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116 votes
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Why can't I light a LED with a 1.5 V battery?

LEDs don't work like ordinary (incandescence) light bulbs. Main differences (a bit simplified for very beginners): They have a polarity, hence they must be powered using DC respecting that polarity....
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109 votes
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Why does this LED bulb have an energy efficiency rating of 'F'?

EU energy ratings get re-done regularly; a rating indicates placement in the field of competitors at the time of rating. Things that were A a couple years ago might now be G; hence, you need to ...
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92 votes
Accepted

What is the purpose of tiny "bumps" on LED legs?

From here this is what the leadframe looks like. This is what they look like when they are molded in small scale production: After this, the parts are separated but the die is made so that it does ...
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85 votes
Accepted

Why did LEDs take so long to appear as light bulbs?

It is not possible to produce white light without an efficient blue LED, either using RGB LEDs or a blue LED + yellow phosphor. The breakthrough was the invention of the high-brightness Gallium-...
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  • 165k
73 votes
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Why are all my yellow 2V/20mA LEDs burning out with 330k Ohm resistor?

Those resistors aren't doing anything at all. All the pins in a row are connected together.
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  • 45k
67 votes
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Easy way to figure out a LED's Vf in order to pick an appropriate resistor

I agree with some of the others here... you're trying too hard. As others have mentioned, the forward drop of an LED varies with its bias current, but for almost every application a hobbyist will get ...
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65 votes
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Why should you use two resistors in parallel on an LED?

Don't look to the arduino designs as examples of stellar electrical engineering. However, there can be a legitimate case for doing this. This part contains 4 resistors. If it was already there for ...
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65 votes

How can I wire a 9-position switch so that each position turns on one more LED than the one before?

Using a regulated current source to light them, wire the LEDs in series and short out the segment which you want to be dark. simulate this circuit – Schematic created using CircuitLab You can ...
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61 votes

Why does my blue LED keep burning out?

Any ideas? Yes. You are feeding the LED with an AC supply. That means that every second half-cycle it will be reverse biased. Most LEDs can withstand about 5 V in reverse. You're applying 24 V AC ...
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  • 160k
59 votes

Why don't LEDs have built-in resistors?

What value resistor would you include? For what voltage? For what brightness? How would you make the resistor when it uses a different process than the LED does? How would you dissipate the heat? ...
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  • 160k
57 votes
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If Infrared not visible, why the red LEDs?

The simple answer is that they are using near IR. LED manufacturers have a good handle on how to make them so they are affordable. Their center frequencies may be invisible to the M-1 eyeball (i.e. ...
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  • 1,714
56 votes

Why can't I light a LED with a 1.5 V battery?

I see Lorenzo has already answered your question directly (+1). Here is what you can do to light your LED and see what you've got. LEDs are diodes, so conduct only in one direction. Unlike a ...
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56 votes
Accepted

Why does an LED have to be a diode?

The existing answers miss the core of the question. An LED needs to be a diode, specifically because the way the charge carriers recombine in the forward-biased diode junction releases the correct ...
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  • 165k
56 votes
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Why use two transistors driving LEDs when one would work?

It's a constant-current circuit. The current should be about 0.7/68 ~=10mA. simulate this circuit – Schematic created using CircuitLab Your circuit with one transistor would allow a highly ...
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55 votes
Accepted

Why can't RGB or bicolour LEDs produce a decent yellow?

Most probably because of short wavelength of your green LED and not as monochromatic green as you might expect (x and y coordinates closer to the center). If you take a look at the CIE 1931 curve and ...
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53 votes
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Why does a longer fiber optic cable result in lower attenuation?

This is where the measurement scientist has to go into full sceptical and investigative mode. First thing. Fibre, as a passive material, is lossy. It absorbs power. Therefore the power arriving at ...
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  • 142k
52 votes

Why hasn't my LED burned out by now?

Properly designed, built and used, today's LEDs have incredibly long lives and the wearout mechanisms are not catastrophic in nature. So instead of using incandescent lamp MTTF statistics, a ...
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  • 1,996
50 votes
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Why are LEDs on most embedded designs inverted?

It's still the case that MCU I/O pins often have weaker drive sourcing current than sinking current. In a typical CMOS MCU output, when they drive LOW, they turn on an N-channel MOSFET; and when they ...
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  • 70.2k
49 votes
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Why are SMD LEDs shipped in a sealed bag containing desiccant?

Through-hole components are generally hand or wave soldered. These apply on local heating to the pads and not to the component itself. SMD parts on the other hand are generally reflow soldered. This ...
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49 votes
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Why did my LED resistor burn while lighting four LEDs in series?

6.8 volts seems awfully high for a single LED. Are you sure that 6.8 is not the number for all four LEDs? That would make it 1.7 volts per LED, which is more reasonable for a red LED. And that would ...
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  • 2,398
44 votes
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What's the intuition for reading transistor/resistor circuits?

What's a step by step way of thinking about the flow of electrons in such a circuit? How can we tell it would provide constant current for instance? The first step is to forget about flow of ...
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  • 160k
44 votes
Accepted

Why is my LED circuit drawing 50% more mA than I calculated?

This is almost certainly within tolerance of expected behavior for these LEDs. A quick way to see this is to do a back-of-the-envelope sensitivity analysis. What's the current if the LED forward ...
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  • 3,379
42 votes
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What are the various 'properties' of a diode?

DC Level 0 Diode The level-0 approximation of a diode is simply an assumed voltage drop across it when forward-based and infinite impedance when reverse-biased. When reverse-biased, no amount of ...
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41 votes
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Are LEDs better than we think?

Yes, this is widely known. Anyone who has tested it knows that. The die manufacturers certainly know it. They don't specify LEDs for more than 5V reverse voltage because it would not measurably ...
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40 votes
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What are the 'knuckles' on through-hole LEDs and photodiodes for?

I have two possible explanations: The 'knuckles' are there intentionally to avoid the pins going all the way through the PCB. Most of the times it is not desired that the pins are going all the way ...
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  • 15.7k
40 votes

How can voltage burn out an LED?

Voltage and current are intimately related. If you attempt to increase the voltage across an LED, the current will increase. Likewise, to increase the current through an LED, you must increase the ...
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40 votes

LED power consumption in theory and reality

Your PSU meter is only has resolution to 0.01 A (10 mA). The actual current could be anywhere between 5 mA and 15 mA for the single LED. Switch your yellow multimeter to mA range, connect the leads ...
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  • 160k
40 votes

Why did my LED resistor burn while lighting four LEDs in series?

I see your problem. Your circuit shows how you're driving a single LED segment. (I presume you then have 7 of these circuits, one for each segment.) The datasheet shows 4 LEDs in series, covering ...
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40 votes
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How are LEDs considered efficient?

You seem to be getting confused between the efficiency of the LED and the efficiency of the circuit to drive the LED. In terms of light output per unit of energy used by the LED they are an efficient ...
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  • 6,792
39 votes
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Two LEDs connected in series don't work

The LEDs both need about 3V to work, so together in series they need about 6V to work, but the IO pin only gives out 3.3V. So there is no way the LEDs could work in series. Also do not connect LEDs to ...
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