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Varrying Forward Voltage? — Parallax Forums

Varrying Forward Voltage?

JedJed Posts: 107
edited 2008-01-17 15:43 in BASIC Stamp
In the specs for the Blue LED for example, the Forward voltage is listed as MIN: 2.8 and MAX 3.2. Does the forward voltage vary during power? or does it ramp up to 3.2 when power is applied? Or does it vary with the current (and if so what are the min and max values for current to get those volts?) How does this factor into designing circuits, do you just ignore the lower value and use the max value?

Comments

  • Mike GreenMike Green Posts: 23,101
    edited 2008-01-17 15:41
    The forward voltage can vary with temperature. It does vary with the amount of current through the LED. There are graphs in the datasheet that characterize this. The forward voltage also varies a little from LED to LED due to manufacturing variations.

    How you use it depends on the circumstances. If you're calculating the value of a series resistor, you might use the minimum value. That way, if the forward voltage is actually higher, you'll get a lower current through the LED than you planned rather than a higher current which could damage something.

    With all of these values, you "play it safe" by using the most restrictive value (lowest gain, highest logic 0 threshold, etc.) With hobby or
    "one of" projects, sometimes you measure the actual value and use that or use a more "typical" value, then check the functioning of the circuit to make sure it works properly.
  • allanlane5allanlane5 Posts: 3,815
    edited 2008-01-17 15:43
    An LED is a highly "non-linear" device. A "linear" device would be a resistor -- as you increase voltage across it, the current through the device increases linearly with the Voltage Law (V == I * R, or Voltage == Current * Resistance).

    Now, an LED is very non-linear.· So, once you get to 2.4 volts, the current through it can vary by quite a lot, while the voltage goes up very little.· This is why it's easy to 'burn out' an LED if you don't put a current-limiting resistor in series with it -- the LED is NOT inherently current limiting by itself.

    The light output of an LED is also very non-linear.· Typically they're 'dim' around 5 to 7 mA, 'fully lit' around 8 to 15 mA, and burn out around 25 mA.· So you've got LOTS of slack to play with.· You really DO NOT want to design at the "max" voltages or currents of the device -- because you burn out a lot of LED's that way, or 'waste' current making the LED 1% brighter.

    Typical values for the current limiting resistor are 220, 330, and 470 ohms.· With a 5 volt driving signal, 1.4 volt RED LED drop, that leaves 3.6 volts for the resistor to drop.

    V == I * R, 3.6 volts / 470 ohms == 7.6 mA.· If your LED will light with this, fine.

    With a 2.4 volt Blue LED drop, that leaves 2.6 volts for the resistor to drop.

    2.6 / 470 == 5.5 mA, 2.6 / 330 == 7.8 mA, 2.6 / 220 == 11.8 mA.· Were I you, I'd try those three, and use the resistor that gave sufficient brightness.

    Note it's a typical newbie mistake to figure your LED at a 3 volt drop, and want to put 25 mA through it.· That, or leave out the current-limiting resistor entirely.· Both of which tend to burn out LED's -- which are VERY long life devices if used properly.

    Post Edited (allanlane5) : 1/17/2008 4:04:36 PM GMT
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