Creating a Variable Brightness LED Panel

Discussion in 'General Electronics Chat' started by cokeguy, Aug 27, 2015.

  1. cokeguy

    Thread Starter New Member

    Aug 27, 2015
    1
    0
    Hello, I've spent countless hours reading through the forums, and i'm having a hard time Deciphering the best way to create a variable brightness LED panel. I do a lot of staged photography for things going on display. I want to create my own lightsource to be able to control the brightness when photographing my items. I want from dull amount of light, to as BRIGHHHHT as I possibly can get.

    - if I wire in 5 sets of LED Bulbs (5V-9V rated) in series (to make one row of 5 lights)
    - then create 10 rows of those 5 bulbs in parallel, I think thats the way I'd want the layout (unless someone says to go all parallel as a better idea)
    - Powersource from a 12V battery (like the UB1250 universal battery)

    - I want to use a potentiometer to control the variable brightness effect.


    How do I lay this out?
    How big of a potentiometer do I need?
    Do I want the lights to go in seriers + Parallel? Or just Parallel them all?

    Is there a formula someone could recommend to help show me the type of potentiometer I would need if I wanted to make the design bigger or smaller based on the amount of lights I want to use? The entire breakdown of math and understanding would bring me to a whole new level of happiness!

    Even if someone could be so kind as to draw a diagram and attach for me that would be amazing :)
     
  2. ian field

    Distinguished Member

    Oct 27, 2012
    4,415
    784
    You would be better off using the potentiometer to adjust a current limit circuit.

    There are 1 or 2 transistor limiters or a variation of the LM317 circuit - all easy to find on the web.
     
  3. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
    4,771
    971
    CRI is important for photography as well as color temperature..
    Most LEDs are lower CRI (compared to "non-led" based studio lighting) and not well suited for your application.
    LED selection is critical.. 85-90 CRI or better..

    But yes as suggested a potentiometer adjusted constant current source is the way to go..
    LEDs need to be driven from a constant current source.. not a constant voltage source like most other "devices"
     
  4. blocco a spirale

    AAC Fanatic!

    Jun 18, 2008
    1,440
    368
    If you want to avoid getting involved in electronics you don't understand; just add switches so that you can switch rows of LEDs off.
     
    Last edited: Aug 28, 2015
  5. wayneh

    Expert

    Sep 9, 2010
    12,155
    3,061
    There are scads of commercially available LED drivers/controllers. I wouldn't think of building one unless you truly cannot find what you need. Using the 12V source opens up even more options and I'd be very surprised if you cannot find what you need.

    Most of those controllers will use PWM (pulse width modulation) to dim the LEDs. I think that is superior for your application, because LEDs shift color slightly if the current level is changed. PWM maintains a constant peak current and varies perceived brightness by turning the LED on and off with a varying duty cycle. An analog dimmer varies brightness by reducing the constant current, and this can cause a color shift. Analog dimmers also waste more energy.

    If the slight color shift and the lower efficiency don't really matter to you, then an analog solution is just fine and would be easier to rig up yourself if you want a DIY solution. But a PWM dimmer is not so tough either.
     
  6. atferrari

    AAC Fanatic!

    Jan 6, 2004
    2,652
    767
    I am afraid that the OP has in mind a rheostat able to handle lot of power. :) The last thing to be of use here.
     
  7. GopherT

    AAC Fanatic!

    Nov 23, 2012
    6,073
    3,846
    @wayneh

    The trouble with purchased LED controllers is that they use PWM. And photographers don't like that since the PWM is generally in the 60 to 600 Hz range and shutter speeds are also in the 1/60 to 1/1000 second range. They could easily clip half a pulse of full brightness one photo and a full pulse at full brightness on the next pose.
     
    #12, Sinus23 and absf like this.
  8. wayneh

    Expert

    Sep 9, 2010
    12,155
    3,061
    Yikes, good point. I didn't realize they were usually that slow.
     
  9. ian field

    Distinguished Member

    Oct 27, 2012
    4,415
    784
    A couple of years ago; someone posted on a Usenet group, asking whether phosphor persistence affected the suitability of white LEDs for use in ignition timing strobes.

    Originally; they used blue LEDs with a yellow phosphor - which probably had a fair bit of persistence.

    AFAIK: they've switched to putting white phosphor on UV LEDs - not sure how that affects persistence, but if I had to guess, I'd say shorter.
     
  10. GopherT

    AAC Fanatic!

    Nov 23, 2012
    6,073
    3,846
    I checked white LEDs up to 1kHz (using a green LED as a detector). Still nice square wave. You are right - short.
     
  11. Bernard

    AAC Fanatic!

    Aug 7, 2008
    4,176
    397
    Have you considered brighter LED's & fewer, brighter than unknown 50.
    Maybe something like Nichia, NFSW157AT, 48 L, 720 mW, 3.2 - 3.6 V, @ 200 mA.
    A pattern of 9 = 432 L. 3 in series, 3 lines would need 3 CC controllers; or with a 5 V regulated supply all in parallel with resistors & as suggested in post #4- 9 switches.
     
  12. GopherT

    AAC Fanatic!

    Nov 23, 2012
    6,073
    3,846
    Put them on full brightness and buy some filters to tone them down as needed.
     
Loading...