminal connected to
the next device's
negative terminal. This
is what you get when you
line up some ordinary
C-cell alkalines (for
example) end-to-end,
like in a Maglite or
other flashlight. This
arrangment adds up the
voltages of the cells.
Such a battery neither
handles more current nor
contains more mAh
capacity than a single
cell. This is the
opposite of a parallel
configuration, which has
positive terminals
joining together and
negative terminals
joining together. An
example is those 3AA>1D
adapters where all three
AA cells' positive
terminals meet at the
top, and all their
negative terminals meet
at the bottom. Such a
configuration has the
same voltage as a single
cell, but can handle
more current draw (or
contains more capacity).
For example, 1AA alk can
push about 500mA at
around 1.5V for about
four hours. 2AA alks in
series can push 500mA at
around 3V for about four
hours. 2AA alks in
parallel can push 1000mA
at around 1.5V for about
four hours (or 500mA for
eight hours, and so on).
Topic 27: Direct Drive
and Regulated
A direct drive (DD)
light is one that has
the battery directly
connected to the bulb or
LED. A regulated light
has some sort of driver
circuitry between the
two. A DD setup is
heavily affected by the
battery size and type.
In a regulated light,
the circuitry will try
to minimize the effects
of the battery. The huge
majority of incandescent
lights are DD. They
start out bright, then
fade over time. The
effect is greatest with
alkalines, which don't
do well in many
situations. The effect
is least noticable with
Lithium-Ions, which
maintain a steady
voltage under relatively
heavy loads. This is why
traditional Maglites,
which are DD by
alkalines, start out
bright for about half an
hour, then quickly fade
out and become dim for
the next few hours until
the battery gives up.
One example of a
regulated incan, which
provides rock-steady
output for the majority
of the battery life, is
Surefire's A2. In order
to drive mostly similar
LEDs with wildly
different battery
solutions, a regulation
circuit allows steady
output for as long as
the battery has power.
As an example, the Fenix
E0 runs on a single AAA
alkaline for eight hours
with no decrease in
output. If it were DD,
it wouldn't light up at
all, much less provide
constant output. An
appropriately DD LED
flashlight would be one
driven by button or coin
cells at somewhere above
the LED's Vf. This
results in a long
runtime with slowly
decreasing output,
determined by the
battery's remaining
power.
Topic 28: Boost and Buck
Boost and buck
circuits increase and
decrease, respectively,
the output voltage of a
battery. This is used
because of Vf
requirements (discussed
elsewhere in the Welcome
Mat). Such a circuit
will usually have
battery + and - inputs
as well as LED + and -
outputs. The interesting
thing about these
circuits is that they
can also be used to
tweak battery current
consumption, as a boost
circuit will draw more
current from the battery
than is flowing at the
output, and a buck
circuit will draw less
current from the battery
than is flowing at the
output. This generally
means that boost
circuits are hard on
cells, while buck
circuits are easier on
them.
For
example, 2AA NiMH
powering an XR-E with a
Vf of 3.7V at 700mA
would require a boost
circuit. If the circuit
was 100% efficient (not
actually achievable),
the following equation
would apply:
3.7V/2.4V*700mA= ~1080mA
This means that we can
use a lower voltage
source like 2.4V, but we
will have to draw over
1A to produce the
desired 700mA at the
emitter.
For
real-life circuits with
efficiencies under 100%,
simply divide the
required battery current
by the efficiency
(expressed as a number
between 0 and 1). For
example, an 85%
efficient boost circuit
applied to the above
situation would result
in the following
equation:
1080mA/0.9= ~1270mA
For buck circuits, the
opposite situation
applies. For example,
powering a 5mm LED with
a Vf of 3.4V at 20mA
with a 90% efficient
buck circuit on a 9V
battery would result in
the following equation:
3.4V/9V*20mA/.9= ~8.4mA
Keep in mind that these
are simplified
situations, with real
flashlights being
influenced by a number
of limiting factors.
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