Dimmer circuit!

Discussion in 'The Projects Forum' started by rougie, May 13, 2009.

  1. rougie

    Thread Starter Active Member

    Dec 11, 2006
    410
    2
    Hello,

    I am not very experienced in electronics... more of a software guy then hardware! However I would like to create a typical lighting dimmer for household use and have a few questions about some circuits I have found on the Internet. I hope someone can help! Please view the following link:

    http://www.epanorama.net/documents/lights/lightdimmer.html#inductive

    I was looking more precisely at the paragraph entitled "Minimal circuit" about a 1/4 of the way down the page.

    I really like the fact that the Q4008LT triac includes the diac. The thing is that I would like to control resistive loads as well as inductive ones. What I mean by inductive is halogen spot lights which feed off of a 120VAC to 24 VAC transformer. In summary I would like to be able to control both types of lighting.

    But what confuses me is if you see the previous schematic under the paragraph entitled "Very basic circuit" it incorporates an RL filter, C1 and L1.

    1) How many micro henrys is L1
    2) Do I really need these two components in the schematic under the paragraph entitled "Minimal circuit" ?
    3) Given the spec sheet at:

    http://www.littelfuse.com/data/en/Data_Sheets/Littelfuse_Thyristor_QxxxxLTx.pdf

    can this triac handle inductive loads as mentioned above or do we have to provide a snubber circuit?

    All helpful feedback appreciated!

    Roger
     
  2. rougie

    Thread Starter Active Member

    Dec 11, 2006
    410
    2
    Hello Alberto,

    Thankyou for your reply... it is very appreciated.

    I have a circuit which dims an incandecent light bulb and I am so surprised that I got it to work :) ... I surely was expecting days and days of trial and error!!!!!! But I got the schcematic from the Internet though. I would never be able to come up with it myself... I have a vague idea of what the components do but that's it!

    The problem is that I am trying to replace the potentiometer with a H11F1M. Basically my micro-controller would output a 0-10VDC which is fed into the
    H11F1M and the output of the H11F1M would be in series with the thyristor's gate (which is also connected to a capacitor) and the safety resistor (2.2Kohm) which is connected to one of the main lines thus replacing the potentiometer all together.

    But it doesn't work! I tried many little tests but I can't seem to get it to work. If I provide you a schematic of my problem, do you think it would be possible for you to give me some insight as to what I am doing wrong.... I have to be honest.... I really don't even know where to start to debug this stuff.

    I am new to this site... because I usually don't play around with electronics so I don't know how to attach a schematic to my post.... but I will look into this and figure it out.

    Hope you can get back to me... it would really help me.

    Thanking you in advance !

    Finest regards
    Robert
     
  3. rougie

    Thread Starter Active Member

    Dec 11, 2006
    410
    2
    Hello,

    I finally was able to get a schematic uploaded. Sorry for the delays.

    If you look at the attached schematic, Fig. A works and I can dim the 100 watt incandecent lamp. However in Fig. B, if I want to vary the light's luminance by remotely varying a resistance by using the H11F1M component (Fairchild) (which replaces the potentiometer all together) the lamp does not dim when I vary the 0 to 10 VDC. Actually, the lamp always stays on!

    I don't know if the H11F1M requres a frequency as opposed to a 0 to 10DC voltage????

    All suggestions appreciated!

    Robert
     
  4. rougie

    Thread Starter Active Member

    Dec 11, 2006
    410
    2
    Hello Alberto,

    Thanks for your reply.

    Excuse my ignorance Alberto.... I see exactly what you are saying.
    I just don't understand why they suggested the circuit at the following link:

    http://www.epanorama.net/documents/lights/lightdimmer.html#inductive

    under the "Simple voltage controlled dimmer" section of the document.

    They replaced the potentiometer by an LDR which feeds off a variable DC voltage source to control the dimming operations of the triac... just as the potentiometer would do ??? Instead of varying the resistance with the potentiometer, they are intending to let the H11F1M do that!

    confused?

    Bear in mind that the diac is already incorporated inside the triac.

    One thing though, when I apply 0Vdc to the H11F1M I measure a high impedence between pins 4 and 6. as I increase my voltage towards 10 VDC, I can see the resistance varying from high impedence down to 0 ohms. Maybe its not perfectly linear but you can see that at about 1 volt I can measure 200Komhs, at 1.2 volts = 150K, at 1.5 volts = 100K, at 1.75 volts = 50 K and so forth.... So it does vary resistance based on an applied DC voltage.

    So if I would use the MOC 3031... acts also like a simple switch, and therefore I am still stuck with the fact that I need a ceratin delay before firing the triac.... right?

    Anyways thanks for your feedback!
     
  5. rougie

    Thread Starter Active Member

    Dec 11, 2006
    410
    2
    Hello Alberto,

    I guess I would opt for (A).

    >A) You build your own optoresistor using a LDR and a 10 volts filament lamp >(you will have a more linear dimming with a lamp). You can incapsulate LDR >and lamp, one in front of other, with shrinking tube, and use the LDR to >replace the pot and supply variable voltage (0-10V) to the lamp to obtain >dimming.

    So you are suggesting something like a:

    http://www.acpsurplus.com/servlet/Detail?no=358

    and get my own light source and put these two components facing each other in a dark enclosure and the vt931 would act as a variable resistor based on the luminance of the light source. <<< And this should replace my pot!

    An LED would do just fine too... right.

    And, are we obliged to build it this way... isn't there a commercially available component that make up the light source and the LDR?

    Okay, I will try this... I will let you know what happens.

    Regards Alberto... talk to you soon!

    Robert
     
  6. rougie

    Thread Starter Active Member

    Dec 11, 2006
    410
    2
    Hello Alberto,

    I have built my LDR with a led. Tapped it up so no visible light can get in it and it works!

    However, I have a very little range as voltage input to the led (light source)
    In other words, at 2 volts the lightbulb is off (0%). At 3 volts the light bulb is on (100%). I was looking more for a wider range something like off at 2 volts (0%) and on at 4 volts (100%).

    However the lightbulb does dim if I vary the dc input to the ldr very carefully from 2 volts to 3 volts.

    My analogue signal is 0 to 10 VDC. so at a 1 volt delta, I have approximately 25 dimming steps. Which I guess it is okay!

    I was wondering though if there would be other ldr's with the led incorporated but that are less light sensitive?

    Anyways Alberto, I thankyou very much for your help... I really appreciated it!

    Finest regards, Robert
     
  7. CDRIVE

    Senior Member

    Jul 1, 2008
    2,223
    99
    You can decrease the sensitivity of the LDR by placing resistance in series and in parallel with it. You can also do the same in the LED side of the circuit by making the limiting resistor larger
     
  8. rougie

    Thread Starter Active Member

    Dec 11, 2006
    410
    2
    Thankyou Cdrive.

    I haven't had the time to try it, but eventually I'll put it to the test.

    Thanks alot buddy!

    Finest regards
    Robert
     
  9. rougie

    Thread Starter Active Member

    Dec 11, 2006
    410
    2
    One last question,

    What is the official name of a component which comprises an LDR and an opto device (LED) all in one compartment?

    Regards
    Robert!
     
  10. CDRIVE

    Senior Member

    Jul 1, 2008
    2,223
    99
    Fairchild calls the H11F1M a Photo Fet Optocoupler.

    http://www.datasheetdir.com/H11F1M+Optocouplers

    Despite what's been stated in this thread this device is AC compatible because it's a Bilateral Fet. It is also not either on or off. The output resistance decreases as the input LED current (brighter LED) increases. Unfortunately the FET output (which acts as a variable resistor) has a max AC voltage limit of 60VPP. The data sheet shows no application as a control for a Triac dimmer but it can probably be done by assuring the the voltage across the FET terminals never exceeds 60vPP. Possibly a resistive voltage divider would achieve this. The problem is that the Diac gate of your Triac probably triggers at 45 volts.

    As far as the name for a opto coupler device that employs a LED and a LDR goes, I've never seen one. This doesn't mean that they don't exist.

    Edit: Actually, I believe I used one about 28 years ago. It would take me days to dig up my old paper records though!
     
    Last edited: May 16, 2009
  11. rougie

    Thread Starter Active Member

    Dec 11, 2006
    410
    2
    Very interesting CDrive.

    >Edit: Actually, I believe I used one about 28 years ago. It would take me >days to dig up my old paper records though!

    Don't mind me my good friend... I'll do some searching and I will eventually stumble on something commercially available.

    Oh yeah, and by the way, a 300 K resistor in parallel with the LDR decreased the sensitivity by 5 volts or so, which is very acceptable. I also replaced the 250 ohm resistor for a 3 k resistor which is in series with the LED. I don't know if adding the latter really decreases sensitivity... doesn't this only change the voltage at which will start to turn on the LED?

    Anyways.... I am beyond happy, I really don't want to change anything now it works perfectly :)

    I thank everyone.... your help was very appreciated!

    See ya around buddy!

    With finest regards
    Robert
     
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