Dimming LED strips?

Discussion in 'General Electronics Chat' started by MrKitty, Oct 6, 2012.

  1. MrKitty

    Thread Starter New Member

    Oct 6, 2012
    9
    1
    Hey guys,

    I was looking to make some kind of rear lighting for my LCD monitor since it's located in a place where the room's light fixture can not illuminate properly.

    I figured doing it is pretty simple, just get a LED light strip, cut it to the proper length and hook it up.
    But I'd also like to add a dimming circuit with a potentiometer so I could dim the LEDs, and I have no idea what is the propper way to do that, I'm assuming I could just cut the LEDs to the lengths I want and just add the potentiometer inline before the LEDs? (obviously my knowledge in Electronics isn't vast, I have done some soldering of headphones and 3.5mm jacks but never messed too much with resistors, LEDs and diodes)

    I want to use a sliding potentiometer like this one here (but of the correct value):
    http://www.ebay.com/itm/Slide-Poten...608?pt=LH_DefaultDomain_0&hash=item2ec2554090

    And here's the LED strip I want to use (or something similar since this one's a bit expensive):
    http://www.ebay.com/itm/5M-Cool-Whi...418476?pt=US_Car_Lighting&hash=item1c247f7a6c

    I would appreciate it if someone could make this a bit more clear to me
     
  2. Sensacell

    Well-Known Member

    Jun 19, 2012
    1,131
    267
    While it is possible to use a variable resistor to dim LED's, you will most likely find that the power levels involved will force you to use a rather large, hard to find and expensive "rheostat"
    (another word for a high-power potentiometer)

    Here are 2 options, it order of complexity:

    1) Buy an off-the-shelf LED dimmer pack. (Google "LED dimmer")

    2) Build your own dimmer, based on "PWM" or pulse-width modulation, which is a fancy term for switching the LED on and off at a frequency high enough that you cannot see the pulses. (200 hz is a good start) Vary the ratio of on / off time to vary the brightness. Most off-the-shelf LED dimmers use this technique to dim LED lights very efficiently, rather than burning power in a resistive element to control the LED current.
     
  3. MrKitty

    Thread Starter New Member

    Oct 6, 2012
    9
    1
    If I build my own PWM-dimmer can I make it so I could still use a resistor?
    Could you point me to a schematic of such a circuit?
     
  4. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    More likely you will need a PWM circuit, dimmers typically do not work well with LEDs.

    Show us you parts please.
     
  5. MrKitty

    Thread Starter New Member

    Oct 6, 2012
    9
    1
    I've already mentioned the LED strips I want to use and the potentiometer that I would like to incorporate in the circuit
    I just don't know how to include a potentiometer in this so called PWM circuit

    Is it possible to control the component that regulates the pulsing frequency with the potentiometer or do I just sound stupid? xD

    Edit:
    Ok so I've done a little search on PWM circuits and I've found out that the do already use a potentiometer (am I correct?)
    my problem now is that the LED strip I'm using is 12V and I intend to run it off a wall supply like this one:
    http://www.ebay.com/itm/AC-100-240V...=Laptop_Adapters_Chargers&hash=item1c1e436af9
    the circuit I've found used the little 3v LEDs and the rest that I've found were arduino circuits. Could someone enlighten me a bit more about these PWM circuits?
    Also what if I wanted to use a PWM circuit to dim CCFLs? Specifically the little 12inch ones that are used in PCs, they have an inverter that converts to a much higher voltage then the input voltage, could I tap in the after inverter and create a PWM circuit to dim the CCFLs? Would it work with such these high voltages or would I end up frying one or the other?
     
    Last edited: Oct 6, 2012
  6. Sensacell

    Well-Known Member

    Jun 19, 2012
    1,131
    267
    a simple analog PWM circuit will typically use a potentiometer to vary the PWM cycle, digital micro-controller based PWM circuits can be controlled by many different inputs.

    CFL's also use PWM, but it will not work to try to drive one PWM circuit with another, that would be like putting a brick on your car's gas pedal and trying to control the speed of the car by pinching the gas line with a pair of pliers.

    If you have documentation on the CFL driver, you may find that it has a DIM input that can be used for this purpose.
     
  7. MrKitty

    Thread Starter New Member

    Oct 6, 2012
    9
    1
    Ok, after doing a little bit more reading I've found this site that has diagrams and schematics for a LED dimmer circuit
    http://pcbheaven.com/circuitpages/LED_PWM_Dimmer/

    What I don't know is would this schematic as it is ( I mean with the exact same components such as the capacitors and diodes) be usable for the 12v LED strips? Can I just take this exact schematic with the exact same value of the capacitors and diodes etc?
     
  8. k7elp60

    Senior Member

    Nov 4, 2008
    478
    69
    Yes the circuit will work but the amount of LED's is limited by the transistor labled T1,approximately 200mA of current.
    I have built a number of dimmers for the LED strips that work from 12VDC and it will control Amps of current.
    Here is a current schematic and construction picture.
     
    Last edited: Oct 7, 2012
  9. MrKitty

    Thread Starter New Member

    Oct 6, 2012
    9
    1
    The physical circuit looks great but I'm having trouble understanding the schematic. I've reattached the schematic with some areas I've marked that I have questions about, and I do apologize in advance if my questions are stupid :)
    1: What's R2 connected to? I see it's going from pin 3 but where does the second lead go to? After D1 and before R1?
    R1 is the 100k POT right? Can I use a 50k POT?
    What/where does C3's second lead connect at? Before R1 and after D1?
    Pin 7 doesn't connect to anything?

    2: What does this arrow represent?

    3: Again this arrow that I don't understand.

    I don't know but your schematic really confuses me...

    About the circuit I linked to, could I just use a different transistor that can supply higher currents?
     
  10. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    1) The other end of R2 is shown connected to the slider pin of the potentiometer R1.
    2) The "arrow" is a common ground point. All grounds are connected together with the negative wire of the battery or power supply.
    3) Another "arrow" common ground point.
    4) Pin 7 of the 555 is shown not connected to anything.
     
  11. k7elp60

    Senior Member

    Nov 4, 2008
    478
    69
    You can use a different transistor but you will have more of a voltage drop. A TIP120-TIP122 series will work.
     
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