Light intensity control for an incandescent light bulb

Discussion in 'The Projects Forum' started by gabrielbaca, Sep 17, 2012.

  1. gabrielbaca

    Thread Starter New Member

    Sep 17, 2012
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    Hi everyone,

    This is my first post, I hope will be the first of many. Currently I am taking a course of Applied Electronics, the professor is not that good and the projects are somewhat difficult to us. Our first project was to design an opamp from scratch, having a diff amp built with BJT's and gain over 600, and the total opamp gain being over 7000. That did not work at all, even though in simulation worked perfectly and control currents were in microamps .

    Anyways, now we need to design a dimmer for an incandescent light bulb, using SCR, TRIAC and an optocoupler isolator. I have done research on those components, somehow understood the optocoupler but not so sure about the TRIAC and SCR. We are not allowed to use any mechanical devices like switches, pots, push buttons, etc. It must be controlled through light, sound, voice, touch, or it may be wireless. Also 5 levels must be managed 0%, 25%, 50%, 75% and 100% intensity.

    I'm not sure what to do, but I do have some ideas. I intend to control through light, using 4 LDR in parallel, each in series with a different resistor. I found the equivalent resistor in darkness is around 10MΩ, so an almost zero current should go through in darkness. When no light is over them, it should give 0%. If I isolate each LDR, and illuminate any of them current should go through, activate the optocoupler therefore activating the AC portion of the circuit. But I'm not really sure if increasing or decreasing the current on the diode side of the optocoupler will actually diminish or increase the current through the bulb.

    Another idea is using a FET as they are sensible to static electricity, but I'm not that sure how to control the levels. Any ideas? Could you please help us out?
     
  2. gabrielbaca

    Thread Starter New Member

    Sep 17, 2012
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    By the way, this is what I will probably use as I had some experience controlling an ON-OFF application for an inductive load. Still my main question is whether the input current in the optocoupler will affect the output, or does it just work as an ON-OFF and I need further control at the output?
     
  3. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    Based on GE SCR Manual, 1972, you could achieve various light levels with light control by selecting R1,2,3,4 to give desired light levels. In series with Rs are 4 ea LDR's. As you said dark resistance several meg ohms, so r valeus are not seriously effected. Use a pot to select values, then sub fixed values. 100% might not be obtainable- more likley 95%.
     
  4. DickCappels

    Moderator

    Aug 21, 2008
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    Gabrielbaca wrote:
    "Still my main question is whether the input current in the optocoupler will affect the output, or does it just work as an ON-OFF and I need further control at the output?"

    Thank you for the succinct question. The answer is that Triacs are to be either or or off. Driving it with less than the needed gate current can result in damage to the triac.

    If you want to vary the average or RMS output voltage or current to the load, you need to do it my adjusting the angle at which the triac is triggered.
     
  5. Audioguru

    New Member

    Dec 20, 2007
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    The triac has an adjustable RC circuit that delays when it turns on for each half-cycle of the AC. When the triac is delayed from turning on then the power to the load is low and a light is dimmed.

    Here are voltage waveforms at the load:
     
  6. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    Opto couplers come in many forms, analog [ LED & LDR combo which by varing LED current can control light ], triac output [on- off output or 0- crossing ], various transistor outputs.

    A single SCR can be used if input AC is first full wave rectified; then unipolar pulse control can be used.
     
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