How To Dim MR16 Dimmable LED Bulb

Discussion in 'General Electronics Chat' started by mpangrekar, May 16, 2009.

  1. mpangrekar

    Thread Starter Member

    May 16, 2009
    14
    0
    Hi,

    I am trying to figure out what is the best way to dim a MR16 LED bulb (attached is datasheet). This bulb has to pins that accept 7V to 15V AC/DC. The mfgr claims, it is dimmable by changing the voltage and the LED bulb has PWM dimming circuit built inside. I have 12 VDC @ 1Amp input supply available to give to this LED bulb. Can I dim it with just a potentiometer? I have tried to use a 5K potentiometer and within 20 degrees of rotation of pot, I get whole dimming, this pot is only 1/4 Watts.

    1. What would be the best way to dim this unit?
    2. The LED bulb is rated for 4 Watts, if I use the 1/4 Watts pot, will the pot eventually heat up and fail?
    3. I have another dimming circuit (PWM) with pot, when I tried this with the LED bulb, it did seem to work, but I saw flickering effect when turning the pot knob, would this because there may be conflict between PWM on my dimmer unit and PWM inside the LED bulb? (Ths bulb did not flicker once pot know was steady)
    4. My goal is to find out if there is an off-the-shelf component (voltage shifter/potentiometer etc.) that I could use to dim this LED without going into designing a special circuit for it.

    Any help, comments are highly appreciated !

    Mayur
     
  2. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    Have you looked at using PWM (Pulse Width Modulation)?

    hgmjr
     
  3. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
  4. HarveyH42

    Active Member

    Jul 22, 2007
    425
    5
    Don't those halogen lighting fixture usually run off 12 VAC? I would guess your LED replacement would be set up to be dimmed in the same fashion as other AC lights, or fans. Believe it involves changing the frequency, not the voltage or PWM.
     
  5. mpangrekar

    Thread Starter Member

    May 16, 2009
    14
    0
    Thanks for your replies. My first preference is not to design a circuit board to control dimming of the MR16 dimmable LED bulb (I am a Mechanical Engr).

    The LED bulb in question runs on 12 VAC/DC.

    All I was trying to find out was is there a standard off-the-shelf voltage regulator/dimming unit that can take input voltage of 12 VDC and supply from 7VDC to 12 VDC to the LED to achieve the dimming.

    Mayur
     
  6. Bricoleur

    New Member

    Jun 16, 2009
    1
    0
    If the bulb has a built in PWM you will not be able to use an external PWM in combination - the flickering is caused by the "beating" of the two frequencies. You will definitely need a variable voltage source. A simple potentiometer could work but it basically discards the excess voltage as heat in inverse proportion to the power used by the bulb (hence it would need to be huge). There are many different types of variable DC supplies on the market but to address your problem it will be necessary to understand where you want to place such a supply and what constraints you have (size, heat, control locations, etc).
     
  7. mpangrekar

    Thread Starter Member

    May 16, 2009
    14
    0
    Thanks for your reply. I suspected that 2 PWMs would be in conflict. I found potentiometers made by Honeywell that are rated for 4 Watts and the LED bulb that I am trying to power up is rated for 4 Watts, so theoretically it should work, correct? I have 2 samples of such pots coming end of this week which I will test.

    I thought about variable DC supplies too, size and heat is not really a concern, because I could select an enclosure based on the size of DC power supply. I would want to put a simple pot on this enclosure to control dimming. I would want to place such DC power supply between 12VDC input supply and he 4 Watts LED bulb. Let me know your thoughts on proceeding with this approach (any sources for such power supplies?).

    Mayur
     
  8. Wendy

    Moderator

    Mar 24, 2008
    20,765
    2,536
    I just reviewed that data sheet, no where was there any reference to a PWM dimmer in there. So you will likely have to build a custom PWM circuit if you want to dim it. There was a reference to the fact that it may not work with some transformer circuits (ie, SCR dimmers) due to low power consumption.

    You could use a straight variable voltage regulator instead, think LM317. Basically this isn't anything but a souped up LED. They do make pots that will do what you want, but you will waste the energy the bulb is trying to save.
     
    Last edited: Jun 16, 2009
  9. BobaMosfet

    Active Member

    Jul 1, 2009
    109
    11
    As a practical matter, it isn't voltage you're concerned with here, so much as the unknown question-- CURRENT. Irregardless of voltage, if the current is wrong, it can fry your light.

    In the other thread, I calculated your current for these lights, based on the standard PIE equation and the information on your datasheet:

    I = P/E
    I = 4W / 12V
    I = 333mA

    Furthermore, the datasheet stated clearly that the device cannot withstand more than 8% voltage above it's maximum rating (in other words 12.96V), so you will also want something to stop voltage spikes- Assuming now PWM device, see the other thread on this on how to set a trim-pot up correctly for this.
     
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