LED lights

Submitted: Tuesday, Feb 11, 2003 at 15:41
ThreadID: 3329 Views:2094 Replies:4 FollowUps:7
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I'd like to run several LED units as general camping lights in my trailer annex and am trying to work out the best power source option. Though many retailers are happy to sell me a varirty of units, no-one seems able to tell me how much current they will draw.

Can anyone help with info on LED current draw?

Thanks
Rohan

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Reply By: plexus - Tuesday, Feb 11, 2003 at 16:13

Tuesday, Feb 11, 2003 at 16:13
As a rule of thumb, each LED will take 20mA of current. You can actually drive them higher (25mA or even 50mA if you pulse them like I do when I do any LED sign circuit designs) but use 20mA as a safe working limit for use on a continuous DC supply. To work out the series resistance you need to get the current limit to 20mA, just use Ohm's Law: R = E/I where I = 0,02 for 20mA. You get the answer in Ohms. For example, if you have a 12 Volt supply, then R = 12/0,02 = 600 so a 600 Ohm series resistor is necessary. Just use the next standard value up, for example a 620 or 680 Ohm is common. Mind you, I'm ignoring the voltage drop across the LED and just using the value of the supply voltage. In reality E would be a little less (say 1,7 V or so). When using more than one LED, you can either run them in parallel or series. The voltage drop across an LED is around 1,7V, ~ 1,9V so if you need to run several LEDs in series, you can quickly get to the point where your 12V supply won't be enough to kick the string over. I prefer to have them all wired in parallel. The disadvantage to this is that you need a separate series resistance for each LED, but on the other hand if one dies, the rest of the cluster will still operate and so if reliability is important, you'll still have light. If you want any more elaboration, feel free to email me.
AnswerID: 12903

Follow Up By: Plexus - Tuesday, Feb 11, 2003 at 16:14

Tuesday, Feb 11, 2003 at 16:14
BTW my email is root@plexus-technologies.com
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FollowupID: 7531

Follow Up By: Diamond - Tuesday, Feb 11, 2003 at 18:19

Tuesday, Feb 11, 2003 at 18:19
wow what the mind me never to ask a difficult question LOL
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Follow Up By: Member - Rohan - Tuesday, Feb 11, 2003 at 20:34

Tuesday, Feb 11, 2003 at 20:34
Thanks. I think I've got that. Putting together the lights is no drama. If I run 3 lights, each running 3 diodes, I should get about 100 hours from an 18 amp battery.

By the way, I found your web site entertaining. If I need any any IC board work done, I know where to come.
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Follow Up By: Brian - Tuesday, Feb 11, 2003 at 23:41

Tuesday, Feb 11, 2003 at 23:41
Try this for the calcs http://www.bit-tech.net/article/68/
Brian
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Reply By: Mark - Tuesday, Feb 11, 2003 at 20:16

Tuesday, Feb 11, 2003 at 20:16
Rohan,
Check out this link http://www.geocities.com/george_tlc/led.html
good info on LED conversions.
Mark
AnswerID: 12918

Follow Up By: Member - Rohan - Tuesday, Feb 11, 2003 at 21:09

Tuesday, Feb 11, 2003 at 21:09
Thanks Mark. George has put together some good stuff here. I might even be inspired to replace a few of the Nissan's standard interior lights.
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Follow Up By: Jack - Tuesday, Feb 11, 2003 at 21:42

Tuesday, Feb 11, 2003 at 21:42
Hi
I hae just taken delivery of some of George's lights for the interior of my 80 Series landcruiser .. and they are brilliant .. in more ways than one. Highly recommended. And George's service is prompt, courteous and efficient.
Jack
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Reply By: David - Tuesday, Feb 11, 2003 at 22:01

Tuesday, Feb 11, 2003 at 22:01
BTW I think you are on the right track with LED's.
I fly for Qantas- we have just taken delivery of three new 747 ER's in the last month or so ( we are THE launch customer) and nearly all the cockpit lights are LED on the new planes!
Not much to do with camping I know, but you get the idea... well sort of?
AnswerID: 12928

Reply By: peck - Friday, Feb 21, 2003 at 16:20

Friday, Feb 21, 2003 at 16:20
Rohan
Plexus' suggestions are Ok except he overlooked the power loss in the series resistors. Putting many LEDs in series such that the total forward voltage drop matches input voltage (say about 6-8 with a smaller series resistor to take care of voltage fluctuation) will provide much more light (the number of LEDs in series times) for the same 20ma current draw ... a much more efficient array!
AnswerID: 13666

Follow Up By: Member - Rohan - Monday, Feb 24, 2003 at 12:14

Monday, Feb 24, 2003 at 12:14
Thanks Peck. I've already put together a board with 3 LED's in parallel. Each LED has a 480 Ohm resistor (pushing 25 mA through each LED). It all works fine but with the highly directional light output, doesn't provide enough general illumination. I'll give the series set-up a go. I assume if I were to use 6 LED's in series, I'd need a single 80 Ohm resistor.

Rohan
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