Problem: burning leds in series

[NicolaBu]

Hello,

Yesterday i was trying to build my first led (in series) board. I used 3 red leds (1.8V, 20 mA) and 4 AAA battery (= 6V).
I calculated the right resistor ( 6V - 1.8V - 1.8V ) / 2 mA = 30 ohm; i should use a 47 ohm resistor but i used a 100 ohm resistor on the positive wire.
I tried the battery on my board and the three leds lighted up, but after 3 minutes suddently the leds turned off.
I checked the current with my multimeter (was 6V) and tried to light a single led with arduino but it didn't worked.
Someone can explain my error? Because i don't understand why using a bigger resistor it can be happen.
How can i correct my board

Thank you so much!

[larryd]

6-1.8-1.8=2.4V
2.4V/.02A= 120 ohms

three leds

3 or 2?

[NicolaBu]

Sorry, my bad

3 leds (6V - 1.8V - 1.8V - 1.8V) = 0.6V / 20 mA = 30 ohm.

But i used a 100 ohm resistor for being sure, and my leds burned. Isn't it strange?

Thank you!

[larryd]

See the drawing for a method you should use.

[NicolaBu]

Thank you for the scheme! Are they leds in parallel right? I use the single led like if it was the only one (right?).
6V - 1.8V = 4.2V / 20 mA = 210 ohm. And with this method my leds won't burn?

But you know why my first method did work only for a few minutes and then burned? I need to know

Thanks so much

[SirNickity]

Two things come to mind.  First, 4xAA is not 6.0v .. it'll probably be over 6.5v.  Not a big deal, just be aware and don't make assumptions.

Second, 20mA is the max of most LEDs.  Using a little less will prolong life.  Now, usually we're not talking lifespans of minutes either way, but still.  Are you sure your LEDs were rated for 20mA continuous anyway?  That's common, but not guaranteed.

[larryd]

Thank you for the scheme! Are they leds in parallel right? I use the single led like if it was the only one (right?).
6V - 1.8V = 4.2V / 20 mA = 210 ohm. And with this method my leds won't burn?

But you know why my first method did work only for a few minutes and then burned? I need to know

The "LED with its resistor" is in parallel with the "other pairs".
Your LEDs will be fine, 210 is not standard value, 220 ohms is (Red Red Brown).
I didn't see how your circuit was wired so its hard to say why they 'burned'. Your resistance was very low.
If you don't follow the current rating of the controller you can damage it!
If you follow the schematic, you are on the way to enjoying a fun hobby!

Edit: Look here for great information: http://arduino-info.wikispaces.com/

[fungus]

But you know why my first method did work only for a few minutes and then burned? I need to know

Because not all LEDs are equal and you weren't controlling the current.

Imagine one of the LEDs needs 1.7V and the other two need 1.9V (perfectly reasonable tolerances).

LED resistance varies with voltage (and changes very sharply at the 20mA point). The 1.7V LED could have presented a much lower resistance than the other two and taken far too much current, enough to kill it. When it died it's resistance dropped and the other two died as well.

Lesson: Don't try to control LED voltage, control the current. If you put in a 20mA limiter instead of the resistor it would have worked perfectly.

[dc42]

Connecting LEDs in series rather than in parallel is generally a good idea (it saves power), provided that you have either sufficient excess voltage that a resistor will give adequate current regulation, or a constant current driver.

In your case you had 3 LEDs each with forward voltage of 1.8V nominal, giving 5.4V nominal in total. This only allows 0.6V for the resistor, which is just 10% of the 6V nominal supply you were using. This means that if the supply is 10% higher than it should be, you get almost double the design current; and if it is 10% lower, you get almost no current at all. Fresh AA alkaline cells normally provide a little over 1.6V, so you were probably at 6.4V total, giving (6.4 - (3 * 1.8))/30 = 33mA if you used a 30 ohm resistor.

However, you used a 100 ohm resistor. So your LEDs should still be OK. Maybe you just had a bad LED, maybe you misread a 10 ohm resistor as 100 ohms, or maybe your batteries are too flat now to light the LEDs. I suggest you test each LED separately.

[Zapro]

Two things come to mind.  First, 4xAA is not 6.0v .. it'll probably be over 6.5v.  Not a big deal, just be aware and don't make assumptions.

In which part of the world are You located ?! Obviously a place where they sell AA Cells with 1,625V on them. Never ever seen a AA cell with more than 1,56V on it, and that's brand new ones...

// Per.

[Grumpy_Mike]

Never ever seen a AA cell with more than 1,56V on it, and that's brand new ones...

Well he and I are in the UK. I have a pack of batteries from a pound shop, that means that they are old or surplus stock.
They are Kodak XTRALIFE AA alkaline batteries, still in the blister pack. I just stuck the probe of my meter through the packaging and measured 1.61V. It looks like you get even older batteries in Denmark.

[Zapro]

Never ever seen a AA cell with more than 1,56V on it, and that's brand new ones...

Well he and I are in the UK. I have a pack of batteries from a pound shop, that means that they are old or surplus stock.
They are Kodak XTRALIFE AA alkaline batteries, still in the blister pack. I just stuck the probe of my meter through the packaging and measured 1.61V. It looks like you get even older batteries in Denmark.

Try replacing the batteries in your multimeter. If they are a bit on the low side, the meter reads too high. I've even seen this on Fluke and Agilent meters, so even the expensive ones can measure too high.

According to datasheets from Kodak, they make no 1,6V cells http://goo.gl/gp2uH

// Per.

[Grumpy_Mike]

Try replacing the batteries in your multimeter

They are new and the meter is calibrated.

According to datasheets from Kodak, they make no 1,6V cells

That is because they are not 1.6V cells. They are 1.5V cells that have a higher voltage when they are fresh.

[Zapro]

Try replacing the batteries in your multimeter

They are new and the meter is calibrated.

According to datasheets from Kodak, they make no 1,6V cells

That is because they are not 1.6V cells. They are 1.5V cells that have a higher voltage when they are fresh.

Exactly. Place a 50mA load on them and they will fall to the nominal 1,5V

// Per.

[SirNickity]

I would rather argue which 9v tastes better on the tip of your tongue than whether AA cells could or should ever read 1.6v.  The point of the statement is and was that a battery won't be a perfect round 1.5v, and assuming this is the highest voltage you'll ever see (times number of cells) may lead to scorch marks.