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Understanding LEDs

Discussion in 'Electronic Basics' started by Jon, Jan 25, 2004.

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  1. Jon

    Jon Guest

    I've picked up a couple of yellow LEDs at Radio Shack to tinker around with,
    and I've got a few questions...

    They are labeled 2.1 volt, 20mA. I recently built a 1.2V - 25V variable
    voltage regulator, so I think I have the power supply that I need. At
    first, I simply set the voltage on my voltage regulator to 2.1 volts, and
    hooked up an LED. Naturally, it got fried in less than a minute. Too much
    current. Then, I did a little math, and realized that if I hooked up two
    220 ohm resistors in parallel, I could get about 19 mA at 2.1 volts, which
    is pretty close to what the LED was labelled. I hooked it up, and it seems
    to work.

    I double checked everything before I hooked up the LED, and sure enough, I
    was getting about 19 mA, as predicted. When I hook up the LED, though, I
    get about 2 mA through the circuit. Also, when I measure the voltage across
    the LED, I get about 1.9V. So, I have a couple of very basic questions.

    1) When the package specifies a voltage (2.1 V in this case), does that
    mean that the voltage drop across the LED should be 2.1 volts, or does it
    mean that I should provide a 2.1 volt source, as I did in the example above?

    2) When an LED is rated for 20 - 30 mA, does that mean that I can pull that
    many amps through the LED, or should I set the current prior to hooking up
    the LED, as I did in the example above.

    3) Any good tutorials on using LEDs, and calculating the R values required
    for proper usage? I guess that I'm hung up on hooking LEDs up in series
    with resistors, and calculating the voltage drops.

    Thanks!!!

    -Jon
     
  2. You need a resistor in series with the LED to limit the current, or a
    constant current source, not constant voltage. You subtract the rated
    voltage on the LED, and subtract that from the supply voltage Use Ohms
    law to calculate the resistance needed for the desired current . You do
    not want to push the current to the max rating, or the LED won't last
    very long.

    Try using Google. There are millions of circuits available online, if
    you take a little time to look for them.
     
  3. LEDs often do not do well when connected directly to a voltage source.
    The current drawn by them could be almost anything, and LEDs tend to
    become more conductive as the temperature rises.
    The normal way to use LEDs is to use a dropping resistor and a voltage
    higher than that of the LED.
    With a 2.1 volt supply and the LED dropping 1.9V, there is only .2 volt
    across the resistor.
    Assuming the LED drops 2.1V at 19 mA and you need 2.1V to push 19 mA
    through the resistor, you need 4.2V to push 19 mA through the series
    combination.
    That is the typical voltage across the LED at specified current. But
    apply this voltage, and the current may be significantly different and may
    vary drastically with temperature.
    That is how much current can safely flow through the LED.
    I have one - http://www.misty.com/~don/ledd.html

    - Don Klipstein (, http://www.misty.com/~don/ledx.html)
     
  4. Gareth

    Gareth Guest

    It means that the voltage drop across the LED will be 2.1V when a
    particular current is flowing through it, for example it may say

    Vf = 2.1V @ 10mA.

    which means if you put 10mA through the LED the voltage across it you
    should see 2.1V, but it will vary slightly with temperature so don't
    worry if you get a slightly different answer. If the current is less
    the voltage will be very slightly less, that is probably why you
    measured only 1.9V with 2 mA.
    That is probably the maximum current that you should put through the LED
    Probably, but there isn't really that much to it.

    When you connect your LED and resistor in series the supply voltage is
    slit between the two components (as it is for any components connected
    in series), so:

    V_supply = V_resistor + V_LED

    You know that the LED has a forward voltage drop of 2.1V because it says
    so on it, that means that the voltage across the resistor will be

    V_resistor = V_supply - 2.1

    You want a current of less than 20mA for the LED, lets say 10mA, that
    should be quite bright and well below the maximum current.

    so to calculate the resistor value you have:

    R = V_resistor/I

    where R is resistor value in k Ohms, and I is current in mA

    R = (V_supply - 2.1)/10

    you see that you need a supply voltage higher than 2.1 for this to work,
    otherwise you get a resistor of zero ohms, then there is nothing to
    limit the current and, as you found, the LED will be damaged.

    Lets say 6V just as an example

    R = (6-2.1)/10 = 0.39k Ohms = 390 Ohms

    Or if you want to use one of the 220 Ohm resistors that you have:

    V_supply = 10*0.22 + 2.1

    = 4.3V, so set the supply voltage to 4.3V

    --
     
  5. The voltage across an operating LED is determined primarily by the
    chemistry of the LED (and the chemistry also determines the colour),
    so you should _never_ connect an LED to a constant voltage source
    (like a regulated power supply).
    Generally, that would be the _maximum_ rated current for the LED -
    greater currents are likely to reduce the LED's life. It is wise to
    operate LEDs at somewhat less than their maximum rating - I find most
    LEDs are bright enough at 8 - 10 mA.
    The normal (and safe) way to operate an LED is to use a supply voltage
    somewhat higher than the LED's rated voltage, and use a resistor in
    series to limit the current to the desired value.

    Normal red, yellow and green LEDs are 1.7, 1.9 and 2.1 volts, or so -
    I'm lazy, so I just say they are all 2 volts, and, as I said, 10 mA
    seems to be a nice current - well within ratings for common LEDs. To
    determine the resistor value, I subtract the LED voltage from the
    supply voltage, and use Ohm's Law: R = E/I. For 5 volts, the resistor
    is then (5 - 2)/.01 = 300 ohms. The resistance is not very critical -
    a higher value will make the LED a little dimmer, while a lower value
    will make the LED brighter (unless you exceed the rated maximum
    current.)


    --
    Peter Bennett, VE7CEI
    peterbb (at) interchange.ubc.ca
    new newsgroup users info : http://vancouver-webpages.com/nnq
    GPS and NMEA info: http://vancouver-webpages.com/peter
    Vancouver Power Squadron: http://vancouver.powersquadron.ca
     
  6. The forward voltage of any diode at any particular forward current is
    hard to predict, exactly. Slight changes, like batch to batch
    variations and temperature cause the same voltage drop to occur for
    quite a range of possible currents. The 2.1 volt spec is a typical
    voltage drop with the full rated 20 milliamps of forward current. The
    way to find the actual forward voltage is to pass the specified
    current and measure the voltage. But if you apply a stiff (well
    regulated) voltage, you will find that a about 25 millivolts change in
    total voltage will double or halve the current. Slight errors in
    voltage correspond to big swings in current. Which is another way of
    saying that big swings in current produce only slight changes in
    voltage drop. And as the LED warms up, the current climbs rapidly
    with a constant voltage applied. These problems are what make it so
    hard to parallel any pair if diodes (have them share the same voltage)
    and expect the current to split equally between them.
    No matter what you do, the current must be kept below the maximum
    rating or the diode will be damaged by heat. If you apply just a tiny
    bit extra voltage, the LED will happily pass way too much current and
    melt down.
    The rule is quite simple, you assume the forward voltage is about what
    the data sheet says, and use a resistor to waste the extra voltage
    while passing the desired current at that resistive) voltage drop. As
    long as that resistor has a lot more ohms than the incremental
    resistance of the diode, it will dominate the current regulation.

    So lets say that the LED will go from 10 to 20 ma as the forward
    voltage goes from 2.1 to 2.140 volts. That implies an incremental
    resistance of (2.140 - 2.1)/(20 ma - 10 ma)=4 ohms. So if the
    resistor is at least about 10 times this resistance, it will
    effectively over ride the current hogging effects of the diode.

    If you want to drive this diode from a 5 volt supply, you assume the
    resistor has to waste the extra 5-2.1=2.9 volts. You choose a 10 ma
    (.01 amp) operating point (if you use a generic engineering safety
    factor of 2) so the resistor has to be about 2.9/.01= 290 ohms. This
    is a lot higher than the minimum 40 or so calculated, above, so it
    should be quite stable. Then you select the nearest standard value,
    270 or 300 ohms. Voila.
     
  7. Jon

    Jon Guest

    Thanks for all the info. It seems so easy after reading all of your
    explanations that I feel foolish for posting the question in the first
    place.

    I thought your explanation was particularly enlightening, John.

    Jon
     
  8. JD Steffen

    JD Steffen Guest

    Okay, so reading though the replys to this post I have learned the proper
    way to
    determine what value current-limiting resistor to use in an LED circuit. But
    I do not
    understand one thing. I always thought that a circuit would consume only as
    much
    current as it needed up to what was available from the supply (until it
    blows a fuse
    or trips a breaker). So you can see where I am a little confused. How does
    an LED
    overconsume current to the point where it blows up? Is it some property of
    semi
    conducters that I am ignorant to? I have no formal education in electronics,
    it's
    more of a passing interest to me. So please excuse me if this is one of
    THOSE
    questions ;)

    Thanks for any input any of you can provide!

    JD
     
  9. Thank you. You have learned something fundamental about diodes of all
    kinds.
     
  10. Lets take a different case. Lets say you want ot test a fuse to find
    out how long it can stand various currents without melting. You have
    an adjustable voltage supply rated for a current well above the fuse
    rating. How would you go about testing a 100 ma fuse to find out how
    it reacts to 200 ma using this supply? The fuse has a cold resistance
    that you can measure, but changes resistance (increases somewhat) when
    it heats up.

    This problem has a lot to do with driving LEDS, in that the device
    being tested has a low resistance, and a rating based on current.

    No matter what voltage you set the supply to, the current will not
    stay at the desired 200 ma. because of the increase in resistance as
    the fuse heats up. The current in any diode will go up as the device
    heats up.

    You will have to add some current regulating mechanism to the circuit
    and provide enough extra voltage to let this mechanism operate in
    order to get a nearly constant desired current through the fuse during
    the test.

    Do you see the similarity between this test circuit and the one used
    to operate LEDs?

    Some devices are voltage operated and draw an appropriate current if
    you apply the appropriate voltage. Integrated logic chips, opamps and
    resistive heaters fall into this category. Other devices deal
    primarily in the realm of current and only operate correctly if
    something else in the circuit controls the available current instead
    of the applied voltage. LEDs and fuses are examples.
     
  11. dB

    dB Guest



    Part of a posting in another forum (the link will take you to the
    particular FAQ) .....


    Have another look at the "generic diode" curve (in the l.e.ds and
    zeners thread) in the FAQ section.

    Both "knees" are similar. If you apply an increasing voltage (either
    forward or reverse) very little current will flow until the critical
    knee voltage is reached.

    We use an l.e.d by biasing it in the forward direction, and use a
    zener by biasing it in the reverse direction.

    You MUST get used to thinking that l.e.ds and zeners
    are CURRENT operated devices, and that the current has to be limited
    with a series resistor.

    We DO pass a current through it and a certain voltage will be
    developed across it.

    We DON'T apply a voltage across it and cause a current to flow through
    it.



    http://pub40.ezboard.com/fbasicelectronicsfrm5.showMessage?topicID=16.topic
     
  12. Olaf

    Olaf Guest

    when considering home-equipment like vacuum-cleaners, lightbulbs etc used
    in the normal way, you are right: each device generates exactly the
    current it needs to operate and not more. But these are devices designed
    to be used safely on a wall-outlet. When considering a single component or
    designing circuits you cannot assume things work out all right. This also
    applies to conductors!

    When a voltage is applied to such a component or circuit it will generate
    a current. But not the current it 'needs', it generates the current it
    can. More precise, it will limit the current only as much as it can. So if
    you connect a simple 5000 ohm (R) resistor to a 10V DC (U) powersource, it
    will limit the current to I = U/R = 10/5000 = 0.002 Ampere. Simple, no
    problem. But now connect a 5 ohm resistor to the same powersource. This
    time the current will be I=10/5= 2 Ampere. 2 Ampere could blow a fuse in
    the powersource, but let's assume it doesn't. Now the resistor will
    generate heath: P = V*I = 10 * 2 = 20 Watts. 20 Watts of heat is quite a
    lot, a normal resistor will be gone before you know what has happened.

    Now for the led: current can only pass through in one direction and when
    connected that way directly to the powersource the led offers no
    resistance to the current, it will not limit the current the same way the
    resistors did in the example above. So the current will become very large
    and something will blow. Could be the fuse, probably it will be the led.
    If you don't want the led to blow you have to find a way to limit the
    current. Resistors are capable of doing this, that's why you connect one
    in series with the led. You've found the calculations in the other part of
    the thread.

    I hope this explains a bit, bye, Olaf
     
  13. JD Steffen

    JD Steffen Guest

    Ah, that makes sense now. Thanks everyone for the responses. It
    has helped alot.

    Thanks,

    JD
     
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