March 22, 2006
Philips Consumer Electronics issued a recall last week of approximately 11,800 flat-panel plasma televisions with the company’s patented Ambilight technology. This according to a statement released by the U.S. Consumer Product Safety Commission (CPSC).
The televisions were reportedly recalled because of a safety risk caused by arcing capacitors inside the left and right side of the their back cabinets. The recall includes several models of 42- and 50-inch TVs manufactured in Belgium and Mexico between April and August 2005.
Philips has received nine reports of overheating caused by the arcing capacitors. In their statement, the CPSC said that, “the results of such incidents were contained within the TVs due to the use of flame retardant materials.” No injuries have been reported.
According to a recent report by The Washington Post, Philips will make house calls to repair the faulty capacitors. A representative from the company said there is no fire risk, but stressed that consumers should disable the televisions’ Ambilight feature and call the company to schedule repair. A spokesperson could not be reached for comment before press time.
Philips introduced consumers to its Ambient Lighting Technology in 2004. Six lighting elements in the rear of the television adjust brightness according to the amount of light in the room while matching the color and intensity of on-screen content. The feature is meant to reduce eyestrain and create a more engaging viewing experience.
Electronic Design Analysis
By Don Tuite
I go back far enough to remember Sylvania black-and-white TVs with "Halolight," which was a lighted bezel around the screen. In that same era, there were also any number of 40-W lamps with conical shades, which were supposed to prevent us kids from going blind from watching TV in totally darkened rooms. So my initial take on Ambilight was that it is a gimmick meant to lessen the sticker shock on a television set that costs nearly $3000.
It turns out I was wrong. A recent independent scientific study actually points to some advantages of the ambient lighting technology (see A scientific study on the effects of Ambilight in flat-panel displays).(One might wish that the university that conducted the research wasn't located in Eindhoven, but I hold the Dutch in pretty high regard for their integrity.)
But what is it about the Ambilight that leads to sizzling sounds emanating from the TV set?
Neither Philips nor anybody else on the World Wide Web will tell us how Ambilight is generated, but we know that it is created by long and narrow things that are located on the back edges of the TV display. We also know that the components of the color output—hue, saturation, and intensity—are variable from totally off to some maximum level of plain white light. Based on that, I’m going to guess that the ambient light is created by sticks of red, green, and blue LEDs.
Let's ask how one drives a string of LEDs. My favorite sources for information about these matters are application notes from companies that have to answer those questions every day. Maxim Integrated Products' Appnote 3256 says, "White LEDs are commonly powered in four different ways: (a) a voltage source and ballast resistors, (b) a current source and ballast resistors, (c) multiple current sources, and (d) a current source with the LEDs in a series connection."
Since Philips' problems are sizzling capacitors, I'm going to guess (d). They're probably driving LEDs in series with a constant current regulator—which inevitably has a capacitor across its output.
With a 3.5 to 4.0 forward voltage drop (Vf) for each LED in the string, it doesn't take too many LEDs before there's a substantial voltage across that cap. LEDs are notorious for wide diode-to-diode variations in Vf, and I suppose a statistician could tell me the odds of exceeding the rating of a somewhat under-specified capacitor. I bet it works out to something like two or three in 12,000. Remember, nobody said 12,000 TV sets were going to fail. That's just the total population that Philips Engineers think could hide a very small number of sparking events.
So kudos to Philips for biting the bullet and recalling their TVs with integrity and good grace—all 11,800 of them.