L.E.D. question..again
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Posted by Palmer on December 16, 2004, 6:32 am
Please log in for more thread options at 20mA. I know that I need to connect them in series right? So I would have the positive wire to the first LED and then the negative side of the LED number 1 to the positive side of LED number 2 and so on and then the negative source wire to the negative side of LED number four. Oh yea ... a 180 Ohm resistor between the positive wire and the first LED. Is this correct? And if my source is a car...which puts out more than 12 volts....is a 200 to 220 Ohm resistor ok to use? Thanks for the help... My be simple for you but I'm trying to learn Screamn1 | |||||||||||||||||||||||||||||||||||||
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Posted by John Popelish on December 16, 2004, 2:09 am
Please log in for more thread options The typical drop of 4 LEDs in series at something near rated current is 4*2.1=8.4 volts. A car produces something like 13.8 volts when the engine is running, so that means you have 13.8-8.4=5.4 volts to burn in the current setting resistor. You want to keep the current below 20 ma for long life, so the minimum resistance would be 5.4/.02=270 ohms. Twice this resistance will produce a just noticeable drop in brightness. -- John Popelish | |||||||||||||||||||||||||||||||||||||
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Posted by Don Klipstein on December 18, 2004, 10:30 am
Please log in for more thread options >Palmer wrote:
>>
>> I'm going to connect 4 L.E.D.'s to a 12 volt source. Each LED is 2.1 volts >> at 20mA. I know that I need to connect them in series right? So I would have >> the positive wire to the first LED and then the negative side of the LED >> number 1 to the positive side of LED number 2 and so on and then the >> negative source wire to the negative side of LED number four. Oh yea ... a >> 180 Ohm resistor between the positive wire and the first LED. Is this >> correct? And if my source is a car...which puts out more than 12 volts....is >> a 200 to 220 Ohm resistor ok to use? >> Thanks for the help... >> My be simple for you but I'm trying to learn >> Screamn1 >
>The typical drop of 4 LEDs in series at something near rated current >is 4*2.1=8.4 volts. A car produces something like 13.8 volts when the >engine is running, so that means you have 13.8-8.4=5.4 volts to burn >in the current setting resistor. >You want to keep the current below 20 ma for long life, so the minimum >resistance would be 5.4/.02=270 ohms. Twice this resistance will >produce a just noticeable drop in brightness. With the 180 ohms proposed by Palmer, the current at 13.8 volts can be 30 mA (more likely a little less since the LED voltage drop will be slightly higher than at the usual 20 mA). Most LEDs have maximum current rating of 30 mA even if their characteristics are specified at 20 mA. But at 30 mA, do not expect 100,000 hour life expectancy unless thermal conditions are especially favorable - both leads of the LED should have a temperature not exceeding about 70 degrees C near the case even if you stay within the ratings. Having 100,000 hours of life with light output staying above 1/2 the "brand-new" amount often requires keeping the junction at a temperature not exceeding 85 degrees C. White LEDs have it much worse due to phosphor wear - skimp on current as much as you can if you need the kind of life expectancy many expect from LEDs. - Don Klipstein (don@misty.com) | |||||||||||||||||||||||||||||||||||||
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Posted by Watson A.Name - \"Watt Sun, th on December 18, 2004, 3:13 pm
Please log in for more thread options
> >Palmer wrote:
> >>
2.1 volts
> >> I'm going to connect 4 L.E.D.'s to a 12 volt source. Each LED is > >> at 20mA. I know that I need to connect them in series right? So I
would have
> >> the positive wire to the first LED and then the negative side of
the LED
> >> number 1 to the positive side of LED number 2 and so on and then
the
> >> negative source wire to the negative side of LED number four. Oh
yea ... a
> >> 180 Ohm resistor between the positive wire and the first LED. Is
this
> >> correct? And if my source is a car...which puts out more than 12
volts....is
> >> a 200 to 220 Ohm resistor ok to use?
> >> Thanks for the help... > >> My be simple for you but I'm trying to learn > >> Screamn1 > >
the
> >The typical drop of 4 LEDs in series at something near rated current > >is 4*2.1=8.4 volts. A car produces something like 13.8 volts when > >engine is running, so that means you have 13.8-8.4=5.4 volts to burn
minimum
> >in the current setting resistor. > >You want to keep the current below 20 ma for long life, so the > >resistance would be 5.4/.02=270 ohms. Twice this resistance will
> >produce a just noticeable drop in brightness. >
be
> With the 180 ohms proposed by Palmer, the current at 13.8 volts can > 30 mA (more likely a little less since the LED voltage drop will be
thermal
> slightly higher than at the usual 20 mA). > Most LEDs have maximum current rating of 30 mA even if their > characteristics are specified at 20 mA. > But at 30 mA, do not expect 100,000 hour life expectancy unless > conditions are especially favorable - both leads of the LED should
have a
> temperature not exceeding about 70 degrees C near the case even if you
output
> stay within the ratings. Having 100,000 hours of life with light > staying above 1/2 the "brand-new" amount often requires keeping the
current as
> junction at a temperature not exceeding 85 degrees C. > White LEDs have it much worse due to phosphor wear - skimp on > much as you can if you need the kind of life expectancy many expect
from
> LEDs.
Wow, that's a new term for me, Don. "Phosphor wear". I'm still running the four white LEDs in series, 24/7, since july 11, which is over 5 months. There are two Nichia NSPW510 white LEDs, which have been on since even earlier, and two Hong Kong LEDs, all in series and running at 20 mA. So far, the Nichias haven't dimmed much, but the HK LEDs are down to less than 1/4 brightness, I can't really say how much because I'm only making this by calibrated eyeball. ;-) That's over 3600 hours, getting close to 4000 hours by next week. Whatever this factor is, it's taking its toll on the HK LEDs. It was much worse at 25 to 30 mA. The HK LEDs lasted only a few months, and then they were really dim. Makes a believer out of me when you talk about the lead temp and such, junction temp of 85 deg, etc. These four LEDs are close together, but I don't think they have ever been warmer than 40 or so degrees C, so I don't think excessively hi should be a factor. I think that Nichia uses some kind of phosphor that's more resistant to degradation over time, and the HK LEDs are just cheap. I'll keep updating as time goes on, and as they dim into nothingness. I may decide to scrap the HK LEDs and put new ones on right after new year's eve, just to start out the new year with a 'fresh start'. ;-) > - Don Klipstein (don@misty.com)
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Posted by John Popelish on December 18, 2004, 7:07 pm
Please log in for more thread options "Watson A.Name - \"Watt Sun, the Dark Remover\"" wrote:
> Wow, that's a new term for me, Don. "Phosphor wear".
> > I'm still running the four white LEDs in series, 24/7, since july 11, > which is over 5 months. There are two Nichia NSPW510 white LEDs, which > have been on since even earlier, and two Hong Kong LEDs, all in series > and running at 20 mA. So far, the Nichias haven't dimmed much, but the > HK LEDs are down to less than 1/4 brightness, I can't really say how > much because I'm only making this by calibrated eyeball. ;-) That's > over 3600 hours, getting close to 4000 hours by next week. Whatever > this factor is, it's taking its toll on the HK LEDs. It was much worse > at 25 to 30 mA. The HK LEDs lasted only a few months, and then they > were really dim. > > Makes a believer out of me when you talk about the lead temp and such, > junction temp of 85 deg, etc. These four LEDs are close together, but I > don't think they have ever been warmer than 40 or so degrees C, so I > don't think excessively hi should be a factor. I think that Nichia uses > some kind of phosphor that's more resistant to degradation over time, > and the HK LEDs are just cheap. > > I'll keep updating as time goes on, and as they dim into nothingness. I > may decide to scrap the HK LEDs and put new ones on right after new > year's eve, just to start out the new year with a 'fresh start'. ;-) I would like ot see something more quantitative if you are going to go through all that trouble, anyway. If you connect a milliamp meter directly across a silicon photo diode, you get a pretty linear representation of the total light that falls on the diode. A large area diode placed directly in front of the LED captures a goodly fraction of its output in a fairly repeatable way. Once you have a few months data, you may be able to curve fit it to an exponential and state the decay rate as a time constant. This photo diode is a few bucks from Digikey: http://www.photonicdetectors.com/pdf/pdbc140.pdf -- John Popelish | |||||||||||||||||||||||||||||||||||||
> I'm going to connect 4 L.E.D.'s to a 12 volt source. Each LED is 2.1 volts
> at 20mA. I know that I need to connect them in series right? So I would have
> the positive wire to the first LED and then the negative side of the LED
> number 1 to the positive side of LED number 2 and so on and then the
> negative source wire to the negative side of LED number four. Oh yea ... a
> 180 Ohm resistor between the positive wire and the first LED. Is this
> correct? And if my source is a car...which puts out more than 12 volts....is
> a 200 to 220 Ohm resistor ok to use?
> Thanks for the help...
> My be simple for you but I'm trying to learn
> Screamn1