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Thread: LED light panel

  1. #21

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    Re: LED light panel

    Quote Originally Posted by Daniel Moore View Post
    Macandal, I'm in Berkeley. Perhaps we could work together on such a project. Summer isn't a good projecty time of year for me though : ).
    I'm available. Let me know what's good for you.
    --Mario

  2. #22

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    Sep 2007
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    Re: LED light panel

    would be a good light source for lf enlargers.

  3. #23

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    Re: LED light panel

    This just occurred to me. Remember, I'm not a master builder but I was thinking, the guy in the video uses something to make sure that the voltage does not exceed 12V, isn't this what a dimmer does? Regulate the voltage and by regulating the voltage the light gets brighter and darker? Couldn't he just use a dimmer?

    (I really hope that wasn't a stupid question. )
    --Mario

  4. #24

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    Re: LED light panel

    Quote Originally Posted by macandal View Post
    This just occurred to me. Remember, I'm not a master builder but I was thinking, the guy in the video uses something to make sure that the voltage does not exceed 12V, isn't this what a dimmer does? Regulate the voltage and by regulating the voltage the light gets brighter and darker? Couldn't he just use a dimmer?

    (I really hope that wasn't a stupid question. )
    The voltage regulator he used has a trim pot to keep the voltage from going too high when turned full on, a regular dimmer doesn't have this so you would have no way to prevent too much voltage from reaching the light if turned up too far.

  5. #25
    jp's Avatar
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    Re: LED light panel

    Quote Originally Posted by macandal View Post
    This just occurred to me. Remember, I'm not a master builder but I was thinking, the guy in the video uses something to make sure that the voltage does not exceed 12V, isn't this what a dimmer does? Regulate the voltage and by regulating the voltage the light gets brighter and darker? Couldn't he just use a dimmer?

    (I really hope that wasn't a stupid question. )
    A dimmer switch is likely more expensive and complicated than a potentiometer (as in the video). They can use different means of modulating power that are undesirable.

    The pot has a wiper (middle pin) that moves along an semicircle track of resistor material (outer pins) and is real simple in operation. A 2.2k pot has 2200 ohms resistance between the outer pins connected to the ends of the resistor material. The circuit probably uses this change in resistance to change the duty cycle of pulses going to the LEDs. LEDs don't respond linearly to changes in voltage but you can modulate the energy going to them.

    As some LED color is from the LED semiconductor and some from phosphors, I think adjusting the output from LEDs would result in color shifts (phosphor glowing while semiconductor doesn't), which is probably the main complaint of cheap white LED construction.

  6. #26

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    Re: LED light panel

    LEDs are current devices; they'll drop around 2 to 2.5v across them pretty much irrespective of the voltage. The excess voltage (e.g. a 12v supply driving a single LED with a 2v drop has an excess of 10v) has to be dissipated across a resistor or a semiconductor current limiting device.

    The proper ways to drive an LED are twofold: either with an adjustable constant current device (i.e. a device which delivers a preset current in use, which is adjusted for the desired brightness) or by over-driving the LED with a higher current (its specification will indicate a maximum pulse current) and adjusting either the pulse repetition frequency or the on-off ratio.

    The second approach has the advantage that it will maintain the same colour temperature irrespective of brightness, but may have issues with strobing on a video shot. The first approach is technically cleaner, and can be more efficient, but it can change colour with brightness.

    One advantage in either approach is that you can place the LEDs in series to reduce the excess voltage - for example, eighty LEDs at two volts each would drop 160 volts, but the current would be the same as for one, so a dropper resistance from (say) 200v would be much smaller and be dissipating less power than just trying to drive a single LED from the same supply.

    Problems of colorimetry are fascinating: trichromic sensors whether film or semiconductor do not necessarily see light the same way the eye does: a mixed source of light with red, green, and blue components may fool the eye that it is seeing a single wavelength, but it's quite easy to find objects which reflect or absorb particular wavelengths which only appear the 'correct' colour under wideband light, e.g. sunlight or continuous spectrum incandescent lights. LEDs tend to be either single-wavelength (or close to it) for coloured LEDS, or phospors activated by UV for some purple and white LEDs. It might be worth playing with strips of red, blue, green, and white with individual dimmers...

    Neil

  7. #27
    (Shrek)
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    Re: LED light panel

    The bottom line is that LEDs simply do not dim. No, a household dimmer will not do the trick.

    In order to fake dimming, the LED is simply allowed to cycle off/on and such a frequency that the human eye cannot see the 'off' periods (yes, this allows the phosphorus-based LEDs to go out of balance, throwing off color rendition). More importantly for photographers, the higher the shutter speed, the greater chance you have of capturing some or all of an 'off' cycle. If it's cycling at 20kHz like modern fluorescents, that won't happen. But if it's a cheaper device cycling at 1kHz, it will happen.

    If you're planning on using LEDs for photography, I suggest avoiding any sort of dimming. Move the light back a few feet instead, or use a higher shutter speed, NDs, or whatever tricks you have. For enlarging, knock yourself out, but you will notice contrast changes in MG papers as you dim.

  8. #28

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    Re: LED light panel

    It's not that they don't dim, it's just that the available dimming methods aren't necessarily suitable for a given photographic method.

    But hey, I'm a natural light sort of chap

    Neil

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