Gradual dimming of ac lamp

Discussion in 'The Projects Forum' started by llawwehttam, Apr 19, 2010.

  1. llawwehttam

    Thread Starter New Member

    Mar 29, 2010
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    I am about to start a project and need a few pointers to get me going. I'm not asking someone to make this for me, I just want a few ideas about how to go about it.

    I want to build a circuit that can be plugged into the mains inbetween a wall socket and the plug to a lamp.

    I want this circuit to have just one or two buttons which when triggered slowly restricts the voltage over 10 minutes or so, slowly dimming the light. This would be perfect for a young child's room.

    It would be best if it just had one button and the circuit reset when the wall socket was turned off and on again.

    Obviously the fact that I am dealing with AC current makes it a bit harder so I will probable be going down the thyristor route, ie TRIAC's etc.

    Any help or guidance would be extremely appreciated.
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
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    You might look at a commercial device like this - http://www.siliconchip.com.au/cms/A_104880/article.html

    It sounds as if it will do what you want. There are circuits that can be found online, but first-time projects that deal with 120 VAC are awfully dangerous to attempt.
     
  3. llawwehttam

    Thread Starter New Member

    Mar 29, 2010
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    It would be nice if I could buy it but this is for a college project. I would consider buying one and opening it up to get some ideas but I think that would be a bit of a waste.

    I have found this:
    [​IMG]

    I think it could work very well with a few modifications.

    I don't really see why an LED and LDR are used when an N-channel mosphet could be used in its place, with a small heatsink of course.

    For safety purposes I will be making this circuit on either 9v or 12v to begin with just to test it.

    Looking at this circuit confuses me a bit as I am used to European symbols.
    Can anyone work out how the time it takes to dim is adjusted. Probably a combination of the capacitor and one of the resistors in the left hand portion of the diagram.
     
  4. retched

    AAC Fanatic!

    Dec 5, 2009
    5,201
    312
    You are talking about a 9v or 12v AC, correct? suppose you could do some sampling with dc circuits, buy controlling an incandescent with a 9v battery would give you very little time to work out the circuit.

    Triac.
     
  5. llawwehttam

    Thread Starter New Member

    Mar 29, 2010
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    Yes I meant ac.
    The left side of the circuit seems to be dc so I could work on that first.

    Thanks for the help so far. Any pointers are still appreciated as I am still getting to grips with this.
    I can build computers and program them but I couldn't design the computer circuitboards to save my life.
     
  6. Norfindel

    Active Member

    Mar 6, 2008
    235
    9
    It seems like the LED/LDR combination is used to isolate the circuit from the power line.

    IIRC, the most basic circuit to dim a lamp is to take the trigger signal as a fraction of the power line (say, by using a transformer and a voltage divider. isn't very practical, however). When the signal is over the trigger voltage, the triac sends power to the lamp. So, with a very simple circuit, you can power the lamp on a duty cycle of 100% to 50%, as the triac will stop conducting when AC reaches 0v. This surely adds a lot of noise to the power line.
     
  7. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    The right side of schematic[ AC side] is fine as is, and is available as a commercial dimmer. When ready ,add LDR in series with provided pot. Left side [DC], operates on low V DC, guess 9 V. With S1 open, dimming is max., just opposite of what we think you want. To develope your control, substitute a visable LED; verry dim will be max dimming, max brightness with LED just medium, around 5-8 mA. Keep LED- LDR. Can use your FET for T1.
     
  8. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    Here is a simple dimmer control. R1 sets max dimming, R3 controls time, LED's 620 Ω controls minimum dimming, max LED current. C1 500 to 2000μF, time about .8 X C1 x R3. When power is applied dimming minimum, to reset again to min close SW for 1 sec, 620 ohm prevents SW contacts from welding.
     
    Last edited: Apr 20, 2010
    llawwehttam likes this.
  9. llawwehttam

    Thread Starter New Member

    Mar 29, 2010
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    Thankyou so much, that is perfect for what I want.
     
  10. llawwehttam

    Thread Starter New Member

    Mar 29, 2010
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    The circuit I posted with the ac part works up to a point but I don't understand why the second resistor/capatitor combination is there as it doesn't seem to affect the circuit if I remove it. Also the circuit does not seem to completely dim even if the LED is off. (the led and ldr are sealed together)

    Any help much appreciated
     
  11. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    R5 & C3 appear to be for electrical noise supression. Try holding an AM radio near, with and without RC. For max dimming did you adjust P1?
     
  12. llawwehttam

    Thread Starter New Member

    Mar 29, 2010
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    Thanks for that. I did adjust P1 but maybe I didn't go high enough.

    I'll try a higher value.
     
  13. bertus

    Administrator

    Apr 5, 2008
    15,648
    2,348
    Hello,

    R5 and C3 make a snubber network.
    Read the PDF in the link for more info.
    http://us.st.com/stonline/products/literature/an/6785.pdf
    These are to protect the triac or thyristor from a to high dV/dt transient.
    Also it reduces noise produced at high frequencies like Bernard said.

    Bertus
     
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