Control the current in a 12V LED lamp for video lighting

Discussion in 'Lighting Equipment' started by yalanliu, Dec 5, 2017.

  1. I'm trying to dim a 12V LED lamp (LEDs specification here however schematics have the real values...) I've build for a video lighting application. My initial thought was to use a potentiometer but it seems that the power consumption of the lamp (7.56 W measured with a multimeter) is to high for a standard potentiometer. Unfortunately I can't found the specification of my Omeg 470 Ohm LIN.A potentiometer so I've tried it and now I think is burnt.

    schematic

    simulate this circuit – Schematic created using CircuitLab

    The lamp is for video lighting, I'm using a camera with a 30-60 fps so I don't know if a can use a Pulse-width modulation because I think it can cause a flickering effect in the video. Further more is for a radiometric experiment and I would like to have a stable output but answer for decent result are however welcome. What are the possible alternatives? Is true that:

    LED’s cannot be dimmed using resistive dimming; it will slowly damage the LED’s.
    ?

    Please note:I'm not interested in power consumption, because is not for standard lighting...EDIT: Answers involving Pulse-Width modulation are welcome if they works for this application
     
  2. As long as the PWM frequency is much higher than the frame rate it'll work. So you're looking at a chop frequency of around 5 to 6 KHz minimum to keep the frame-to-frame variation within 1%.

    At that sort of frequency you might run into non-linearity problems due to capacitive loading.

    All that's really needed is to limit the current into the LEDs, and this could be done with a transistor (or mosfet) or two attached to a suitable heatsink.

    Wirewound pots can be bought in a range of power-handling ratings BTW. You just need to look harder! 470 ohms across 12volts dissipates only 0.3W, but maximum dissipation occurs at lower resistance values. Anyway, you can get rheostats that dissipate tens of watts.

    Another, and simpler option is to attenuate the LED output with ND gels in front. Maybe a perforated screen would work. Or two offset slitted screens that can be overlapped to provide variable attenuation. Or crossed polarisers - there's loads of ways to do this.

    "LED’s cannot be dimmed using resistive dimming; it will slowly damage the LED’s?"

    - I've not heard that before. LEDs are designed to run over a range of current, and resistive limiting is the commonest way to control current. It's more likely that PWM could cause damage through repetitive momentary capacitive current surges.
     
    Last edited: Dec 5, 2017
  3. Keep it simple. ND is cheap and quick. I also tend to find that light frost does a good job and you can make fine adjustments by clamping the frost over the doors and varying the opening of the doors. Other methods? Iris down the camera or move the light further back. Dimmers are mainly used on large sets where it would be difficult to get to the lights easily.
     
  4. I can build a constant current power supply with the LM317T voltage regulator and can regulate the current up to 1A. However, I am afraid the color temperature would change.
     

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