# Is there any benefit of making lamps with high wattage LEDs but low wattage driver circuits other than cost?

Over the past few years I have bought a number of GU10 mains voltage LED bulbs and have generally been disappointed with their brightness. The 9W LED bulbs I bought (supposedly 460-590 Lumen) produce far less light than the 50W Halogen bulbs they replace (specified as 350 Lumen on the box, but I think 800 Lumen would be more typical give 16L/W).

Testing them with my Fluke I found that they draw far less current than I would expect from their "specification", so I have always assumed that the limiting factor was the mains power circuit and that if this circuit were permanently being driven at 100% it would be far more likely to fail than the LEDs themselves.

True to form, over the past few months, two of these 9w (3x3w) bulbs have died, the first almost 12 months to the day from when it was first fitted. I estimate that this bulb was used for approximately 2000 hours over that year, far less than I would expect from a LED bulb. In both cases, the failure manifested as the bulb pulsing light at between 1 and 3 Hz rather than providing continuous illumination.

Opening up one of the failed bulbs, I found that it contained a BP3102 High Precision PSR Constant Current LED Driver and from what I can see of the PCB, the circuit is the pretty much the reference circuit in the data sheet. Given that the data sheet says explicitly that "the output power of system should be less than 5W", I cannot see that it is honest to call this a 12W bulb.

Looking for datasheets for the LED itself, it looks like each of the three LEDs should be putting out 160-190 lm at 3W, so 480-570 lm is pretty close to the specified 460-590 lm), however the power draw suggests that the actual light output at 5W should be more like 266-300 lm, which seems about right.

Putting aside the dishonesty of sellers rating a bulb on the potential output of the LEDs rather than on the actual output of the bulb, I wonder if there might be a technical reason for overrating the LEDs but under rating the power circuit.

There is an obvious cost advantage to the manufacturer to using lower spec parts, but I would have thought that failing to meet the advertised specifications and reduced lifetime would not be considered a good tradeoff.

Similarly, I would have expected that the LEDs would only draw as much power as they need, so overrating the power supply would never be a problem, and the ideal would be for LEDs and power supply to be matched in their power requirement and output.

Is there something I am missing about how a circuit like this should be designed, or is my cynicism just realism?

• 1. Because they can. 2. The LEDs were cheap at the time. (3) Read 'Wild Swans" by Jung Chang - or at least skim it. If anyone in the company management is >= 50 years old then they have been through a hell you cannot imagine and the while concept of specification sheet means something quite different to them and you. If anyone is > 60, think yourself lucky to have a spec sheet. Don't expect it to be for your product. (4) The above is, of course, drivel. But alas quite possibly not utterly so. If you do not have trustworthy provenance in your supply chain then anything can happen, and may. ... – Russell McMahon Jan 26 '14 at 11:55
• I have dealt with an LEd manufacturing company with a test lab of instrumentation, LED burn in arrays and more. And impressive spec sheets. Which were utter rubbish. When my 'rather different' test results were submitted for their consideration they told my client that I appeared to be doing things wrongly.| I now use ONLY 'name brand' LEDs, and still look sideways at them when we pass. – Russell McMahon Jan 26 '14 at 11:58
• The trouble is @RussellMcMahon, many of these sellers stick brand name LEDs alongside their own cruddy power supplies and that seems to be where the problems start. I think what is most annoying is that some sellers show pictures of their lovely labs, complete with integrating spheres, yet don't add realistic luminous output specs. They apparently have the equipment, so why not use it to tell us the specs of the products they are selling. Knowing the actual power draw can tell you a lot about the expected luminous output, but nowhere near as much as a real measurement in a calibrated IS. – Mark Booth Jan 28 '14 at 20:04

The LED power may be limited by thermal considerations. Both the power supply and the LED are prone to failure. The former because of possible shoddy transient protection and inevitable death of filter capacitors (exacerbated by the typical high temperatures). LED life will also be greatly foreshortened by high junction temperature.

These things are a trade-off. My suspicion is that the LED rating of 3W you mention is not really achievable (with any reasonable lifetime) in the environment of the light bulb, and everything (driver and LED) is really close to the line. Here's a snippet from a Chinese "3W" LED datasheet:

The Power Dissipation is absolute maximum 3W at $T_A = 25°C$. It will have to be derated above 25°C at the bulb, so I think you can see that calling that thing a "3W" LED is somewhere between irrationally optimistic and an outright lie. While the bulb maker might be perfectly happy to run it right at 3W, if they die so fast that they are getting 40' container loads of lamps dumped back at them, they'll not be so happy.

The only thing I really expect to be true of the bulb is the (initial) lumen output (you can expect them to fade over time). If the lumen output falls short, especially out of the box, that is false advertising and I would return them. So far, from reputable makers such as GE, I have not been disappointed. I can't directly measure lumens, but I use lux meter to check light levels on work surfaces and so on, and can reasonably well compare halogen and LED bulbs with similar angles.

• Thanks Spehro, that brings up some interesting points. I know that when I first started using 'high power leds' in metrology applications we put big heatsinks on them. As it is the thermal coupling inside these bulbs appears pretty atrocious. Few Ebay sellers even list lumen specs, even this I can tell that they just take the LED lumen spec and multiply it by the number of LEDs, so it is definitely dishonest. – Mark Booth Jan 25 '14 at 20:02
• I bought some E14 base (European standard candelabra for a chandelier acquired in Dubai) LED lamps and several types of MR11 (small halogen type) LED lamps from eBay sellers, and most of them were sub-standard. Sticking to brand name (where possible) from here on in. – Spehro Pefhany Jan 25 '14 at 20:10
• P.S. Only a few data points, but the trend (for the no-name stuff) seems to be for more impressive specs for the same performance and worse build quality. – Spehro Pefhany Jan 25 '14 at 20:17
• The trouble is that brand name kit is 5x the cost of equivalent cheap stuff from china, and prices are still falling, so hopefully by the time the current crop of bulbs fail the gap in price will have narrowed substantially. The art is interpreting the specs of ebay sellers to guess what the actual specs are. – Mark Booth Jan 25 '14 at 20:34
• I don't see that much difference at the 60W equivalent level, an performance, safety approvals, etc are worth at least a 20-30% premium. – Spehro Pefhany Jan 25 '14 at 21:50

Underdriving could be crappy power supply or just deception .Others have said this and I am inclined to believe this.One reason to underdrive is efficiency .The LEDs that I have looked at are less efficient at thier max rating .Another reason to underdrive is product life.If you run a decent power supply that runs cool and is not full of electrolytic caps then your LEDs will limit the life of the product .If you want 100000 hours at realistic internal temperatures then underdriving is the way to go .I think that underdriving is more expensive because you need more LEDs for the same light output .

• Sadly, under driving the LED doesn't help if you are overdriving the power supply electronics, hence my '10 year' LED bulbs blowing in just a year. – Mark Booth Apr 9 '16 at 16:35