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The new led strips

jonwjonw Posts: 67
edited 2011-09-28 15:17 in BASIC Stamp
The guide lines allow for some non parallax chat and i feel parallax members are some of the best,so here goes with my questions.
I read that while regular leds require constant current sources the new led strips need a constant voltage source.
Q1. Is a regulated power supply the same thing as a constant voltage source?
Q2. Does any one know how to explain the two in a way that is easy to understand.
Q3. And if a constant current source is required for regular leds,,,then why do we all use regular power supplies ,,to light them,,,and it seems to work fine. ( ie what may the writer have meant to say that)
Thanks , jonw

Comments

  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2011-09-27 22:16
    A1. Typically, yes.

    A2. The forward voltage of LEDs can vary a little from part to part, from batch to batch, or with temperature. The current vs. voltage curve takes a steep excursion around the forward voltage -- as it does with any diode. So regulating an LED's voltage is a risky proposition, due to the slope of the current curve near Vfwd. For that reason alone, it's always best to regulate the current instead of the voltage.

    A3. The reason "regular power supplies" work with LEDs is due to the resistor always placed in series with one (or a string) of them to limit the current.

    -Phil
  • davejamesdavejames Posts: 4,047
    edited 2011-09-28 09:55
    Phil Pilgrim (PhiPi) wrote: »
    A2. The forward voltage of LEDs can vary a little from part to part, from batch to batch, or with temperature. The current vs. voltage curve takes a steep excursion around the forward voltage -- as it does with any diode. So regulating an LED's voltage is a risky proposition, due to the slope of the current curve near Vfwd. For that reason alone, it's always best to regulate the current instead of the voltage.

    So the voltage would be set to overcome the highest Vfwd in the string, thus ensuring a desired current ("current compliance")?
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2011-09-28 10:37
    No. That would fry the LED from too much current. You need to either limit the current with a series resistor or regulate via feedback. Just providing a regulated voltage to a non-current-limited string of LEDs will not work.

    -Phil
  • davejamesdavejames Posts: 4,047
    edited 2011-09-28 10:59
    "...limit the current". That's the key.

    So, no matter what Vfwd might exist in the string, the current through the string will be limited to 'X' amount.

    But won't that still result in variations of brightness, and isn't that what is being sought to avoid?
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2011-09-28 12:14
    If the voltage across the current-limiting resistor is large w.r.t. the total possible forward voltage variation of the string, the variation in current from string to string will be minimal. Of course, this presumes that the lumens-vs-current curve for each LED is identical, which it won't be. In fact, it tends to vary a lot from batch to batch, since LED manufacturers are not able to control their processes to the extent necessary for predictable output. Typically, high-brightness LEDs are binned after manufacture into brightness categories, but the output can vary even within one bin. The only way to guarantee constant lumens is with a light sensor in the LED driver's feedback.

    -Phil
  • jonwjonw Posts: 67
    edited 2011-09-28 12:58
    So putting a series resistor in, basicly makes it a constant current supply.

    But I still do not really under the difference between CC and CV supplies. Is it the feedback or the way its regulated that makes the difference.

    One person explained that a constant current source will have a constant current ( sounds reasonable right ) But does that mean the voltage varies to keep the current steady?

    Thanks for all the kind replys.
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2011-09-28 13:04
    jonw wrote:
    One person explained that a constant current source will have a constant current ( sounds reasonable right ) But does that mean the voltage varies to keep the current steady?
    Yes, that's exactly what it means.

    -Phil
  • jonwjonw Posts: 67
    edited 2011-09-28 14:50
    Phil Pilgrim (PhiPi) wrote: »
    Yes, that's exactly what it means.

    -Phil

    So phil the voltage will change,,,to keep the current steady? ( sorry to repeat )
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2011-09-28 15:17
    With a properly-designed current regulator, yes.

    -Phil
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