| Electronics > Repair |
| Why/how do these LED lamps fail so fast? |
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| Zero999:
--- Quote from: IanB on September 21, 2023, 01:37:17 am ---Actually, this is not entirely true. Constant current supplies like to kill loads connected to them, and they are more deadly when the current is controlled more tightly. Here's an experiment I did: I took a length of nichrome resistance wire and connected it to a variable power supply with voltage regulation ("constant voltage"). I was able to gradually turn up the voltage until the wire glowed red, orange, yellow, quite brightly. Everything behaved fine. Then I took the same length of wire and switched the power supply to current regulation ("constant current"). This time, when I gradually turned up the current the wire glowed red, started to be orange, and then quickly burned out and failed with a molten hotspot. --- End quote --- --- Quote from: T3sl4co1l on September 21, 2023, 02:44:32 am ---Actually, this is not entirely true. Constant voltage supplies love to kill loads connected to them, and they are more deadly when the voltage is controlled more tightly. Here's an experiment I did: I took a string of LEDs and connected it to a variable power supply with current regulation ("constant current"). I was able to gradually turn up the current until the LEDs glowed painfully bright. They got warm and that was about it. Then I took the same string of LEDs and switched the power supply to voltage regulation ("constant voltage"). This time, when I gradually turned up the voltage, the LEDs glowed dimly at first, but as they started to heat up, they got brighter and brighter, until suddenly eclipsing the previous test and then nothing. I tested the LEDs and found one had failed open. --- End quote --- Both statements are correct, but it depends on the load. If it has a positive voltage vs temperature coefficient, like a piece of nichrome wire, then it will be more likely to be killed by a constant current supply, whist if it has a negative voltage corefficient, such as an LED, it will be more vulnerable to a constant voltage power supply. As the nichrome wire heats up, its resistance will increase, but the currrent is constant, so the supply will increase the voltage leading to progressively more power dissipation. As the LED heats up, its foward voltage will reduce, for the same current, but the power supply will maintain the same voltage, so the current will increase, leading to increasingly highr power dissipation. The nichrome wire heater should be powered from a CV power supply, whilst the LED off a CC power supply. |
| David Hess:
--- Quote from: T3sl4co1l on September 21, 2023, 03:08:05 am ---Anyway, regarding LEDs: they are extremely long lived, in and of themselves, when ratings are respected. The phosphor is actually the higher wear element, as it tends to fade over time. The power density on it is quite impressive, for just a powdered mixture of ceramics that have this one magical property (fluorescence). The blue LED chip itself also declines, but slower. But that's just the LEDs themselves. What's powering them, is another matter. Lifetime can always be lower, and when it is economical to do so, it absolutely will be done. Electrolytic capacitors are the most obvious and worst offender, with 2khr 85°C parts being common picks. Put that in a stuffy lamp fixture and it might last mere months! --- End quote --- Exactly, the "50,000 hour operating life" specification is a fantasy based on the operating life of the LEDs to reach 50% brightness and has nothing to do with operating life and reliability of the LEDs and ballast. When California passed their LED bulb mandate, they based the economics on that 50,000 hour operating life saying that LED bulbs would actually cost less over the long term. (1) After a few years, they admitted in a report that that operating life was grossly exaggerated (I knew that) and that the economics were so bad that incandescent bulbs were better; the extra cost of the LED bulbs was greater than the extra cost of the power for an incandescent bulb. (2) Unfortunately, but not surprisingly, California has since censored that report and removed access to it, because fuck you citizens. A reliable electronic ballast needs to not use operating life limited parts like aluminum electrolytic capacitors, be significantly derated, and include protection circuits which themselves will not fail. Because of operating temperature, it probably also needs to use hybrid construction. (1) I do not believe that politicians are stupid enough to fall for this. They did it deliberately for purposes of rent seeking, so fuck them. (2) I have had the same experience with government mandated electronically commuted motors which replace less efficient shaded pole motors. Their operating life is short and replacement cost is high so their total cost of ownership is higher despite the power saved. |
| soldar:
OK, I have had a closer look and the circular strip of leds is composed of 102 discrete units which may have more than one led per unit. I do not know how they are connected. Three contiguous leds are blackened and, sure enough, they remain dark when the others light. Anyone know how they might be connected? If I know this maybe I can try a repair by taking them out of circuit or by shorting them or by substituting a resistor.... |
| coppice:
--- Quote from: T3sl4co1l on September 21, 2023, 03:08:05 am ---Anyway, regarding LEDs: they are extremely long lived, in and of themselves, when ratings are respected. --- End quote --- Some are. Some aren't. There used to be a huge number of LED lamps that gradually dimmed to a level of uselessness over a year or two. That problem seems to have improved a lot, but some still rapidly dim. |
| Zero999:
Have you measured the voltage across the LEDs which do work? |
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