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LED Switching Regulator

Discussion in 'Electronic Theory' started by Hero999, Aug 12, 2008.

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  1. Hero999

    Hero999 Banned

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    Here's a floating version of this circuit.

    This can be used in the car with a chassis return.

    It also has the advantage of being short circuit proof and of being more efficient since the current driving the transistor isn't completely wasted.
     

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  2. Hero999

    Hero999 Banned

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    Another thing you should be aware of is that, if poorly designed, this circuit can fry LEDs if a brownout occurs. When the input voltage drops below a certain level, it will stop oscillating, this will mean the LEDs will overheat if they can't withstand the peak current continiously. The solution to this is to design the circuit so the peak current never exceeds the absolute maximum rating or add a low voltage disconnect circuit to protect it from a brownout.
     
  3. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hi there,


    The idea is to light the LED and also get good regulation
    with fluctuations in power supply voltage and LED voltage.
    Many converters like this draw more current as the power
    supply voltage rises, and sometimes we want to keep the
    output power at some predetermined level even if the
    voltage changes drastically.
    Perhaps you can do a quick study to see how much the
    LED current changes with power supply voltage, but
    you also need to add in some resistance in series with
    the inductor and in series with the power supply.
     
  4. dave

    Dave New Member

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  5. Hero999

    Hero999 Banned

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    I've it at different voltage and you're right the current regulation isn't perfect. Generally the current drawn from the supply reduces with increasing voltage and the LED current increases slightly.


    I don't understand why you would want to do this, the inductor already has a series resistance and the current sense resistor adds to this; why would you put another resistor in series with the power supply which would only make it less efficient?
     
  6. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Yes, the regulator ic's usually have some form of regulation built in
    to keep the output at some preset level, and it stays that way for
    a wide range of input/output conditions.

    The series resistor for the power supply is when you are using
    batteries for the actual real life power supply, i apologize if you
    are not doing this. The inductor series resistance also affects
    regulation sometimes and even circuit start up. If it's low
    however it wont be a problem, but i just thought i would mention
    it.
     
    Last edited: Nov 1, 2008
  7. Ubergeek63

    Ubergeek63 Well-Known Member

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    Sorry I missed this before... I usually post quickies unless I have something well formed already or my wife is out.

    So it sounds like you had that end right... it is just often even the dissipation in the rectifier is missed, never mind the inductor losses.
     
  8. Hero999

    Hero999 Banned

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    This circuit has regulation too, it just isn't perfect.

    It uses a comparator as a schmitt trigger with preset on and off threasholds which vary very little as the power supply voltage changes.

    I still don't see why you need a series resistor regardless of what the power supply is. It's true that you might want to incorperate over-current protection but a simple fuse or PTC resistor would do that.
     
  9. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Yes, while many regulation chips are made to be very close over
    a wide range of operating conditions. They have some extra circuitry
    built in for the sole purpose of regulating some parameter. This
    extra step is what puts them in a class where you start looking
    at little things like percent voltage regulation with line and
    with load. The dc accuracy is made to be very good on purpose,
    not as a by product of some other function for the same amplifier.
    This is often done with an integrator, added in just for that.

    When batteries are used as power sources in real life there is always
    some internal resistance, while in a spice environment you have to
    add it yourself. The point is that some power converters work
    very well with a zero impedance source, but as soon as you add
    some real world series resistance they dont regulate as well.
    This is why i suggested trying a circuit with some series resistance
    if it is going to be run on batteries.
     
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