LEDs facts and fiction

When Chuck Smith asked me to put together a column for PlanetChristmas magazine that was a little more on the technical side of things I was nervous at first. Would average readers want something “more technical” or would this level of detail bore them? I agreed to give it a shot, but I wanted to present technical concepts in a way that everyone could understand. I hope I’ve achieved that goal. Let’s start off with a simple statement:
LEDs will save your life.

Really. They will also help you all but eliminate your electric bills, find the car keys, restore your faith in religion, and cure cancer. Oh, and they never go bad – they last forever. OK, so perhaps I am exaggerating just a tad. OK, maybe a lot. But are some of those claims really that far from some of the ones you’ve seen and heard?
The Light Emitting Diode (LED) itself is not new technology. First discovered in the 1920’s, the LED reportedly made its initial debut as a practical technology in the early 1960’s. LEDs have been used as indicator bulbs, for example, for many years. They’ve been used in calculators, clock radios, microwave ovens, and other devices that needed illuminated indicators or alpha-numeric displays. Given its actual lifespan the widespread use of LED as a source of lighting is a relatively recent mass-market development. The realities of LED lighting are still being defined, and there is not yet enough reliable data to make any claims other than the technology is in a state of constant improvement. Based on the trend, the gap between many of the claims that are being made today and reality will likely close as the technology matures.

It took many years for traditional incandescent and fluorescent lighting to become as affordable and predictably reliable as it is today. Much of the evolution of those technologies came from constant reinvention of manufacturing techniques and investment in materials research that would help develop more efficient and more reliable materials. However, like most technologies, there are always some physical limitations that, once mature, cannot be exceeded. At that point mass-production techniques and market competition ultimately maximize the overall price/performance point. This is the point where the products become commoditized and purchase decisions tend to follow a trend of convenience versus small differences in retail pricing.

LED lighting is nowhere near that maturity level today. The commercial hype and misinformation about LED lighting is rampant. This situation is further compounded by the continuous sharing of opinions that are often passed off as facts. Those of you who know my background know the reasons I became so infatuated with LED lighting. All the hype surrounding the technology got the best of me. In a weak moment several years ago when I actually bought into some of the marketing hype I made a major purchase of LED product. When I first received the product I was pretty pumped about it. It wasn’t until I started performing some real-world testing (once a nerd, always a nerd) that I realized I had dropped thousands into something that wouldn’t last a week in my display. Several thousands of dollars wasted, I decided others should not have to go through what I went through. So the pilgrimage began. And, I went public with my test results.

Since then I have been contacted by many LED lighting vendors and manufacturers. They all want the “LED KILLER” to test and “certify” the products they are considering marketing to avoid bad customer experiences and ultimately damage to their reputation as vendors. Many of the vendors in the Planet Christmas community have contacted me, and I’m happy to report that as a result many products have NOT made it to your display. Several of them have undergone design modifications that in turn provided you with a better, more usable product. And of course many of them tested just fine out of the box. As a result, I have a whole collection of one or two “light things” in the test lab, many of which will serve only as a reference on how not to build lighting products.

I’ve observed numerous discussions out there related to [let’s call them] “Christmas LEDs”. Topics include energy consumption, light output, removable bulbs versus non-removable bulbs, color consistency, dimming capabilities, life expectancy and return on investment (ROI). Some of those topics might someday materialize into articles of their own. Perhaps the most discussion related to Christmas LEDs is the argument of half-wave rectification (HWR) versions full-wave rectification (FWR). There is a lot of confusion over the difference and what that difference means to the Christmas light enthusiast. At the recent California Christmas Lights Workshop I had the honor of delivering two presentations on LED lighting in general, and it was obvious that few fellow enthusiasts understood the differences technically and even fewer had the opportunity to see the differences firsthand. This article’s focus will be on differences between HWR and FWR, with the hopes that you will not only understand what HWR and FWR are, but why you might care. Let’s start with some basics:

Rectification: Simply put, rectification is the conversion of Alternating Current (AC) to Direct Current (DC). LEDs are DC devices, meaning that they only operate (create light) when connected to the proper polarity. You can test this theory with a simple red LED and a 1.5 volt battery. The LED will only light if connected one way and not the other. Alternating current, (the stuff that comes out of your home’s standard electrical outlets), viewed visually, is best represented by a sine wave (the green line) in the picture above.

In figure 1 on the previous page the yellow horizontal line represents zero volts (0V), and the light green wave illustrates how the current coming from your standard wall outlet varies repeatedly between positive (above the yellow line) and negative (below the yellow line), hence the term alternating current. In the United States this cycle (a cycle is represented by the time between any two red markers) occurs 60 times per second, or 60 Hertz (Hz). Direct Current (DC) is essentially a flat line (see Figure 2) that represents the difference between 0V (the yellow line) and the actual measured voltage (the green line).

Figure 2. Pure DC signal (no wave)

An incandescent light has a tiny metal filament inside the bulb. When enough voltage is applied to that filament it gets hot enough to generate visible light. Filaments are neither AC nor DC devices, per se, and will light up regardless of what direction current is flowing. For reference, you use filament bulbs routinely in fixtures within the home, all running on AC power, and you use them in DC power source situations, such as flashlights and automotive lights. They are interchangeable in that respect.

Getting back to the AC waveform in figure 1, if the yellow line represents 0 VAC that means the bulb is not generating any light at the point where the green line crosses it. But as the sine wave approaches its maximum positive and negative peaks the bulb glows brighter and brighter. So in effect this means that when connected to an AC power source a filament bulb actually turns itself on and off 120 times per second – on during the positive and negative peaks, and off when the curve crosses zero volts. So why don’t we see the bulb blinking? There are two main reasons for this.

The human eye operates much like a movie camera and has its own “frame rate”. It essentially takes lots of still photos per second and the brain blends and merges them into what it perceives to be continuous movement. This tends to minimize the blinking effect somewhat. However, because the filament generates light based on heat it has an inherent startup and shutdown time, meaning that it takes long enough to stop glowing as it approaches zero volts that it starts glowing again as it gets further from zero cross on the other half of the cycle. This also minimizes the effect.

LEDs have a much fast startup and shutdown time. You can easily prove this by setting up an incandescent string right next to an LED string and telling your lighting controller to shimmer both strings. The shimmer effect will be more pronounced in the LEDs that don’t glow through their zero cross point as much as the incandescent bulbs.
As stated earlier, rectification is the conversion of AC to DC. The rectification circuit’s job is to take the rather curvy sine wave from figure 1 and make it a flat line like the one from figure 2. Getting a perfectly flat line involves a lot more circuitry than most retail or consumer LED lights can afford to pack in, so some concessions are made to get “close enough”. Half-wave rectification circuits typically involve the use of a single diode in line with the light(s). Diodes are electronic devices that only allow current to pass in a single direction, so when a half-wave circuit is implemented the original AC waveform ends up looking like the one in figure 3.

Figure 3. Half-wave rectification of AC signal

What’s happening here? Well, the diode is simply doing its job. It is allowing the positive portion of the AC waveform to pass and blocking the negative portion. This is basic half-wave rectification. Your reaction to this might be “well that’s just throwing away half of the power I send to it”, and you would be right. Remember that LEDs turn on and off much faster than incandescent bulbs. The net effect is that most people can see the LED lights flickering when they are half-wave rectified. The flickering is actually magnified if the lights are moving because the flickering is not fast enough for the eye-brain combination to stitch the images into smooth motion. The effect is almost like having a bunch of strobe lights. Throw a lot of these HWR strings out in the yard, each with a bunch of strobes, add a little wind, and you’ll likely have some disappointed spectators – though (and here’s the catch) they may not know why they feel that way.

What if you didn’t want to simply discard half of the input power’s waveform but rather put it to use? Full-wave rectification is the answer to your prayers. Figure 4 illustrates what happens when you simply add 3 more diodes to make what is called a bridge rectifier. Bridge rectifiers are full-wave by their design, because they literally invert the negative portion of the AC signal into a positive portion. Though this is not pure DC it is close enough to reduce the flicker effect to the point where it is cannot be perceived by the human eye-brain combination.

Figure 4. Full-wave rectification of AC signal

If you have access to sample HWR and FWR strings, preferably of the same size and color bulb, you can see the difference for yourself firsthand. Put one bulb of the HWR string in your left hand and one bulb of the FWR string in your right hand. Hold them both about a foot or more in front of your face with the tops of the bulbs facing each other – you will be looking at the sides of the bulbs. Then, move the two bulbs up and down in a synchronized motion, going both up and down in a wide arc (about 2-3 feet) in about one second. Repeat this motion continuously and observe the differences in how you perceive the two bulbs. The combined effect of your eye’s natural shutter speed and the differences in the flickering of the bulb should make the HWR in your left hand will appear as three or four lights going by. Your right hand will look like twice as many (six to eight) bulbs, because the FWR string has twice as many waveform peaks due to the negative to positive current conversion.
In the end it is up to you to decide which string of lights you prefer. For some reason HWR strings are still lower priced than FWR strings. The actual difference in cost to manufacture them is not at all significant and certainly not as much as the retail price difference. In the long term I don’t expect much of a market for HWR strings, but they’ll be around for a few years until the market drives the demand for FWR strings.

The good news for bargain hunters is that you can convert a string to FWR if you are willing and able to do little soldering and work with heat shrink tubing. You can buy small bridge rectifiers at your local electronics store very inexpensively and insert them into the circuit feeding the string. Remember though that you are working with potentially lethal voltages and currents, so the work needs to be done properly.

Now that you know all about half-wave and full-wave lighting I’m sure you’ll start noticing who bought their lights from a reputable FWR retailer and who bought theirs at a half-price post-Christmas sale at a local big-box retailer. I apologize in advance if this ruins your viewing pleasure.

About the author: Fabian Gordon has a long history as a professional musician and audio production engineer, and has been a strategic and architectural computer software and services design consultant for almost 30 years. He has been an electronics hobbyist since age 10 and studied Electrical Engineering at Northeastern University. He is currently Chief Technology Officer (CTO) at Ignite Technologies in Frisco, Texas, and has served in that role for three other companies since relocating to the Dallas area from New Jersey in 1992. Fabian spends much of his time dealing with emerging technologies, their potential practical implementation, and the vision of how they may fit into “the larger scheme of things”. Christmas lighting is something he was fascinated with since a very early age, and he has been at the forefront of the transition to quality LED lighting. His efforts in this area have earned him the nickname of the “LED KILLER” by fellow Christmas enthusiasts. The Gordon family display www.GordonLights.com was the 2008 Christmas season Light-O-Rama category 5 winner.

From the July 2009 edition of PlanetChristmas Magazine

by Fabian Gordon