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:



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 serious LED buffs!

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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