A few bits about LEDs, including a response to Ron, ZL1TW's question about dimming over time.
LEDs do dim over time. Since they usually do not suffer catastrophic failure that stops all output altogether (like an incandescent bulb does when the filament fails) their end of life is typically measured as the point at which the light output has dropped to 1/2 the original design value. That operating life is generally in the vicinity of 100,000 hours. LEDs can harm your eyesight! Peering directly into an LED can cause eye damage. They are NOT lasers, but they can be very bright - much brighter than an incandescent bulb of that size. The issue is that the light is being emitted from a much smaller point source than other types of lamps, including incandescent bulbs. You have nothing to fear from incidental light from an LED, but some manufacturers warn against looking directly into an LED at close range. LEDs are current driven, not voltage driven. That is, the current through the device determines the brightness and, unfortunately, the temperature of the semiconductor inside. Higher than normal currents produce device heating that will reduce the light output and decrease the operating life. Like most epoxy-encased semiconductors, LEDs can EXPLODE when subjected to serious over-currents. The bang is small, but the lamp sprays epoxy shrapnel around. It could raise havoc with an eyeball that is close to it when it happens. Actually, that's true of many types of transistors when subjected to big over-currents, but LEDs seen to be the ones people are peering at when they go off! There's nothing to worry about if they are being operated at their proper current levels. The only danger is when the current-limiting resistor is not correct for the voltage source. LEDs can be damaged by ESD. Actually ANY semiconductor junction can be: it's a matter of how big of an ESD charge reaches is. Most diodes are pretty hard to hurt, so you won't find warnings to wear a wrist strap when handling them in the Elecraft manuals. Still a jolt can do damage to an LED junction. It's no where near as robust as, say, a power supply rectifier diode. Ron AC7AC -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of AA5CK Sent: Wednesday, July 12, 2006 10:07 PM To: ron_w Cc: [email protected] Subject: Re: [Elecraft] K2 backlighting brightness Hi Ron I went through this a few weeks ago. After trying several different value resistors in parallel with R 10 I ended up with a 39 ohm one. This dropped the the total resistance to about 18 ohms and increased the current from the original 24 mA to 34 mA. Still not as bright as #5523 but close. Further increase in current didn't seem to increase the brightness much so I went with the 39 ohm resister but YMMV. I don't know about LEDs dimming but an increase in current may shorten their life. Hopefully 5 mA per pair won't have a significant effect. Maybe we'll get a more definitive answer from the list. This is really not a big problem and I am certain I would have never noticed the difference in brightness had the two rigs not been setting side by side and even then it took several days before I noticed it. Good Luck....Ted...aa5ck ron_w wrote: > > Both K2's go remarkably the same, but the older K2 has a noticeably > dimmer display than the new one which raises the questions :- > (1) Do the back light LED's dim with time? ......the older K2 *has* > had a lot of use. > (2) Would changing R10 to (say) 22ohms have any detrimental effect on > the life of the back light LCD's? > . > Cheers.......Ron ZL1TW _______________________________________________ Elecraft mailing list Post to: [email protected] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com _______________________________________________ Elecraft mailing list Post to: [email protected] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
