For what reason do LED lamps that have a yellow led strip turn off without prior warning whilst led lamps with rows of stand alone Led lamps loose their brightness gradually? Just curious
LED Hand lamps
- Thread starter toad a caravan
- Start date
- Mar 14, 2005
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If you have a strip of Leds, the forward voltage voltage combines and may exceed the battery voltage, so to make them work the lamp probably includes a boost regulator, which will provide a very even voltage or current output voltage, but as the voltage of the battery drops, as soon as the regulator can't boost it enough it stops completely.
Even though the lamps may have the same number of LEDS, just wired differently, the boost circuit needs some power so the battery will deplete more quickly. where as if the leds are fed off the battery directly there is nothing to control the discharge and the battery may fall below optimum voltage but it will still continue to deliver some current hence the dimming of the leds and lasting longer.
Another reason is that the LED semiconductor chip itself suffers from overheating over time ( they sometimes pulse the leds above their 'normal' voltage to achieve a higher brightness) and this degrades their light output eventually) The multiple LED versions have several parallel strings of LEDs( in series with a current limiting resistor). If one of the LED has a slightly lower forward voltage drop it will cause that string to take slightly more current than the other strings and will cause the rest of the LEDs in the string to warm up causing a slow thermal runaway and loss of brightness ( Way back when , I served my apprenticeship at Ferranti they had an LED development and production department - for military devices and I got instructed all about LEDs right from the manufacture of the chip itself ,connection to the lead frame , encapsulation and testing , which included colour, forward voltage and luminance matching).They have made great strides since then in the brightness, colour range e.t.c but those fundamentals are still valid . Some of the cheaper brands on the market may not fully test for endurance of their products. I've even seen the Alko ATC green/red LED suffer such a fate , even though its completely sealed inside the plastic encapsulation its light output had dropped away so much you could only see it at night! What Prof said about the drive ICs is also valid . I have several workshop LED lamps and they do have a habit of suddenly switching off once the battery voltage dropped beyond a certain value (these IC's are constant current drivers rather than the buck boost ICs.
The COB (Chip on board) has multiple LEDs on a single backplane ( which also serves as a heatsink) Two of my high intensity , focusable LED torches have four ultrabright LEDs on board and at one focus setting you can actually see the pattern of the LEDs and the gaps between -it looks like dark cross on white background. I've recently seen 300W COB LEDs advertised for sale how times have moved on
The COB (Chip on board) has multiple LEDs on a single backplane ( which also serves as a heatsink) Two of my high intensity , focusable LED torches have four ultrabright LEDs on board and at one focus setting you can actually see the pattern of the LEDs and the gaps between -it looks like dark cross on white background. I've recently seen 300W COB LEDs advertised for sale
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