Lighting control with PWM

Discussion in 'The Projects Forum' started by eric_s88, Aug 15, 2013.

  1. eric_s88

    Thread Starter Member

    Apr 20, 2011
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    Hi everyone :)

    I'm planning to start this project.A circuit that sens the light intensity and decide the pulse width (pwm) to increase or decrease the light of light bulb.

    my question : Is is correct to use SSR relay? I mean I want to connect the pwm signal to input of ssr , and connect the output of it to light bulb.

    is this device (ssr) valuable to use in this project? :confused:
     
  2. eric_s88

    Thread Starter Member

    Apr 20, 2011
    157
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    is there any idea if using SSR is correct or not ?? :)
     
  3. ScottWang

    Moderator

    Aug 23, 2012
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    Does the light bulb needs AC or DC power?
    How about the voltage/current of light bulb?

    light bulb type.
     
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  4. eric_s88

    Thread Starter Member

    Apr 20, 2011
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    the light bulb uses 220V AC power. and its current is about 0.5 amps. a 100 watt

    sth like this :

    [​IMG]
     
  5. MrChips

    Moderator

    Oct 2, 2009
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    Nice photo of a bulb.
     
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  6. tubeguy

    Well-Known Member

    Nov 3, 2012
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    +1 terrific photo..

    Yes, You can use an AC SSR for dimming the lamp. The trick is to synchronize the control signal to the actual AC wave. You want to trigger the lamp at the same point in every AC cycle.

    Earlier is brighter, later is dimmer.
     
    Last edited: Aug 15, 2013
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  7. eric_s88

    Thread Starter Member

    Apr 20, 2011
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    thanks for your response , but I didn't get the reason of your trick :-??

    I just want to connect the pwm signal (for example with 3Khz fr.) to the input of the ssr , and when the pwm is in high state the light is on, and when its in low state the lamp is off ( its fast enough that human eye cannot recognize it )

    am I right ??

    and what about the working frequency of ssr ? is it fast enough to handle with a 3Khz signal ?
     
  8. MrChips

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    Oct 2, 2009
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    Why 3KHz?

    The SSR can switch at 3KHz but it will not turn off at that rate because of the AC line frequency, 50 or 60Hz.

    The SSR will only turn off when the current reduces to zero and this will happen when the AC power crosses zero. Light dimmer circuits work by controlling the phase angle at which the SSR is turned on, not by PWM.
     
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  9. eric_s88

    Thread Starter Member

    Apr 20, 2011
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    thanks , so if I want to control the light by pwm signal, whats your suggestion ?
     
  10. tubeguy

    Well-Known Member

    Nov 3, 2012
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    Eric_s88,

    Please don't get stuck on the idea of using PWM for this particular situation. PWM IS the thing when controlling voltage/power for many applications, but not necessary in this case.

    Google "How light dimmers work"

    Do you already have the PWM signal and must use it? Because, yes, it can be done using that signal with a bit more circuitry :)
     
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  11. eric_s88

    Thread Starter Member

    Apr 20, 2011
    157
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    hi tubeguy :)

    yes you are right, but this is the thing I have to do.. I have a project with this definition: close-loop pwm control in order to control ambient lighting .. so any idea ? :(
     
  12. eric_s88

    Thread Starter Member

    Apr 20, 2011
    157
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    exactly , this is what, I must do. so do you have any idea to use pwm to control an ac load like a light bulb ?
     
  13. ScottWang

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    Aug 23, 2012
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  14. eric_s88

    Thread Starter Member

    Apr 20, 2011
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    thank you

    3KHz was just an example :)

    can I use uC and its pwm signal in the circuit you have posted the link ?? :confused:
     
  15. MrChips

    Moderator

    Oct 2, 2009
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    You can use the PWM signal to control a photocoupler that drives the diac or triac.
    I have made my own photocoupler using an LED and an LDR.
     
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  16. tubeguy

    Well-Known Member

    Nov 3, 2012
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    I can't do a schematic for a while. Only a minute.

    One Idea - just the basic's:

    First, you need an isolated sync signal from the AC line. An AC optocopler or AC transformer with diodes for example. Make a ramp signal from this. Feed that in to a comparator.
    Feed the pwn signal through a low pass filter to convert it to a variable DC voltage, then into the other input of the comparator.
    This should make a voltage controlled trigger signal synchronized to the AC line. Then use this signal to trigger the AC SSR.
     
    Last edited: Aug 15, 2013
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  17. ScottWang

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    Aug 23, 2012
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    Last edited: Aug 15, 2013
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  18. richard.cs

    Member

    Mar 3, 2012
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    If you want to use PWM rather than phase angle then the simplest way is to full-wave rectify the mains without smoothing and feed the lamp pulsey dc. Not smoothing is important as that would increase the rms voltage the lamp sees.

    You can then switch the lamp at kHz with a single n-channel mosfet in the negative side. Note that the negative side is not at earth potential but instead bounces between 0 and -340V relative to earth. This means that you should provide optical or transformer isolation between your PWM source and the mosfet. The only exception is if you treat the whole circuit as live and insulate apropriately, only chose this approach if you really know what you're doing.

    An alternative is to put only the mosfet inside the bridge so that the load still sees a.c. For a filament lamp it doesn't really matter which method you use.
     
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  19. eric_s88

    Thread Starter Member

    Apr 20, 2011
    157
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    thank you all for your replies :)

    @to MrChips: is this schematic correct for my work ? dear ScottWang posted this.

    can I use it as a dimmer with for example 3Khz PWM signal ?

    schematic

    [​IMG]

    and another question, in this schematic if when pwm signal is low the lapm will be off ?? I mean in this circuit for the turning the TRIAC off, is it enough to turn off the optocoupler ? don't we need any zero-cross detection , right ?

    confused :confused:
     
  20. eric_s88

    Thread Starter Member

    Apr 20, 2011
    157
    1
    according to this link which dear ScottWang posted :
    http://electronics.stackexchange.com/questions/45035/triac-dimmer-circuit-design-help-resistive-load

    we have this :

    so pwm is useless :( I'm really confused
     
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