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Simple LED circuit

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PDubya

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What's the best way to determine how many LED's I can safely power with an L7805A power circuit?

Maybe the better question is, what's the best way to power 15 LED's (5xRGB)?

Thanks!
 
Your LM7805 setup will much more likely be limited by the allowable heat. This is simply (Vin-5v)*I. We can't tell you the dissipation without knowing the size of the heatsink. If you have no heatsink, a TO220 pkg reg example has a max die temp of 125C and a rise of 65 deg C/watt. So with a room temp of 25C (it'll be different for a hot car) you can dissipate an absolute max of 1.54W IF there is open airflow around the reg. It is unwise to run the reg this hot for very long.

So if your reg was running off a car and the reg has no heatsink, at 14.6v (engine running, fully charged battery) it can handle 160mA. It would be wise to limit it to about half that to keep the temp reasonable. Even better, add a heatsink.
 
You don't need a voltage regulator when LEDs are operating from a wall-wart. Each LED just needs a current-limiting resistor.
With a 9V supply, the voltage for each red LED is about 1.8V so 3 can be connected in series and in series with a 180 ohm current-limiting resistor.
The red and green LEDs might have a voltage of 3.5V each so 2 can be in series and in series with a 100 ohm current-limiting resistor.
 
I have an question.. What type of LEDs do you have?
Standard LEDs have 2-3 volts drop, they would be fried when put on a 5V regulator..

If you connect them in series, you can drop the current to a reasonable level.
Othervise, you'll have to put a serial resistor on each parallel LED (300-500 ohms) to limit the current.
 
Beowulf > My initial thoughts were to just use 5 LED's of each color in channeled groups. So I'd have 1 group of 5 red, 1 group of 5 green, 1 group of 5 blue.

I assumed I'd be connecting a resistor to each LED. Never even thought I could connect them in series as Audioguru mentioned.

audioguru > I wish I could use this solution, but I'd like to be able to control each "group" of LED's with a microcontroller, so the clean 5v is sort of a necessity for my project. Sorry about not mentioning that, I assumed it's consumption would be neglegable in the grand scheme of things in comparison to the LED's
 
A microcontroller doesn't have enough output current to drive more than 1 LED brightly, or maybe 2 red ones in series. The LEDs would need series current-limiting resistors to limit the current to less than the 25mA max output of the microcontroller.

Since you have more LEDs than the microcontroller can drive, add a transistor to drive more of them for each color from 9V.

I don't know if your microcontroller will overheat if it tries to drive 15 LEDs with each one on a separate output port.
 
PDubya said:
Could I use one transister for each group, or would I need one for each led?
A 2N3904 little transistor can drive up to 10 LEDs at 20mA each.
A 2N4401 little transistor can drive up to 30 LEDs at 20mA each.
A 2N3055 big transistor can drive up to 750 LEDs at 20mA each.
Many big Mosfets can drive up to thousands of LEDs at 20mA each.
 
Connect two LEDs in series and you can double the number of LEDs!
 
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