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Discussion in 'LEDs and Optoelectronics' started by Hpy, Aug 20, 2018.

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  1. Hpy


    Aug 20, 2018
    Hello everyone!

    I hate having to ask stupid questions, I'm not an expert in this at all and after quite a bit of research I'm now stuck on a few basic issues.

    I'm trying to design and put together a dimming by PWM table lamp with a rotary potentiometer type switch that would dim the light and switch it off. I looked into getting a BuckPuck LED Driver with a 5K potentiometer -

    However I'm now looking at getting a Mean Well LDD-L (WIired version) Step Down Driver which seems to do the same thing, it's slightly cheaper, smaller and I can get it at my local tech store (pdf attached with driver spec).

    My LED spec is 7W / fV 11.0~14.0V / 550mA

    So my questions are:

    - What power supply do I need for the driver to work with this LED? I'll be using the 500mA driver and I assume the power supply output needs to match the LED input? So would a AC-DC 12v 0.5A power supply work?

    - How would I wire up a potentiometer to the driver for the PWM dimming to work? What kind of resistance would the potentiometer need to have? Could the Potentiometer also be used as an on/off function? - Not at all knowledgeable about potentiometers here...

    Again, sorry for the basic questions or if they've been asked before. I'm new to this.

    Thanks in advance for any help!

    Attached Files:

  2. dave9


    Mar 5, 2017
    The driver has at least 4V drop (difference between minimum input and output) and LED has up to 14.0V forward drop, so adding them you need at least a 18V PSU. To ensure long life from the PSU and ease keeping it cool running, you want significant overhead for current. Since 1A is a common value that seems a good choice, so between 18V and 36V (driver input upper limit) and 1A or higher.

    Unless I'm reading the datasheet wrong, only the 1000mA, 1200mA, or 1500mA versions would allow using a POT for analog dimming by pulling the analog dimming pin voltage down to ground.

    A POT won't turn the lamp off. Even if it shuts down current to the LED to an ineffective level you still have the AC-DC supply getting power, so I'd put a power switch before it. The Buckpuck seems the easier ready to use solution but I wonder if there isn't some dimmable AC-DC driver out there that eliminates getting a separate AC-DC supply.

    7W LED array and an AC-DC dimmable driver is about what you get in the average $5 dimmable LED bulb in the US, then it just has an LED compatible dimmer switch on the wall (possibly placed in the lamp instead, I don't know what form factor you're targeting for the lamp). However, they usually use a series of LEDs operating at higher voltage and lower current. Besides that difference you could literally buy a bulb and repurpose the components, keeping in mind that without the bulb shell it may need supplemental heatsinking.
    Last edited: Aug 20, 2018
    Hpy likes this.


    May 20, 2017
    You have to mine all the way to the bottom of the spec sheet where it tells us that PWM (dimming control) pin requires 0.5 to 2.5V with respect to 0V to effect the dimming function. You could conceivably pot down the power supply to give the required voltage range.
    Hpy likes this.
  4. kellys_eye


    Jun 25, 2010
    The LDD device won't dim with 'just' a potentiometer. You need a pulse signal (variable) to do the dimming.

    Your original choice device would be the one you need.

    Change the potentiometer for one with an integral on-off switch and wire the supply to the LED control module through the switch contacts such that at 'low' setting, any further anti-clockwise makes the switch operate and disconnects the power.
    Hpy likes this.
  5. Hpy


    Aug 20, 2018
    Hey Guys!

    Thank you for the replies! That's definitely cleared some things out for me.

    I stupidly didn't realize that LDD driver requires an external PWM generator with a 5V source for it to dim that way. Also that the 1000mA upwards drivers are the only ones that could be used with a POT and analogue dimming.

    So it looks like I'm going back to the BuckPuck driver. I'm quite restricted with space in the design of the lamp, I've attached as reference a lamp which is the result that I'm looking for, as in everything's contained within a cylinder base, although the attached is using a halogen bulb which makes it a lot less complicated - mine would be an aluminium tube (aprox. 80-85mm diameter and same in height) containing the heat sink for the one 7W LED and the BuckPuck driver with a potentiometer sticking out of the base.

    So now that I know what PSU to use, what type of potentiometer do I need and how do I choose? Kellys-eye you mentioned a Potentiometer with an integral on-off switch? That would be ideal! Any reference as to which and where to get it from?

    This is the diagram for the BuckPuck datasheet. I can't read these things properly, but I assume I'm looking to do something like figure 13?

    (Also on a side note, Dave9 did you have something like this in mind when you mentioned an AC/DC dimmable driver? )

    Thanks again guys I really appreciate any help given...


    Attached Files:

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