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### Topic: Resistor with 6V LED? (Read 1 time)previous topic - next topic

#### schoolsterz123

##### Jun 19, 2012, 06:19 pm
If I have a 6V LED and run power from the 5V pin through the LED to ground would I need a resistor at all because it's a 6V bulb? What is the point of using the 5V pin when you could one of the I/O pins and set it as an output and HIGH?

I am not sure what would be the best approach. Are you not supposed to use the 5V pin? On the Ardunio Uno info page it says, "Supplying voltage via the 5V or 3.3V pins bypasses the regulator, and can damage your board. We don't advise it." I'm not sure why it is there if they advise you not to use it?

Thanks for the help!

#### pylon

#1
##### Jun 19, 2012, 06:28 pm
What is the maximum current allowed through your LED? You must take care of that. What does the 6V mean. Is this the voltage drop of your LED? You're writing about a bulb, is it an LED alone you're trying to drive or a whole circuit with an LED and some other parts (which typically are within a bulb)?

#### schoolsterz123

#2
##### Jun 19, 2012, 06:38 pm

What is the maximum current allowed through your LED? You must take care of that. What does the 6V mean. Is this the voltage drop of your LED? You're writing about a bulb, is it an LED alone you're trying to drive or a whole circuit with an LED and some other parts (which typically are within a bulb)?

I am using one of these green bulbs "http://www.superbrightleds.com/cgi-bin/store/index.cgi?action=DispPage&Page2Disp=%2Fspecs%2Fba9s_1led.htm" that is 6V for an A-B Pushbutton that I want to light using my board. I'm not to sure about the voltage drop as I'm still trying to learn all of this. Eventually I want to have a panel with various 6V bulbs and switches but for now I'm just trying to figure out what is needed to just light the bulb.

#### pylon

#3
##### Jun 19, 2012, 06:58 pm
Although there's no datasheet (at least I cannot find one) I guess from the description these bulbs are complete circuits (LED plus current limiting resistor for the voltage stated). So you probably could just drive them with the 5V pin. To be on the safe side use a multimeter to get the resistance value of the bulb. If it's in the range of 200 to 400 Ohm you can connect it directly between 5V and GND.

#### DVDdoug

#4
##### Jun 19, 2012, 07:00 pmLast Edit: Jun 19, 2012, 07:03 pm by DVDdoug Reason: 1
That "bulb" already has a resistor inside.

Quote
If I have a 6V LED and run power from the 5V pin through the LED to ground would I need a resistor at all because it's a 6V bulb?
No.  6V is the "rating".  The actual voltage depends on the power you connect.  Running from 5V and adding a resistor does not increase the voltage.   It decreases the voltage (to the lamp).   Your lamp will run slightly dimmer at 5V (compared to 6V), but it should be fine.  If you add an external resistor, some voltage gets "dropped" across the resistor... the 5 volts gets divided between the resistor and lamp, so there would be less than 5V across the lamp.  It would be even dimmer.

Quote
What is the point of using the 5V pin when you could one of the I/O pins and set it as an output and HIGH?
If you need to switch the lamp on/off with software, use an I/O pin.   Otherwise, I'll turn the question around...  What's the point of using an I/0 pin if you are not going to turn on/off the lamp with the I/O?   If you use an I/O pin, you'll have to program it.  And, I/O pins are more "fragile" than the power supply, and if you accidently short-out an I/O pin you could damage the chip.

Quote
On the Ardunio Uno info page it says, "Supplying voltage via the 5V or 3.3V pins bypasses the regulator, and can damage your board.
They are talking suppliying to the Arduino board....   You are talking about supplying power from the board to an LED lamp.   It's OK to "pull" a little power from the board to power a few LEDs, etc.

#### schoolsterz123

#5
##### Jun 19, 2012, 07:14 pm
Thanks for the help guys! Makes a lot more sense now.

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