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Unsure What Some "Maximum Ratings" Terminology Means — Parallax Forums

Unsure What Some "Maximum Ratings" Terminology Means

Jonathan MorrisonJonathan Morrison Posts: 23
edited 2005-07-21 16:47 in Robotics
I am having some trouble understanding the ratings on a component I just bought (a 10 segment LED). It has the following info - stuff in red (notice it's all red [noparse]:)[/noparse] ) - I have no clue about and would be really grateful if someone could shed some light on them:

Absolute Maximum Ratings (Per Element):
Power Dissipation: 75mW
Forward Current: 20mA
Reverse Voltage: 5V
Reverse Current: 100 uA

Electrical Characteristics:
Forward Voltage: 2.0V (typ) 2.8 (max)
Forward Current: 20mA
Peek Wavelength: 697 nm

Thanks for any clarifications on these. Are some of these ratings just synonyms for stuff I already know - like R V and I? Thanks!

Comments

  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2005-07-21 16:23
    Jonathan,

    ·· The power rating in mW is based on the power and current.· 20mA is a standard upper range current draw in LEDs.· Most run at 20mA fine.· In your case, since you have the forward voltage and the current, you can easily calculate the resistor required to run this from a 5V source, such as a Stamp I/O pin.

    ·· So for an LED with a Forward Voltage of 2V on a 5V power supply drawing 20mA would require a 150 ohm resistor in series.· Although you could easily use a 220 aqnd draw less current.· Most of the other information like wavelength you won't need to worry about.· That's it's color output in the visible spectrum.· If it's red, it's red, if it's green, it's green.· Most people don't need to worry about the exact wavelength.

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    Chris Savage
    Parallax Tech Support
    csavage@parallax.com


    Post Edited (Chris Savage (Parallax)) : 7/21/2005 4:26:15 PM GMT
  • Jonathan MorrisonJonathan Morrison Posts: 23
    edited 2005-07-21 16:32
    Thanks! So what is the deal with forward and reverse voltage? Does forward mean "from battery to VSS"? What is reverse voltage and reverse current? Thanks.
  • allanlane5allanlane5 Posts: 3,815
    edited 2005-07-21 16:41
    Well, an LED (Light-Emitting Diode) IS a 'diode', so it has a forward biased and reverse biased direction. In forward bias, the voltage drop across it is 2.0 to 2.8 volts ("Forward Voltage" value -- it you put the LED and a current-limiting resistor in series, with 5-volts across the two. The LED will 'drop' 2.8 volts, no matter what the current is.) In this state, the LED will easily conduct enough current to destroy it -- which is why that current limiting resistor is in there. By the way, most LED's I've worked with are fully on at 15 mA, and destroy themselves at 25 mA -- 15 mA is a much better value than 20 mA.

    In reverse bias, the LED will take up to 5 volts before destroying itself, and will 'leak' 100 micro-amps in that condition. We don't usually reverse bias LED's, but there are some clever designs (bi-color LED's for instance) which take advantage of this. "Reverse Bias" here meaning you turn the diode around, put +5 on the cathode and ground on the anode.
  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2005-07-21 16:47
    Jonathan,

    ·· Actually those meanings are meant to be intuitive...Forward voltage and current are the voltage drop and current consumption when the device is connected properly.· As for the remaining items, instead of explaining, I will send you a link that might explain it in more detail so you understand.

    http://www.americanmicrosemi.com/tutorials/diode.htm



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    Chris Savage
    Parallax Tech Support
    csavage@parallax.com
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