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Thread: Power supply driver help please

  1. #1

    Default Power supply driver help please

    I have a project where I need to drive a LED from an AC power supply.
    I've been looking at some of the AC drivers on LEDSupply page, but it's been a long time since I tinkered with electronics.
    I need to supply a single LED to light a small area for photograph purposes, and an LED seems the perfect fit.

    So I have a few questions that maybe people here can help with.
    - I see some of the AC drivers are dimmable, does that mean using a normal wall dimmer for ceiling lights? https://www.ledsupply.com/phihong-ac-led-drivers
    -We have been using my Zebralight right angle up to this point, but we'd like something that doesn't use batteries. We'd like to be roughly around 50-60 lumens @ 4000k (middle setting on my ZL). I see they have 4000k single LED's here: https://www.ledsupply.com/neutral-wh...igh-power-leds what current amount would it take to roughly get the 50-60 Lm output? I'm not sure how flux ratings convert to lumens.
    -Can I connect one single LED to the AC driver output, or do I need resistors in the circuit?

    I'm sure this is absurdly simple for some people here, I just would like some guidance before smoking some components. Thanks in advance.

  2. #2
    Flashaholic* DIWdiver's Avatar
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    Default Re: Power supply driver help please

    Modern illumination grade white LEDs generally give about 100-150 lumens per watt. So what you want is around half a watt. Vf is going to be around 3V, so you need about 1/6 Amps, or 170 mA.

    The driver will have an output voltage range. If the range includes 3V, then you can use it for a single LED without additional components. If not, then I would suggest probably zener diodes instead of resistors, as they would work better with the dimming control.

    You need to be careful in selecting a dimmer. There are several different kinds, and most of the drivers you are looking at are NOT compatible with the standard incandescent dimmers.

  3. #3

    Default Re: Power supply driver help please

    Thank you! That info gives me something to start with, much appreciated.

  4. #4

    Default Re: Power supply driver help please

    So I'm looking at the Single 1 Up option of this:
    https://www.ledsupply.com/leds/cree-...high-power-led
    The forward voltage is around 3v.

    And looking at this constant current supply, 350ma option:
    https://www.ledsupply.com/led-drivers/phihong-pda006a

    The spec sheet for the power supply says output voltage is 9v-17v.
    Will I smoke the LED connecting it to this power supply? Or what do I need to simply connect the one LED linked above to this power supply?
    thank you!

  5. #5
    Flashaholic* DIWdiver's Avatar
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    Default Re: Power supply driver help please

    That LED would be a good choice. The driver isn't a great match for it though. I think you'd probably burn up the driver first, then maybe the LED after the driver fails. To prevent this, you'd have to drop the additional 6V across some kind of series load (I've suggested zener diodes), but that would mean wasting 2/3 of the power coming out of the driver. At the levels you're talking about, that's only around 2 watts, so that's actually pretty reasonable. Also, that driver is not dimmable, so you'd be stuck with either twice the desired output, or wasting even more power.

    I suspect that at such a low power level, you will not find suitable drivers that operate straight off the mains. You might have to resort to a DC power supply, followed by a driver with a low-voltage DC input. An example might be these parts:
    https://www.digikey.com/product-deta...232-ND/4031883
    https://www.digikey.com/product-deta...153-ND/4488532

    Note that the driver I've shown supports analog dimming by a rheostat (aka Potentiometer, pot, or variable resistor). Many dimmable drivers support PWM dimming only, which requires something a bit more complicated than a pot.

  6. #6

    Default Re: Power supply driver help please

    thank for the ideas!

  7. #7

    Default Re: Power supply driver help please

    Got all my parts, thanks again. Another question, what is the purpose of using Zener diodes to just a regular diode for something like I'm describing?

  8. #8
    Flashaholic* DIWdiver's Avatar
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    Default Re: Power supply driver help please

    When forward biased, Zener diodes behave similarly to regular silicon diodes. When reverse biased, they block current up to a certain voltage, then start conducting. This occurs in a predictable, repeatable and non-destructive way. A standard diode will do much the same thing, but the breakdown voltage is less predictable, and breakdown tends to be destructive.

    A 6.2V 3W zener diode would, in a single part, drop the additional 6V you need, and handle the 350 mA current. To do the same thing with regular diodes, you'd need to run them in the forward direction, and so you'd need around 10 of them to get to 6V. Another thing about the zener is that its voltage varies less with current than a string of regular diodes. So again, more predictable in this application.

    Of course, you could also string 3 LEDs in series, and just block the light from two of them.

  9. #9

    Default Re: Power supply driver help please

    thank you for the great answer!

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