why we use resistor with LED

Discussion in 'General Electronics Chat' started by shreekar27, Oct 26, 2011.

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  1. shreekar27

    Thread Starter New Member

    May 14, 2011
    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...:)
  2. KJ6EAD

    Senior Member

    Apr 30, 2011
    anhnha likes this.
  3. t06afre

    AAC Fanatic!

    May 11, 2009
    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
  4. Jon Wilder

    New Member

    Oct 25, 2011
    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).
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  5. Adjuster

    Late Member

    Dec 26, 2010
    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.
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  6. Lundwall_Paul

    Active Member

    Oct 18, 2011
    It is all about current limiting.
  7. Barasha Mali

    New Member

    Oct 13, 2010
    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 .
  8. gerty

    AAC Fanatic!

    Aug 30, 2007
    And a led without a resistor wastes leds.:D
  9. samin


    Oct 14, 2011
    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.

    :D :D
  10. Wendy


    Mar 24, 2008
  11. cabraham

    Active Member

    Oct 29, 2011
    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.

    Last edited: Oct 29, 2011
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  12. Wendy


    Mar 24, 2008
    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.
  13. Wendy


    Mar 24, 2008
    I have split the hijack off and moved it to Resistors and LEDs. We will carry on the conversation over there.
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