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### Topic: LEDs in series (Read 665 times)previous topic - next topic

#### gizahuna

##### Dec 15, 2012, 12:09 am
I'm new to all this and I have two LEDs (super bright white) from Radio Shack connected in series.  When I connect the circuit to the Arduino the lights turn on but very dim.  Is it because I am drawing too much power then the 5v from the arduino?  Basically my students want to create headlights for their robot.  I figured two LEDs in series would do the trick and not take up an extra pin on the board.  Is there a better way to do this?

The specs for the LED are below
Reverse voltage: 5V
Continuous forward current: 30mA
Peak forward current: 100mA
Power dissipation: 120mW
Operating temperature range: -25°C to 85°C
Storage temperature range: -25°C to 100°C
Forward voltage: 3.6V
Reverse current: 60 ?A
Luminous intensity: 16,000 mcd
Peak emission wave length: CIE coordinates typically X: 0.3, Y: 0.31
Viewing angle: 22°

#### Grumpy_Mike

#1
##### Dec 15, 2012, 12:10 am
Quote
Forward voltage: 3.6V

So two in series require at least 7.2V to drive them. The arduino output is only 5V.

#### dhenry

#2
##### Dec 15, 2012, 12:15 am
Quote
I am drawing too much power then the 5v from the arduino?

The voltage isn't sufficient to get both of them to conduct sufficiently.

#### krupski

#3
##### Dec 15, 2012, 12:34 amLast Edit: Dec 15, 2012, 12:36 am by Krupski Reason: 1

I'm new to all this and I have two LEDs (super bright white) from Radio Shack connected in series.  When I connect the circuit to the Arduino the lights turn on but very dim.  Is it because I am drawing too much power then the 5v from the arduino?  Basically my students want to create headlights for their robot.  I figured two LEDs in series would do the trick and not take up an extra pin on the board.  Is there a better way to do this?

The specs for the LED are below
Reverse voltage: 5V
Continuous forward current: 30mA
Peak forward current: 100mA
Power dissipation: 120mW
Operating temperature range: -25°C to 85°C
Storage temperature range: -25°C to 100°C
Forward voltage: 3.6V
Reverse current: 60 ?A
Luminous intensity: 16,000 mcd
Peak emission wave length: CIE coordinates typically X: 0.3, Y: 0.31
Viewing angle: 22°

The only things that matter to this discussion are:

* Continuous forward current: 30mA
* Forward voltage: 3.6V

Two in series requires 7.2 volts, so you need to power each one individually because 5 volts isn't enough.

Also, 30 mA is too much for an Arduino port to supply, so you will need a driver transistor to turn the LED on and off.

Since the forward voltage is 3.6 and you want to use 5.0 volts, you need a resistor R=(5-3.6)/0.03 = 46.6 = 47 ohms.

So, you would need TWO 47 ohm resistors (one for each LED) and two driver transistors if you want to control each LED separately, or one if you want to control both at once.

Lastly, paralleling the two LED's is not a good idea because their forward voltage drop is not exactly equal and one will surely draw more than the other, giving an unbalanced amount of light.

To be sure... the Arduino will drive the switching transistors only, the transistors will control the LED's. You cannot connect the LED's directly to the Arduino ports (they draw too much current and will destroy the chip).

Of course you know that you need an appropriate resistor between the Arduino port and the base of the transistor as well......

Hope this helps.

Gentlemen may prefer Blondes, but Real Men prefer Redheads!

#### gizahuna

#4
##### Dec 15, 2012, 12:46 am
Do they make a lower voltage LED so two can be hooked up in series?

#### oric_dan

#5
##### Dec 15, 2012, 12:58 am
Yes, but not in white color. Red Leds may be as low as 1V or so.

What most people will probably do at this point is run the Leds [in series] off the
main battery, and use an NPN or n-channel MOSFET inverter to drive the Leds.
For only 30 mA, almost any NPN will do, like 2N2222. Choose the collector R value
to push the correct Led current and make certain to saturate the transistor.

http://www.kpsec.freeuk.com/trancirc.htm

#### krupski

#6
##### Dec 15, 2012, 01:01 am

Do they make a lower voltage LED so two can be hooked up in series?

To my knowledge, most of them are at least 3 volts forward. I don't recall ever seeing ones with a 2.5 volt drop.

If you want to make LED "headlights", then you only need one transistor, two LED's and 3 resistors (two 47 ohm and one around 560 to 680) to go between the base of the NPN transistor and the Arduino port.

Isn't that do-able?
Gentlemen may prefer Blondes, but Real Men prefer Redheads!

#### krupski

#7
##### Dec 15, 2012, 01:04 am

Yes, but not in white color. Red Leds may be as low as 1V or so.

What most people will probably do at this point is run the Leds [in series] off the
main battery,
and use an NPN or n-channel MOSFET inverter to drive the Leds.
For only 30 mA, almost any NPN will do, like 2N2222. Choose the collector R value
to push the correct Led current and make certain to saturate the transistor.

http://www.kpsec.freeuk.com/trancirc.htm

Hey that's an idea... if the main robot power is more than 5 volts (which I'll bet it is), then run the LED's from the unregulated power source. Just put the LED's in series, choose the right resistor and use a little NPN transistor to turn it on and off (2N2222, 2N3904, etc... type).
Gentlemen may prefer Blondes, but Real Men prefer Redheads!

#### oric_dan

#8
##### Dec 15, 2012, 01:27 am
Also, when the headlights start getting dim, you know the battery is running down,