LED FAQ
Your frequently asked questions about LED
I collected in LED Faq answers to your most commonly
asked questions about LED lights. Suits visitors with little
or no technical knowlege or the do-it-yourselfer.
LED
general
NEW!
Low
voltage or High voltage saves more energy? ... >>
AC
LED - What is it? ... >>
Light
Emitting Diode
(LED) - what is
it? ... >>
"My
LED is 5 Volt" - what
does it mean? ... >>
LED
array - what is it? ... >>
Circuitry
NEW!
What is Ohm's Law? >>
Adaptor
-
What is it and Why ... >>
Inverter
- Why I need it >>
LED driver
-
what is it and what type ... >>
Dimmer
-
Can I use with my LED? ... >>
LED
internal
dimmer - what is it and how ... >>
Resistor
- what type ... >>
Resistance
of a
LED - what is it and how ... >>
Operation
Exact
current: If I know the exact voltage across my LED can I ... >>
LED
meltdown
- what is it and how ... >>
Maximum
operating point
- what is it? ... >>
I
heard about safe
operating point - what is it? ... >>
What is PWM? ... >>
Distribution
Q. What is OEM
A. An abbreviation
that stands for
Original Equipment Manufacturer.
NEW!
Q. What is Ohm's Law?
A. In short:
I
= V / R (This is the original formula.) It establishes a
reversed relationship between Resisance (R) and Current or Amperage
(I), which can also be understood as a kind of traffic flow of
electrons (negatively charged particles.) Current is said to be
stronger if more charges pass through the same section of a conducting agent -- like a copper wire in the cable.
V is Voltage in the formula.
But lately, we speak of another formula too. One which establishes
relationship between power (W) and voltage (V.) It goes like this:
I = W / V; where I = (W / R)
2
What is important from this relationship, is that
as a rule a low voltage light
draws higher amperage for the same brightness (measured
with 'W' ) unless resistance is less by a power of two (or a square
root.) But that rarely happens, here's WHY:
To operate from the mains low voltage lights need a transformer (
adaptor in techno speak), which
has a coil in it to work. Current passing through that coil meets high
resistance (you hear the typical low humming of a transformer), and quite of a lot of the
power (W) generated turns into heat. The rest powers your low voltage outlet.
NEW!
Q. Low voltage or High voltage LED
light saves more energy?
A. First,
they both use a
magnitude less energy then incandescent and halogen lights and still
less then compact fluorescent lights.
But between the two types of LED
lights? - Low voltage lights actually draw higher
amperage (refer
Ohm's Law.)
This means that you
ONLY
save more energy with a Low voltage LED light than with a High voltage
one if you do one of these:
- Use maximum possible length operated from the same
transformer -- best exploited with a low voltage LED rope light; OR
- Power your LED from a low voltage source such as a 9V
battery.
LED Faq General
Q. AC LED - What
is it?
A. It is a LED with a built in
converter.
Q.
Light Emitting
Diode (LED) - what is it?
A. LEDs
are semiconductors, diodes in particular.
A
LED should have high hole mobility inside the semiconductor crystal
emitting
the light. But high hole mobility also means that the full content of
any section of the crystal is unpredictable to a large extent. So your
bright LED is quite unpredictable as to
exact
Resistance (R), also.
See Ohm's Law for Resistance's role >>
more
Q. I have a
"5 Volt LED" - what does it mean?
A. Such
specification refers to the maximum operating voltage. As a rule the
LED described should mostly operate below that maximum. 5
Volt is its
top bearable limit also called the
maximum operating point.
It is not to be reached or reached only occasionally for very short
periods of time.
Q.
LED array - what is it?
A. LEDs can be single chip
based, or LEDs can populate many independent LED chips built as one
package called array.
Circutry LED Faq
Q. Adaptor (or converter) -
What is it and Why I need it with my LED?
A. Adaptors are
also called converters or power adapters. For
low voltage LED lights (12V, 24V) you would use an adaptor. It
transforms
your high voltage main (120V, 240V etc) to the low voltage at which your LED
can operate.
Q. Driver - what is it
and what type should I use with my LED?
A. A
LED driver is the circuitry (a self-contained supply of power) that
powers your light source with the needed drive current. But this
current is still quite lumpy so we need to finetune the output that
reaches the LED. For this to be accomplished we also need a
resistor
built in the driver circuitry.
Drivers
should be current regulated, so they deliver a consistent
current over a range of load voltages.
Actually, your LED can be
overdriven past their continuous operating currents and
generate higher peak light outputs (within some limits) by operating
them at a
reduced duty cycle. This will hold the average current and therefore chip heating within the continuous operating limits.
Q. Can I use
a dimmer with my LED?
A. The
short answer is No. The longer? Yes and no. You may not use an external
dimmer if your LED is not equipped with an internal dimmer.
See LED dimmer >>
more
Q. LED
internal dimmer - what is it and how it works?
A. Internal
dimmer is a LED driver equipped with a dimming control. A LED driver
can create dimming by by way of pulse width modulation (called PWM)
circuits.
Q.
Resistor - what type should I use with my LED?
A.
Resistors regulate current to keep it to a limit
or an
acceptable range before it reaches your LED. Current and voltage in a
resistor are linearly related (see
Ohm's Law.) For this reason it is best to use a current-limiting resistor instead
of a voltage-limiting resistor. -- might be pricier...
See LEDs resistance >>
more
Q. LEDs
resistance - what is it and how it behaves.
A. Unlike
conductor materials, LEDs are semiconductors and don't have one fixed
resistance, they have varying resistances. The current flowing in an
LED is an exponential function of voltage across the LED.
This
means that a small change in extra voltage can result in large change
of current.
See resistor for role in LED circuit >>
more
See resistance in Ohm's Law >>
more
LED Faq - LED light Operation
Q.
If I know the exact voltage across my LED can I determine the exact
current?
If so, how?
A. No
you can't unfortunately, not exactly. If you know the voltage across
the LED that doesn't mean you know the exact current flowing thru your
LED at that time. LEDs are funny things. They have high mobility inside
the semiconductor crystal emitting the light. This means they are quite
unpredictable as to finding out the exact resistance at a certain
voltage - therefore the exact current flowing thru your LED. All you
can do is spot checks in various discrete stages. At this stage we do
not have a formula. Once we'll have one, it'll still likely to be too
complex to be used by an average do-it-yourselfer.
Q. LED
meltdown - what is it and how it "works"?
A. The
thermal runaway effect ultimately killing your LED is popularly called
LED meltdown. The current through the
junction will
tend to increase as the temperature rises. This in turn will heat you
junction further. Beyond a certain maximum point this can not be
reversed. Your LED will melt down and die. Current-regulated drivers
can counter this tendency whereas voltage-regulated drivers tend not
to.
See proper LED light heat management >>
more
Note:
I wrote this about LED bulbs, but they
also apply to other LED products
Q.
I heard about safe operating point - what is it?
A. When
you buy an LED, it should come with a rating that looks similar (not
identical) to this: 3.3V @ 20 mA typical. I reality, that "typical"
data only gives you
one single point along the
operating curve of various known and measured data points. Usually,
that single point is the safe operating point.
Q. Maximum operating point - what is it?
A.
You may get
a maximum operating point either as current or voltage. For example,
when you read that your LED is "
3 Volt" you are actually given a
maximum operating point expressed in voltage. What it means is that
your 3V LED usually operates below that voltage and has a
maximum limit
of 3V.
Q. What is PWM?
A. PWM is short for
Pulse
Width Modulation. It is digitally
encoded light
output control that regulates the cyclical peaks and lows of
power -
called duty cycle - released to your LEDs via a special program. It is
often
hard-coded in a separate device called
controller or micro
controller.
When PWM is used, it is to
exceed the maximum continuous output available from your LED.
See PWM for LED circuitry >>
more
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Enjoy my Web article clippings. This one for the
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LED
FAQ Glossary, Tutorial
Good glossary: http://www.nuhorizons.com/glossary/optoelec.asp
Very Basic LED tutorial (but well
illustrated.) I recommend for the beginner do-it-yourselfer as an
additonal to this LED Faq.
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