I am an engineering student and have performed various practicals related to logic gates in which LED is my output device.I always have used 330Ω/470Ω resistor in series with the LED.But I don't know the reason behind that.I even know that without that resistor, LED sometimes malfunctions, causing it to stop working. But yet don't know how that resistor helps in protecting LED?

Any help from anyone will be of great benefit for me...

 

This is a pretty good explanation:
http://led.linear1.org/why-do-i-need-a-resistor-with-an-led/.

 

Then it comes to LEDs always think current not voltage. A LED can easily conduct a current that will destroy it. So you must always use some sort of method to limit the current. You may build a constant current source. But the simplest way is to use a series resistor to limit the current. Else you may burn out the LED very quickly. Take a look here. It may help you http://www.societyofrobots.com/electronics_led_tutorial.shtml

 

I seriously hope this doesn't turn into an engineering student trying to convince us that a resistor in series with an LED "wastes energy" (actually had one express that as an opinion).

 

Series resistors do "waste" energy, but it is a necessary waste unless some more efficient scheme like a current-mode SMPS is used.

In the same way, ordinary friction brakes and clutches on a car waste energy, but they are necessary unless more sophisticated systems such as regenerative braking or continuously variable transmissions are available.

 

It is all about current limiting.

 

A led requires very less current (about 15 mA).So if large amount is supplied to it it may be damaged.In order to limit the large amount of current we use resistors in series to the LED depending on the supply we apply .

 

a resistor in series with an LED "wastes energy"

And a led without a resistor wastes leds.

 

I am an engineering student and have performed various practicals related to logic gates in which LED is my output device.I always have used 330Ω/470Ω resistor in series with the LED.But I don't know the reason behind that.I even know that without that resistor, LED sometimes malfunctions, causing it to stop working. But yet don't know how that resistor helps in protecting LED?

Any help from anyone will be of great benefit for me...

Resistors limit current. In a typical application, a resistor is connected in series with an LED makes Enough current flows to make the LED light up, but not so much that the LED is damaged.
So it’s vital to stay within the limits of the LED. If you would attach an LED to a 5 Volt power supply directly, you would burn it instantly. The high current would destroy the pn-gate. That’s the point where the current limiting resistor comes in.

And a led without a resistor wastes leds.

 

A LED tutorial...

LEDs, 555s, Flashers, and Light Chasers

 

Ideally, an LED driven by a current source would operate safely without wasting power. Many OEM vendors offer SMPS units which output constant current for LED drive. The SMPS would output a safe steady current to the LED & the voltage generated by the SMPS would equal the LED forward drop. No wasted power would be incurred since no resistor is used.

But if the LED is to be supplied from a CVS (constant voltage source), then a resistor is mandatory. A loss will be incurred, but can be minimized by using a CVS whose voltage value is not much greater than the LED maximum forward voltage drop.

If the LED has a max Vf of 2.2V, a 3.3V supply plus dropping resistor works well without too much power lost. But variations in Vf have a strong influence on forward current. A 5V supply reduces said variations but more power is lost in the resistor.

It's a trade off, but definitely avoid using a 24V supply as much power is wasted in the resistor. A current source can be made from discrete parts. A pnp current source can drive a 2.2V LED from a 3.3V source with very little variation in current as Vf varies, while keeping power loss low.

You have options, & creativity can really help. Just take my advice. Never place an LED directly across a voltage source, because that is usually certain doom for the LED. An LED is a current-driven device, & direct voltage drive means death for the LED.

Claude

 

If you are talking strings of LEDs, then 24VDC is exactly what you need. One resistor can control the current for 8 or so LEDs.

 

I have split the hijack off and moved it to Resistors and LEDs. We will carry on the conversation over there.