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 Post subject: LEDs develop heat
PostPosted: 26.02.2003 02:27 
I finished a project today - the glowpad table. It is basically a small glass table illuminated by two LEDs. I'll send in some pictures to the gallery very soon.
The two leds are 1000 MCD blue 4,6V 20mA. I have connected them to an old battery charger from a radio controlled car. (The battery charger is 11,6V and 150mA). With some simple calculations, you will see that I am using to many volts for the two LEDs (connected in series). Does the mA have anything to say? Do they get to much mA?
Well - I felt the leds just now, and they had developed quite alot of heat. Almost enought to melt the glue from the hot glue gun. Why do the LEDs develop heat? Is it because I am giving them too much current? If I don't insert a resistor, will the LEDs burn out quickly?
Hope you can answer my questions, and as I said - I will get some pictures up soon. (Personally, I think it's the coolest thing I've ever made...)


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 Post subject:
PostPosted: 26.02.2003 03:04 
Here are some pictures:
Image

Image


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 Post subject:
PostPosted: 26.02.2003 09:29 
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Joined: 24.01.2002 10:09
Posts: 4683
Location: Finland
Milliamperes flowing through the leds depends on the voltage they are having. Higher the voltage -> higher the current -> more heat. Just calculate a proper current limiting resistor to that circuit.

R=U/I = (11.6V - (2*4.6))/20mA = 120 ohms.

Just add that resistor to the circuit and you are set. Btw. Nice looking table. :)


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 Post subject: baard
PostPosted: 26.02.2003 11:28 
Thanks! I'll add a resistor to it.


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 Post subject:
PostPosted: 26.02.2003 11:33 
Quote:
R=U/I = (11.6V - (2*4.6))/20mA = 120 ohms.


Japala, shouldn't I divide it by 40mA when I have two LEDs? So it would be like this: (11.6V - (2*4.6))/40mA = 60 ohms ?


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 Post subject:
PostPosted: 26.02.2003 11:47 
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Joined: 24.01.2002 10:09
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Nope. When connected in series the same current will flow through every components so 20mA is correct. When you connect the leds in parallel they both need to have that 20mA at the same time so the supply will need to supply 40mA. You should read my Basic Electricity for modders to learn more of this. Really simple and easy document to view through: http://metku.net/index.html?sect=view&n ... /index_eng


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