LED conversion of Zone VI VC head?

Discussion in 'Enlarging' started by BradS, Feb 3, 2009.

  1. BradS

    BradS Subscriber

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    I was thinking that the Zone VI variable contrast head might make a really nice LED conversion. There is plenty of room in the head itself and the little control box could be adapted to the same purpose as the original cold light design.....


    Has anybody else attempted this? Any ideas / suggestions?
     
  2. domaz

    domaz Member

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    People have done it. You can search the forums here and find a few examples. The easiest way to do it is to get some 1w or so Luxeon LEDs, some thermal epoxy and a power supply. Find an arrangement of LEDs that covers the negative evenly and you are good to go. If you want to try to use RGB LEDs to modify the color of the light to do VC that is much harder. Calibration being probably the hardest part.

    A note regarding power supplies: To power the LEDs do not use resistors, it's a very bad idea if you want consistency and for the LEDs to last any time at all. Instead use a constant current source regulator. Something like a BuckPuck would be ideal. The BuckPuck can handle 32V so one unit could power about 8 1w LEDs. Get two of those units and a 32V power supply and you would be set for 16 LEDs.
     
  3. BradS

    BradS Subscriber

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    I'm curious. Why are resistors a bad idea? I don't understand. Can you elaborate on that?
     
  4. Curt

    Curt Subscriber

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    I just read that Power LEDs are better than using resisters, I still don't know exactly why, but the article said the LED output was more consistent.

    I ran across this interesting video on how to make an LED circuit board, even if you don't intend to make a board it does give some insight into how a custom LED mounting is done. In this case for a custom LED display of computer activity.

    Optioelectronics is a complex study, good luck.

    http://www.youtube.com/watch?v=nm79uwwnY-0
     
  5. domaz

    domaz Member

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    Reasons for not using resistors:
    1. Constant current sources are way more efficient and thus generate less heat and take less power. This means you don't have to worry about finding high wattage resistors or buying a bigger DC power supply.
    2. With a resistor any change in supply voltage will cause a change in current to the LEDs. LED light output is determined by current. Take a look at the datasheet on page 11. You can see how LED current and Luminous Flux (light output) go together. With a proper LED power supply small changes in voltage do not effect the current output.
    3. If you overdrive an LED, which is easy to do with a resistor as there is no protection, you could change it's light output permantely. Even a power surge from unplugging or plugging in your DC supply could cause this.
    4. Most LED power supplies shouldn't suffer from the "slow-off" problem that you get when powering LEDs from a standard wall-wart supply and resistors. That is- you unplug the wall-wart the LED slowly dims off because it's draining capacitors.

    Also it's important to note that light output does change with LEDs depending on temperature as well (pg 9 of Datasheet). So that's something to consider too. The best way to handle that would be to make sure you have plently of heatsinking and cooling on the LEDs, that way the LEDs don't heat up much over the course of an exposure. You could also leave the LEDs on until they reach their operating temperature and leave them on the entire printing session, using a shutter of some sort to time exposures.
    One of the best things about LEDs though is that you get instanenous light- no ramp-up time like you get with bulbs. So even exposing film at say 1/125 could be possible with a proper timer.
     
  6. BradS

    BradS Subscriber

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    This is simply not true. A constant current driver simply replaces the current limiting resistor with a series pass power transistor and adds electronics to control it. The power transistor still has to do the same job as the current limiting resistor. It has to drop the same voltage as the current limiting resistor would have and still generates the same heat.




    I think you mean "affect" not, "effect". :smile:




    You've made some interesting points but, frankly I still do not see any reason to favor an expensive current controlled power supply over a decent regulated supply and current regulating resistors. Maybe, I just have to build one and find out for myself...I feel like there is a lot of marketing going on and many folks are getting sucked in by it....but, like I say, I need to try it myself and find out what the real truth of the matter is.
     
    Last edited by a moderator: Feb 10, 2009
  7. domaz

    domaz Member

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    Yes but the regulators I'm talking about are switching. So they switch the voltage on and off very fast to regulate things. This is a much more efficient approach than a resistor which just drops voltage by dissappating it as heat. So both the resistor and the switching regulator do the same job- one just does it better.

    They are not really expensive as I said you can get a Buckpuck for $15. Resistors will probably work ok if you are careful with your source voltage configuration and LEDs. The only bummer will be the LEDs will probably "dim-off" so you won't be able to use a standard darkroom timer.