LED Throwies

Step 2: Test the LED

Test your LED to determine color, brightness and functionality. Pinch the LED legs, or leads, to the battery terminals. The longer LED lead, called the anode, should be touching the positive terminal (+) of the battery and the shorter LED lead, called the cathode, should be touching the negative terminal (-) of the battery.

Note that the positive terminal on the battery has a larger contact surface than the negative terminal. The positive terminal extends around the sides of the battery. Don't let the cathode lead of the LED accidentally touch the positive terminal of the battery. This will create a short and cause the LED to function improperly.

For more information on LEDs click here.
For more information on batteries click here.

Comments

[genericanonymity]

"Note that the positive terminal on the battery has a larger contact surface than the negative terminal. The positive terminal extends around the sides of the battery. Don't let the cathode lead of the LED accidentally touch the positive terminal of the battery. This will create a short and cause the LED to function improperly."

Is this particularly common? I'm hoping to let people choose their own colors for an LED project, so I'm not intending to attach all LEDs to a battery in advance of the project.
If you know any good tricks to avoid letting the cathode lead from contacting the positive side of the battery, what are they?

[wjustin]

(removed by author or community request)

[enemigo]

coat the battery with silicone or tape to only expose the actual front and back sides and not the wrap-around edge.

[Linkster999]

i think u have ur info wrong Cathode is positive and Anode is negative

[humming]

No cathode is definitely negative and anode is positive.

[tegan44]

It actually depends on who teaches you - Physics in high school and college taught me cathode is negative, anode is positive, whereas engineering classes in college taught me the opposite.

In the end it doesn't matter as long as you keep your conventions consistent and know how to correctly wire up circuits in practice.

[shilpi97]

you r correct.......

[WearJDV]

As I read this ... I am trying to think of something really cool to do with these ...

[k247br]

i think that is true.maybe you should think about it.
if it were entirely based on current limitation and not voltage, wouldn't hooking a 12V LED up to a car battery which is capable of pushing 300 CCA require a massive amount of resistance, compared to, say, hooking one up to 4 CR2303 batteries in series?

this is you web site :http://www.magical-led.com

[gammaraptor]

Well, all my LED's are rated for around 2.2V. This sucks because i can't use just one 1.5v battery. I can't use 2 either, because it melts the chip. Anyways, what you need to do is to find a resistor....

[zack247]

cr2032s are 3v...

[intellijel]

Why are no current limiting resistors used? Does the CR2303 already have built in current limiting?

[DavidKaine]

The only time you need current limiting is if the maximum voltage drop across the LED is less than the forward voltage of the battery. In this case, the CR2303 is 3.0V forward, and most of the LEDs are 3.0V rated, with 3.6V max.

[bearsinthesea]

If you put in a resistor, would it make the battery last longer? (with a dimmer LED) Or would it waste power as heat dissipated through the resistor?

[svnt]

That explanation is at best limited and pretty inaccurate. It works with a CR2303 because of the internal resistance of the battery. The fact that the battery is 3V helps by not requiring the battery or LED to dissipate too much power. For a (potentially dangerous) counter-example, find a nice power supply that has a 5 A + current limit and set it to 3V. Then hook up your LED and look at the current consumption (probably 100mA+). Then look at the maximum rated current in the datasheet (probably 20mA). Then think about how long that LED will last.

[DavidKaine]

Every battery has an inherent internal resistance due to the materials that it is made from, not because it is manufactured that way. I was trying to stay away from being too technical with my description, but I guess I didn't do so well, lol. The actual answer is a combination of mine and SVNT's. If it were entirely based on current limitation and not voltage, wouldn't hooking a 12V LED up to a car battery which is capable of pushing 300 CCA require a massive amount of resistance, compared to, say, hooking one up to 4 CR2303 batteries in series?

[Void Schism]

Sounds to me like people are having problems with semiconductors. Being they do not follow Ohm's law. A semiconductor junction (such as the P-N junction in the LED) will drop a constant voltage in it's normal region of operation. Thus, if you have an LED with a 3V forward bias voltage, and a 5V battery, you have 2V over a negligible resistance. Ohm's law does apply to the tiny wire inside the LED connecting the top face of the semiconductor to its respective "leg", and so the current it is conducting rockets up until the wire burns out (much like a fuse). Providing the battery voltage is close enough to the diodes forward bias then the current drain is set by the junction (usually about 20mA) and the LED emits light as expected. If you added resistance to this then voltage would be dropped across the resistor, and if this voltage is too high, then the LED will have insufficient Voltage to emit light.

[intellijel]

DK's explanation had me scratching my head :P svnt's explanation makes more sense and is what I basically assumed: the CR2303 essentially limits current output internally.