Recently in Lighting Category

EDN.com has a great two part teardown of the new Philips AmbientLED 12.5W lightbulb. I use these A19 style bulbs in my house. They have a nice even white-light output, dim much better than the other LED bulbs I've used. All around, great players. And CL&P has an instant rebate for them in CT that makes them $20 each.

• Teardown Part 1: http://www.edn.com/blog/PowerSource/40511-Philips_LED_bulb_Tear_down_Part_I_light_patterns_.php
  
• Teardown Part 2: http://www.edn.com/blog/PowerSource/40512-Remote_Phosphors_Philips_LED_bulb_Tear_down_Part_II.php
  

What's really neat about them is that they use remote phosphor technology. The yellow coating on them fluoresces under the blue LED used internally in the devices. This allows for the diffuse white quality of the light. This is in contrast to the CREE LEDs that have the secondary phosphor built into the LED shell itself and a diffuser in the A19-style shell. Their luminous efficacy is actually much better than CFLs for a change.

Unit	Watts	Lumens	lm/W	Price
Eco-Smart	8.6 W	420 lm	49 lm/W	~$20
Philips AmbientLED 12.5W	12.5 W	800 lm	64 lm/W	~$20 AIR
Compact 16W CFL	16 W	800 lm	50 lm/W	~$2

When I was in Japan, I replaced a couple of my mother-in-law's ceiling lights with LED bulbs. They are the big thing now in Japan, which is trying hard to meet the Kyoto protocol guidelines.

Coming back to the States, I thought I'd replace the few remaining incandescents and some of the CFLs (compact fluorescent lights) in my house with LEDs.

In comparison to incandescents:

• Much better efficiency
• Produce much less heat
• Much longer life (10,000 hours vs. 1000 hours)

In comparison to compact fluorescents, LED have:

• Marginally better efficiency in their current iteration*
• Require almost no warm-up time to maximum light**
• Work great in the cold
• Have no toxic chemicals such as mercury in their construction
• Generate no UV or IR rays
• Much longer lifetime (50,000 hours vs. 10,000 hours), especially in situations where there is a rapid on-off cycle which is very harmful to CFLs

I'm experimenting with a few different sources. I bought some E27 (edison screw-in) bulbs from China, have a PAR30 "can" size reflector on order from an eBay vendor  and just ran down to Home Depot and got their new $20 EcoSmart LED bulbs.  I'll be putting them through the wringer and let you know what I think.

So far, the EcoSmart (pictured right) with 420 lumens @ 8.6 watts (49 lm/w) seems a good price/performance leader at $20 a bulb. The light it emits is a nice warm white, rather than the very bluish light that the Chinese bulbs are giving me. And it's dimmable, which is surprising given its low cost. It's working fairly well with the Lutron dimmers in my house.***

 

* Everyone thinks that LEDs are much more efficient that CFLs. That's true as a general proposition under optimal conditions (Cree is getting 200 lm/w in the lab) but the LEDs in these household bulbs are usually overdriven in order to put out as much light as possible. The LED bulb above is only getting 49 lm/w  where as a 16 watt CFL usually puts out 800 lumens, which would be 50 lm/watt.  In comparison, an incandescents gets around 7-24 lumens per watt.

** While LEDs themselves light up at 100% brightness instantaneously, I've found that must of the bulbs I have require around 0.25 seconds for the LEDs to turn on from when the switch is thrown. Most likely the internal LED drivers require a minute amount of startup time.

*** My standard Lutron dimmers (Maestro series) expect a certain amount of resistive load. They're rated for 600 watts, but if you use all LEDs (or CFLs), they may see less than 50 watts on the  circuit. The Lutrons end up flickering at low dim levels -- or acting bizarrely. So I've had to retain a single incandescent on my dimming circuit. This doesn't make me happy, I'd like to switch to all LEDs -- but in order to do so, I'll have to switch out my dimmers.

An anonymous poster criticized my use of LM317 chips as current regulators for high-power LEDs. While I think they are great for 350 mA single LEDs, they are clearly inappropriate for the 12 watt LED that I was playing with.

My LED was taking around 800 mA @ 12 volts. A little searching around reveals that a good alternative is the AP8803 which is a LED buck driver which can handle 1 amps @ 8-30 volts. The nice thing (aside from its 92% efficiency) is that you can set up a dimmer circuit on it using minimal parts -- or PWM control from a microcontroller.

Well, my favorite discrete component of the week has to be the lowly LM317 voltage/current regulator. As one of my previous posts showed, I'm using it to current regulate some high power LEDs and I also use it as a voltage regulator.

Here's the quick and easy way to wire up an LM317 as a voltage regulator:

where the values of R2 and R1 are calculated as follows to give Vout:

R2 is usually set to 240 ohms and you can ignore Iadj to a point. Rearranging the equation gives you:

So if you want a 5 volt output, then R2 = 720 ohm (and R1 = 240 ohm). The TO-220 form factor of the LM317 that I'm using can provide up to 1.5 amps of output current and can be paralleled if I need more.

Late update: or you can just use an online calculator: http://www.jlab.org/~hansknec/index.html

DealExtreme is one of my favorite online stores. It's a distributor of inexpensive electronic gadgets based in China. I'm always finding something new there. The latest treasure is this little-but-very-bright bare LED:

DealExtreme lists it as a 10 watt LED (SKU 5876). Unbelievably it's just under $12 with shipping included!

Looking at the die shows that it is 9 discrete high-powered white LEDs in a single package. DealExtreme is bad about specs, but the comments in the DX forum seem to suggest that 700 mA at 12 volts is a reasonable spec for this LED. This would yield 8.4 watts.

(I'm wondering though if it isn't 3 x 350 mA @ 3.5 serial LEDs in a 3 parallel strings, which would be 1050 mA @ 10.5 volts. But for now, I'll run it at 700 mA).

DealExtreme lists it as 500-600 lumens @ 6500K color temperature.

As with most LEDs, you need a good current regulated driver circuit since you can't just run these things off a resistor. I decided that the easiest and simplest driver would be one based off the amazingly versatile LM317 chip.

As before, these sites have good javascript based circuit diagrams for calculating LED driver circuitry:

• http://diyaudioprojects.com/Technical/Voltage-Regulator
• http://www.reuk.co.uk/LM317-Current-Calculator.htm

Plugging my values (700 mA) into them yielded the need for a 1.8 ohm resistor with my LM317. Here's the schematic that I designed around those figures (courtesy of ExpressPCH):

Bodged together and plugged into a li-ion pack from my model helicopter and voila, an amazing amount of light. I'm thinking of using it on the headlight of my Piaggio (which currently uses a 3-watt LED) or to replace the bulb on my old 15-watt Niterider headlight, which has seen happier days.

(More photos and photometric testing after the jump)

Found a great article that describes how to make a LED voltmeter using a chip designed specifically for that, the LM3914. http://www.evconvert.com/article/led-bargraph-battery-monitor

Happy new year! A new decade!

I've been playing with LEDs for my EV and robotics projects. It doesn't seem to make sense to use incandescent bulbs in an EV build -- it'd ruin the whole concept of going green.

LEDs are tricky to deal with though, especially the high-output "star" type LEDs that are emerging. Rather than voltage regulation, you have to regulate the amount of current that goes through them. This isn't fixed, because as an LED heats up, its resistance goes down (unlike an incandescent filament whose resistance goes up as it heats up, thus self-regulating). If it gets too hot, it goes into thermal runaway and you soon have what ledophiles call a Dark Emitting Diode (DED) -- dead, get it?.

So, you need some form of a current regulating system. For small 3mm or 5mm LEDs, people just use a fixed resistor since the current demand is rather small, around 20 ma. This limits the maximum current that can go through -- but it also limits the max brightness because you have to put in a safety factor and you can't easily adjust for fluctuating voltage.

Here's a good javascript calculator for series/parallel LED resistors:

http://led.linear1.org/led.wiz

The problem with 5mm LEDs is that the clear plastic casing limits the amount of heat that the LED can output (and yes, LEDs do produce waste heat, although not as much as incandescent lights). Heat control is one of the main factors affecting the output of LEDs and the reason why manufacturers went to the star configuration, which allows you to directly back the LED with a heatsink -- which lets the LED current jump from 20 mA to 350 mA with a concomitant light output.

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Now, if you want to use a high output Luxeon or Cree star, you also have to current regulate as mentioned before. The typical white high-output LED takes 350 ma with a forward voltage drop of 3.5 volts. With a light output of 120 lumens, this is good enough for a moped, scooter or bicycle headlight.

Cree even has a high-power star that consists of four of their 120 lumen LEDs mounted on a single die. This produces 480 lumens, although you'll need to regulate four x 350 ma. There are other stars that bundle 2 x 120 or 3 x 120 lumen LEDs. More than enough to blind you -- or for a car or motorbike headlamp.

For high power LEDs, the LM317 seems a good choice for current regulating at a low cost. Here's a good javascript calculator for that:

http://diyaudioprojects.com/Technical/Voltage-Regulator/

and some more info on why current regulation is necessary:

http://users.telenet.be/davshomepage/current-source.htm