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Thread: Difference between voltage & current regulation?

  1. #1
    Enlightened
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    Default Difference between voltage & current regulation?

    Hi! Can anyone explain to me the difference between a voltage regulated flashlight and a current regulated flashlight?
    What are the pros & cons of both regulation systems?

  2. #2
    Flashaholic
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    Default Re: Difference between voltage & current regulatio

    For a LED light, current regulation is better. The forward voltage drop of a LED can vary from unit to unit, but the maximum safe current sould still not be exceeded. So if you design a voltage circuit for one LED, and then drop in another LED, the current through it could be a LOT more or a LOT less.

    However, with that being said, voltage regulation is usually "good enough" for an LED as long as you are careful, especially if you put in a small series resistor.


    For an incandecent, you probably want voltage regulation. The current through the bulb will start off VERY large until the filament heats up. Bulbs are designed for a certain voltage, and the current is what it is.

  3. #3

    Default Re: Difference between voltage & current regulatio

    While most incandescent bulbs are rated by voltage, I suspect they would be better off driven by a current regulator. This should increase their life and still maintain a constant output. Some aircraft halogens are rated by current.

  4. #4
    Flashaholic
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    Default Re: Difference between voltage & current regulatio

    It's worse than that with LEDs.

    As an LED heats up, the forward voltage drop goes down, which means if you are using a voltage regulated supply, the current goes up.

    When the current goes up, the LED heats up even more. The forward voltage drops even more. More current goes in.

    Pretty soon the LED goes pop. This is known as thermal runaway.

    I put a 5W Luxeon (mounted on a CPU heatsink) on a power supply, and adjusted the voltage to give 700mA. Within a short period of time (on the order of a minute?) the current was over 800mA and climbing.

    You definitely want a current regulated supply when working with LEDs. You can probably get by without one if you are only driving a couple of low-power LEDs within their specs.

    The guy who designed the Action Light did a bunch of bench testing a few years ago running Nichias at 30mA and found that unless you could really get the heat out fast, they would go into thermal runaway on a voltage regulator.

    Scott

  5. #5
    Flashaholic* idleprocess's Avatar
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    Default Re: Difference between voltage & current regulatio

    Incadescent filaments are effectively resistors.

    Supply them with voltage and they regulate their own current - depending on feed voltage.

    If you supply them with more voltage than they're rated for, they burn up and fail due to exceeding their watt rating, just like a resistor.

    Heat also kills LEDs, but their "resistance" is dynamic. If you limit the current and your current supply can provide enough voltage for the LED's Vf, it will run fat and happy for its theoretical lifespan. LEDs generally can't dissipate as much power as a resistor (with the exception of high-power LEDs), so they die from overheating quite quickly.

  6. #6
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    Default Re: Difference between voltage & current regulatio

    [ QUOTE ]
    Ocelot said:
    The guy who designed the Action Light did a bunch of bench testing a few years ago running Nichias at 30mA and found that unless you could really get the heat out fast, they would go into thermal runaway on a voltage regulator.

    [/ QUOTE ]

    These 'guy' .-) sorted the LEDs into 7 bins for their voltage at 20mA and even then they are only simialr at this current and pretty different at lower levels. There would be more to tell, but will be somewhat OT here.

    There was also the idea to drive an incandescent bulb at constant current and therefore having no need for an additional 'soft start'.

    To look at it from the other side:
    A battery is more or less a constant voltage source with a max current.
    A bicycle dynamo is a constant current device (= current regulation, with a max voltage depending on the rpm), which confuses people who are not use to them. I had some little 'flame wars' in CPF some years ago with people who did not understand the concept (but where quite good in electronics).

    That means, a bicycle dynamo is the perfect source for LED lighting and is therefore used with incandescents .-)

    That means also, that when your front bulb blows, the voltage at the rear light is not just a little bit higher (as it would be with a battery), it will reach the max voltage which could be pretty high and most likely will damage the back light very soon.

    I hope this helps a little bit with the difference in between volatge and current regulation.

  7. #7
    Flashaholic* MrAl's Avatar
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    Default Re: Difference between voltage & current regulatio

    Hello there,

    In addition to the other great posts...


    Let's try to answer a few questions about running LEDs properly
    in an order that might clear things up a little...


    What is a constant voltage source?
    What is a constant current source?
    What is the difference between the manufacturers 'voltage' specification
    and the manufacturers 'current' specification for an LED?

    We'll find that the answer to this last question really helps us
    find the answer to the best way to run an LED, but knowing the
    other two helps also :-)


    A constant voltage source is a source of energy that will keep the
    voltage level ACROSS an object the same regardless of how the object
    changes internally, with NO REGARD for what the current level is.

    A constant current source is a source of energy that will keep the
    current level THROUGH an object the same regardless of how the object
    changes internally, with NO REGARD for what the voltage level is.

    Now the manufacturer of the LED gives us various guidelines to
    go by when designing circuits for LEDs. Some of these guidelines
    include specifications that MUST be followed VERY CLOSELY and
    others are just general guidelines or typical running specs
    which arent exact for every single LED but just typical ratings.
    This means there are two basic types of specs given by the
    manufacturer so we can run LEDs in circuits:

    1. Specs that must be followed closely.
    2. Specs that are just typical of most of these kinds of LEDs.

    Now we are ready to ask the question:
    Is it better to run an LED with a constant current or a constant voltage?

    We look first at the LED specs, sorting them into one of
    the catagories above...

    THE 'CURRENT' SPEC:
    The manufacturers CURRENT spec for an LED says what the led can
    stand in terms of current through the device, and if it goes over
    a certain LIMIT the led might blow out. This is very important to know!
    We put this into catagory #1 because it has to be followed
    very closely. If we dont get this right or something about
    the led changes with time we blow out the led long before its
    useful lifetime. The current through the device must therefore
    be correct at all times, or at least below a certain LIMIT point.

    THE 'VOLTAGE' SPEC:
    The manufacturers VOLTAGE spec for an LED says what 'typical'
    voltage MIGHT be present when the led is in operation, especially
    around the recommended operating current level.
    We put this into catagory #2 because it's just something about
    the led that is likely to occur when running the led in a typical
    circuit. We should also note that this is NOT A SETPOINT!
    We cant really set the voltage at a particular level and expect
    the current to be correct, because the operating voltage
    changes with temperature as well as from package to package.


    The answer to our last question should be obvious now...
    Since the 'current' specification is so important to running
    an LED properly (if it changes by much it might blow out)
    we really have to regulate the current through the device,
    not the voltage across it. The voltage the LED will operate
    at might change a little while running but we dont really
    care that much as long as the current stays at a safe level.
    With the constant current source we set the current at a certain
    level and know it will always be correct regardless of what
    happens to the voltage, so we can be assured that the
    device will operate correctly for a long, long time.
    If we try to regulate the voltage what happens is we get
    the correct current level while we are setting it, but then
    later when the temperature changes we find the current
    has changed! Not only that, if it decides to change to
    a much higher level the LED blows out!
    Needless to say, this is not what we want, so we dont regulate
    the voltage.

    BTW, the LED is known as a 'current operated device' because
    the current is the most important parameter and is the
    parameter to set when designing a circuit for an LED.
    The current is the 'primary' parameter, while the voltage
    is the 'secondary' parameter.
    Yes, it's good to know what the voltage is, but it's better
    to know what the current is.


    Good luck with your LED circuits!

    Take care,
    Al

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