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Current Limiting Resisitor for LED / Optocoupler — Parallax Forums

Current Limiting Resisitor for LED / Optocoupler

ArchiverArchiver Posts: 46,084
edited 2000-10-09 19:02 in General Discussion
I am in need of some assistance in determining the
correct series resistor value to use for driving an
LED or an Optocoupler.

For an LED, I am assuming from all my reading that a
330ohm resistor in series is sufficient. But, if I
refer this back to calculations, I have a problem. If
I know the forward voltage and current of the LED, how
do I calculate the current limiting resistor value?

Appreciate any advice,

Jack

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Comments

  • ArchiverArchiver Posts: 46,084
    edited 2000-10-09 03:37
    In a message dated 10/8/00 9:32:14 PM Central Daylight Time,
    jscasserra@y... writes:

    > For an LED, I am assuming from all my reading that a
    > 330ohm resistor in series is sufficient. But, if I
    > refer this back to calculations, I have a problem. If
    > I know the forward voltage and current of the LED, how
    > do I calculate the current limiting resistor value?

    Subtract the forward voltage (Vled) from five, then divide by the forward
    current (iled).

    R = (5 - Vled) / iled
  • ArchiverArchiver Posts: 46,084
    edited 2000-10-09 14:18
    Check out my web page, I have complete information on just how to calculate
    exactly the correct current limiting resistor for your needs.

    Gary

    www.microcontrollertricks.cjb.net
    Original Message
    From: "Jack Scasserra" <jscasserra@y...>
    To: <basicstamps@egroups.com>
    Sent: Sunday, October 08, 2000 9:31 PM
    Subject: [noparse][[/noparse]basicstamps] Current Limiting Resisitor for LED / Optocoupler


    > I am in need of some assistance in determining the
    > correct series resistor value to use for driving an
    > LED or an Optocoupler.
    >
    > For an LED, I am assuming from all my reading that a
    > 330ohm resistor in series is sufficient. But, if I
    > refer this back to calculations, I have a problem. If
    > I know the forward voltage and current of the LED, how
    > do I calculate the current limiting resistor value?
    >
    > Appreciate any advice,
    >
    > Jack
    >
    > __________________________________________________
    > Do You Yahoo!?
    > Yahoo! Photos - 35mm Quality Prints, Now Get 15 Free!
    > http://photos.yahoo.com/
    >
    >
    >
  • ArchiverArchiver Posts: 46,084
    edited 2000-10-09 17:10
    Hi Jack,

    In addition to the other suggestions, you might want to read:
    http://www.al-williams.com/wd5gnr/basiccir.htm

    This page covers dropping resistors, voltage dividers, Thevinin equivalents,
    and power transfer theorem in a practical way.

    Regards,

    Al Williams
    AWC
    * Floating point math for the Stamp, PIC, SX, or any microcontroller at
    http://www.al-williams.com/awce/pak1.htm


    >
    Original Message
    > From: Jack Scasserra [noparse]/noparse]mailto:[url=http://forums.parallaxinc.com/group/basicstamps/post?postID=5L2W1ycjLbXBfFcxzjblbpXJaFoZPXpJcpg6XJfhVB_znG4icv_hXmBUTb7mlvL7P9zQV5wa3v7EizbJkLzl]jscasserra@y...[/url
    > Sent: Sunday, October 08, 2000 9:31 PM
    > To: basicstamps@egroups.com
    > Subject: [noparse][[/noparse]basicstamps] Current Limiting Resisitor for LED / Optocoupler
    >
    >
    > I am in need of some assistance in determining the
    > correct series resistor value to use for driving an
    > LED or an Optocoupler.
    >
    > For an LED, I am assuming from all my reading that a
    > 330ohm resistor in series is sufficient. But, if I
    > refer this back to calculations, I have a problem. If
    > I know the forward voltage and current of the LED, how
    > do I calculate the current limiting resistor value?
    >
    > Appreciate any advice,
    >
    > Jack
    >
    > __________________________________________________
    > Do You Yahoo!?
    > Yahoo! Photos - 35mm Quality Prints, Now Get 15 Free!
    > http://photos.yahoo.com/
    >
    >
  • ArchiverArchiver Posts: 46,084
    edited 2000-10-09 17:53
    >If
    >> I know the forward voltage and current of the LED, how
    >> do I calculate the current limiting resistor value?
    >

    As I see it, wouldn't it be simply:

    +5 o
    /\/\/\/\/
    |>|
    (Ground)
    source Rlim LED

    Assume, for example, that the LED should be limited to 10 mA. Then

    Rlim = 5/0.010 = 500 ohms.

    Steve

    Steve Roberts: sroberts@s...
  • ArchiverArchiver Posts: 46,084
    edited 2000-10-09 19:02
    In a message dated 10/9/00 11:57:11 AM Central Daylight Time,
    sroberts@s... writes:

    > As I see it, wouldn't it be simply:
    >
    > +5 o
    /\/\/\/\/
    |>|
    (Ground)
    > source Rlim LED
    >
    > Assume, for example, that the LED should be limited to 10 mA. Then
    >
    > Rlim = 5/0.010 = 500 ohms.

    There is an error here -- you need to subtract the LED's forward voltage from
    the supply. Many LEDs have a forward voltage of 1.2 volts. If this were the
    case, the correct value would be:

    Rlim = 3.8 / 0.01 = 380 ohms

    A 330 ohm resistor would give you 11.5 mA and a 470 ohm would give you 8 mA.
    Both are "standard" (5%) resistor values.
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