Help me understand LEDs

[Dick in Wisconsin]

After a reading a number great threads about LED lighting on the GJF as well as the information available on the web sites of a number of LED sellers, I think I might have LEDs figured out. Please correct me if I'm wrong.

1. Basic LEDs are either 4 to 12 volt or 16 to 24 volt.
2. The individual LEDs are one of two sizes: 5050 (5.0mm x 5.0mm) or 3528 (3.5x2.8). I think either size can be either voltage.
3. To operate on something different that their native voltage, the circuit the LEDs are on must have the voltage stepped down to the native voltage (like 120volt to 12 volt). Typically a transformer or a resistor is used to do this.
4. LEDs are either dimmable or non-dimmable. Dimmable have special circuitry built into the LED to facilitate the dimming.
5. Individual LEDs are usually inexpensive. What makes a 120volt LED light bulb expensive is that there is a transformer or resistor circuitry in the light to step down the voltage.
6. As the need for brighter "light" increases, the number of LEDs in an array or light are increased.
7. The LEDs themselves create very little if any heat. The heat from a 120volt LED light bulb comes largely from the transformer/resistor circuitry stepping down the voltage to w either 12 or 24 volts.

Do I have this correct?

 

[zendriver]

Probably.

Regarding No. 5 LED bulbs are (were) expensive, IMO, because of price markup, not cost of manufacture. Transformers and ICs are dirt cheap also.

There is an "older" video out there, where Jay Leno is interviewing a manufacturer of LED lights . The guy quoted a price of about $74, for a 4' fluorescent replacement tube.

They are now available for about $8-10, or so. Everybody is the world is making LED lights-of all types, now, so expect them to become cheap commodities, before too long.

A good thing.

 

[cbusters]

Converting LED lights to run on AC current seems like a baby step to actually save energy. Sort of like just reducing the bulb wattage in a regular light socket. Isn't this just a compromise? To truly use the features of LED lighting, shouldn't you just have LED tape in tubes (for protection) hooked to a 12v battery and then charged with a solar panel? After the initial cost, the only expense is defective parts or replacing the battery every few years. It seems that a true DC system is the direction to go. Transformers would eat a lot of electricity. Or do I have an AC socket and a hole in the ground confused?

 

[rockwithjason]

You get more light for less energy. That is no baby step

 

[cybrdyke]

After a reading a number great threads about LED lighting on the GJF as well as the information available on the web sites of a number of LED sellers, I think I might have LEDs figured out. Please correct me if I'm wrong.

1. Basic LEDs are either 4 to 12 volt or 16 to 24 volt.
2. The individual LEDs are one of two sizes: 5050 (5.0mm x 5.0mm) or 3528 (3.5x2.8). I think either size can be either voltage.
3. To operate on something different that their native voltage, the circuit the LEDs are on must have the voltage stepped down to the native voltage (like 120volt to 12 volt). Typically a transformer or a resistor is used to do this.
4. LEDs are either dimmable or non-dimmable. Dimmable have special circuitry built into the LED to facilitate the dimming.
5. Individual LEDs are usually inexpensive. What makes a 120volt LED light bulb expensive is that there is a transformer or resistor circuitry in the light to step down the voltage.
6. As the need for brighter "light" increases, the number of LEDs in an array or light are increased.
7. The LEDs themselves create very little if any heat. The heat from a 120volt LED light bulb comes largely from the transformer/resistor circuitry stepping down the voltage to w either 12 or 24 volts.

Do I have this correct?

If we limit the conversation to LED's for lighting, then....
1) Alot of LED's are made to run in these ranges, but there are exceptions.
2) There are hundreds if not thousands of different sizes of diodes available.
3) LED's need a driver, just like fluorescent needs a ballast. The driver changes supply voltage into the voltage required for a specific diode (or group of diodes) PLUS it delivers the appropriate drive current so that the diode will emit a certain amount of light. This is the function that most people have trouble grasping. An LED is not like an incandescent bulb that only comes on to one level of brightness.
4)All LED's will dim, but the dimming is done with the driver, not the diode. If the driver is capable of being dimmed, it can dim the diode.
5) Diodes and drivers are all coming down in price. They are a system. Neither works without the other.
6) To increase lumen output, a diode can be pushed to it's limits by increasing drive current. In modern LED systems, the diodes are not pushed to their limits. Adding more diodes costs too much money. It's cheaper to just push them closer to their limit.
7) The junction of the positive and negative substrates on the diodes is where the heat is created. The driver, unfortunately, is typically a heat sensitive item and will suffer as a result of the heat created at the diodes. In modern LED's, however, the heat issues of the past have largely been mitigated and are hardly a problem anymore.
I wish it was as simple as you thought, but it's really ******** complicated. Just like my girlfriend.
CD

 

[Dick in Wisconsin]

If we limit the conversation to LED's for lighting, then....

I wish it was as simple as you thought, but it's really ******** complicated. Just like my girlfriend.
CD

What else can LEDs be used for besides lighting.

Your comments were very insightful. Helps round out what I thought I knew. Thanks!

 

[ishiboo]

After a reading a number great threads about LED lighting on the GJF as well as the information available on the web sites of a number of LED sellers, I think I might have LEDs figured out. Please correct me if I'm wrong.

1. Basic LEDs are either 4 to 12 volt or 16 to 24 volt.
2. The individual LEDs are one of two sizes: 5050 (5.0mm x 5.0mm) or 3528 (3.5x2.8). I think either size can be either voltage.
3. To operate on something different that their native voltage, the circuit the LEDs are on must have the voltage stepped down to the native voltage (like 120volt to 12 volt). Typically a transformer or a resistor is used to do this.
4. LEDs are either dimmable or non-dimmable. Dimmable have special circuitry built into the LED to facilitate the dimming.
5. Individual LEDs are usually inexpensive. What makes a 120volt LED light bulb expensive is that there is a transformer or resistor circuitry in the light to step down the voltage.
6. As the need for brighter "light" increases, the number of LEDs in an array or light are increased.
7. The LEDs themselves create very little if any heat. The heat from a 120volt LED light bulb comes largely from the transformer/resistor circuitry stepping down the voltage to w either 12 or 24 volts.

Do I have this correct?

1. A "basic" LED is a single diode. They use just a few volts. Some more advanced on-chip diodes use more, but they are multiple diodes in one package connected in series.
2. Or a bunch of other formats as well. 5050 and 3258 are both SMD, and there are a bunch of other SMD ones as well. The "bulb" type are often T1, T1-3/4, etc.
3. Maybe. Each LED has a certain forward voltage. If you connect the correct number in series, there is no transformer or resistor required. IE, on a 120v Christmas light string, there may be 60 2V diodes - adding up to 120v. The AC waveform provides some natural on/off and is what causes some strings to appear to "flash" or flicker at a (to your eye) high frequency - you are seeing the raw 60HZ flash. LEDs respond almost instantly (which makes them GREAT tail lights), so they do not "smooth out" the incoming AC signal like an incandescent bulb does.

Any "powerful" LED light typically uses a LED driver, which combines a switch-mode or other power supply with a PWM-based LED driver. This "flashes" the LED at an imperceivable frequency... so the peak brightness is the same, but the LED is not on all the time. This reduces heat and energy consumption, thus making the LEDs last longer.
4. A LED itself can be dimmed in various ways - changing the PWM, voltage, lowering current on a constant current driver, etc. The way a LED is driven means some are set up for dimmers and some aren't.
5. Yes, but see #3. The best ones use LED drivers and that adds cost. So does the heat sink.
6. Or higher-wattage diodes are used.
7. LEDs create plenty of heat. The driver if efficient creates almost none. LEDs are many times more efficient than incandescent which generates way more heat than light, but their byproduct is still heat.

 

[cybrdyke]

What else can LEDs be used for besides lighting.

Your comments were very insightful. Helps round out what I thought I knew. Thanks!

Haha...well...I guess since LIGHT is in their name.....
But, I meant that we were talking about general illumination of spaces. Other uses are signaling, as in traffic lights and cross walks, airport markers and even coloring of buildings.
But other uses are medical...for instance, they use LED light to "set" tooth fillings when you have a cavity. Indicator lights, televisions, and even your cellphone has an LED screen.

 

[pedrodagr8]

Since I saw this late I will chime in.

1. Individual LED dies are typically between 1.2-3V depending on the technology used, InGaN (Blue to UV and white LEDS) tend to be 2.2-3V while GaAs (red-green colors) tend to be 1.6-2.2V. These can be strung in series to make the various voltage combinations you frequently see. So 4x 3V LEDS in series would make a 12V series, while 8x in series would make a 24V forward voltage.
2. They come in a variety of sizes, from the gargantuan down to as small as a period on a piece of paper. The forward voltage is strictly dependent on chemistry of the semiconductor used in the LED. Any variation from these low values means that they are daisy chaining more than one LED die in series.
3. This is where we get into a point of contention that I must bring up. LEDs are NOT voltage controlled devices, they are current controlled devices. What this means is that the output varies approximately linearly with current. The current across an LED can change exponentially with small changes in voltage. For example, at 3.4V an LED might draw 100mA, at 3.5V it might be 200mA, at 3.6V it might be all the way up to 400mA! So in 200mV you increased the output by 300%. When you see the voltage of an LED listed, this is simply the voltage at which the LED conducts the rated current. Most drivers do TWO functions, they step down the voltage to a reasonably controlled value and the limit current. For low current LEDs this isn't a big deal but for high power LEDs (like CREE XM-L) this is a huge deal. This is because the large LEDs produce enough heat that they can enter thermal runaway. When this happens the LED heats up, which causes the resistance of the LED to drop, which allows more current across the LED, which causes it heat up more, which drops the resistance more, on and on until the LED burns out. If the LED is current limited, even if the LED heats up, the constant current driver just adjusts the voltage to keep the current within regulation. Smaller LEDs are less of an issue, because they can remain reasonably cool just by heat transfer to air.
4. See my discussion above, by varying the current to the LED you can vary the output. The driver has to be designed to vary the current, otherwise it will not dim. In fact, using a triac based dimmer (a chopping dimmer) to dim a non-dimmable driver can kill the driver.
5. Low power LEDs are already commodity pricing. You can get 1000 bulk white LEDs for under $10 and even less in larger bulk or in SMD. They will be unbinned and low quality but the fact is, the basic LEDs are already at commodity pricing. You do see some signficant cost increases for LEDs with consistent color profiles and performance, because each LED has to be characterized. Even then, you are looking at US$0.05-US$0.10/LED or less. The driver does add cost, though even this cost is not substantial. Most of the cost right now is in the engineering and testing of the bulb and in the heatsinking. As these get more commmoditized as well we will see the price drop quite low, near commodity levels. There were always be a steep premium for the best performers. My guess is that low CRI will remain commodity for a while, with high CRI LEDs commanding the price premium.
6. You can use more efficient LEDs, higher-power LEDs or more LEDs in the array or a combination of all of the above. Which solution is chosen will often depend on the application and power output needed. Bulk lighting in a building will use a high number of LEDs to spread the light out, outdoor spot lights will use a smaller number of higher power LEDs to have a much brighter output in a smaller area. There is a limit on how many LEDs can be crammed in a space before needing a heatsink, so this combined with efficiency, often limits the max number of LEDs in a strip.
7. LEDs still produce heat (see thermal runaway) and high power LEDs can produce a LOT of heat. Just a lot less than an incandescent bulb which is operating at 100's, if not over 1000 degrees. The issue is that LEDs are much less tolerant of heat just 125oC is usually enough to kill the LED. So a lot of heatsinking must be there to wick the heat away.

 

[Dick in Wisconsin]

Since I saw this late I will chime in.

forward voltage

The issue is that LEDs are much less tolerant of heat just 125oC is usually enough to kill the LED.

Better late than never!

Very helpful. Learning more.

Please explain "forward voltage". New term to me.

125*C = 257*F. That seems "hot" to me, but probably much lower than what an incandescent light can get to.

What about LED tolerance to cold? Say 0*F to -35*F. Does anything special need to be done to an LED to make it tolerant to relatively cold temps? Cold temps impact life of LED?

 

[pedrodagr8]

LEDs are VERY cold tolerant, much more so than heat tolerant. Usually the driver (or specifically the capacitors in the driver) are the most senstive to cold but even then you are talking more about cryogenic levels of cold. LEDs at liquid nitrogen temps can color shift but unless you live on the surface of Pluto this isn't an issue you will encounter This is why they are so much better in cold environment than fluorescent lights. In fluorescent lights, there is a vaporized gas inside, which in cold temperatures can condense some and need to warm up enough to revaporize. There is no such issue with LEDs.

125oC is HOT but keep in mind that is the temperature of the LED die, not the entire LED case. The die can be much hotter than the case itself, depending on the thermal resistance of the case. Thermal resistance is how efficiently something transfers heat. If the case has a high thermal resistance, the die might be 125oC and the case is only 50oC, on the other hand if the thermal resistance is low and the die is 125oC the case might be 110oC (way hot enough to burn you). In general most LED strips are good enough to keep the LEDs cool, modules can vary pretty wildly, same with bulbs. As for the temp of a filament, that runs around 2500oC (yes that is thousands of degrees Celsius). This is because filaments are black body light sources, meaning the color temp of the light output is actually the temp of the filament.

Forward voltage quite simply is the voltage rating at the specified current. It is ONLY valid at the listed current i.e. 3.0V @ 20mA. They could choose any reasonable current (say 2.8V @ 10mA or 3.1V@40mA) and use that voltages as the forward voltage. It is called forward voltage because LEDs are diodes, this means they have both a forward and reverse voltage. Forward voltage is the standard voltage drop across the LED that produces light. On the other hand, like most diodes (excluding zeners) hitting the reverse voltage will kill the LED. Diodes behave like a one-way valve and the reverse voltage is basically the voltage applied in reverse where this valve breaks down and can no longer hold it back. As expected, if the diode isn't designed for it (zener diodes are one of the few types designed to handle this) this will damage the diode.

 

[GarageSpider]

Better late than never!

Very helpful. Learning more.

Please explain "forward voltage". New term to me.

125*C = 257*F. That seems "hot" to me, but probably much lower than what an incandescent light can get to.

What about LED tolerance to cold? Say 0*F to -35*F. Does anything special need to be done to an LED to make it tolerant to relatively cold temps? Cold temps impact life of LED?

With a resistor, voltage drop is proportional to current ala ohms law. Incandescent bulbs are basically resistors. More voltage, means more current, bulb gets brighter.

LEDS have a non linear voltage to current curve. So they pick a point on the curve and say it's voltage drop is Y at current X. This curve is why LEDS are driven by current drivers.

 

[theoldwizard1]

.
This is where we get into a point of contention that I must bring up. LEDs are NOT voltage controlled devices, they are current controlled devices. What this means is that the output varies approximately linearly with current.

Thank goodness somebody actually brought this up so I did not have to !

I could not have done as good a job of explaining it a pedrodagr8 !

 

[theoldwizard1]

One of you young wipper-snappers had better correct me if I am wrong because it has been WAY TO LONG since i have done this

Forward voltage quite simply is the voltage rating at the specified current. It is ONLY valid at the listed current i.e. 3.0V @ 20mA. They could choose any reasonable current (say 2.8V @ 10mA or 3.1V@40mA) and use that voltages as the forward voltage. It is called forward voltage because LEDs are diodes, this means they have both a forward and reverse voltage. Forward voltage is the standard voltage drop across the LED that produces light.

In a simple application where an LED is going to be acting as an indicator light, the trick is limiting the current draw, based on the (DC) supplied voltage, say 5V. Typically a "current limiting" resistor is used so we do not exceed design point for LED operation is 2.8V @ 10mA.

Remember Ohm's Law; V=IR or V/I=R

If we want to drop 5V at 10ma (5.0/0.010) of total resistance is 500Ω. How much of that is the LED ? (2.8/0.010) 280Ω. So we need a 220Ω resistor in series.


(God, I hope I did that right ! I haven't "ridden this bicycle" in a long time !)