LED drivers, an application decision?

Discussion in 'General Electronics Chat' started by Cr_rydah02, May 19, 2011.

  1. Cr_rydah02

    Thread Starter New Member

    Apr 18, 2011
    it seems to me that the need for a "driver" is pretty dependent upon the application of your LED's.. Can somebody tell me if that completely ignorant or if im on the right track? Im not using a battery, my circuit is on a DC, and the voltage drop of my (4) LEDs is slightly less than my supply voltage... ive wired them in series and im currently running them with a small resistor and it works great. But i am looking to replace these with some higher output LED's. The voltage drop of the (4) LED's im considering would still be lower than my supply voltage. What, if any, are the advantages of running drivers with these?


  2. KJ6EAD

    Senior Member

    Apr 30, 2011
    By driver, I'm assuming you mean a constant current supply of some kind. If your LEDs are configured to use most of the supply voltage, your resistor power loss will be minimal. If the power supply doesn't drift, all is well.

    The value of a driver is most noticeable with battery supplies where the supply voltage may change by 50% during discharge (1.5V per cell down to 0.8V per cell), where external control such as a PWM are being used or in cases of very expensive LEDs where a small fluctuation in the power supply could cause expensive damage.

    With battery supplies, I often use an LM317LZ in current regulating mode. When I need external control I usually use a CL320 or similar.

  3. #12


    Nov 30, 2010
    Another definition of "driver" is a current amplifier used to help, for instance, a timer chip that doesn't have enough ability to supply current. This seems to not be important in this case. I'm just saying this in case Crydah has some confusion going on about what a driver is.
  4. SgtWookie


    Jul 17, 2007
    If the LEDs are low current (<70mA or so) then a simple resistor works OK with a fixed supply voltage.

    You really need to use a driver with high-power LEDs, as when the LEDs heat up, their Vf decreases, and the high-power LEDs are usually priced high enough to justify the cost of a driver IC. The low current LEDs also heat up and drop their Vf, which is why you should de-rate their current (use a somewhat higher resistance than called for).

    Switching LED controllers are very efficient, and don't require many external components; most just need a resistor or two, a cap or two and a suitable inductor. The inductor takes the place of the series resistor; the switching controller attempts to keep the current flow through the inductor constant.

    While there are more parts involved than a linear regulator, and certainly more than a simple fixed resistor, you avoid wasting the power that would otherwise be dissipated in the regulator or simple fixed resistor, so you reduce the waste heat and expense of operating the LEDs - and you also increase their likely life span.

    There are ready-built LED controllers available; "BuckPuck" is one popular brand.
    Here is a page showing some of the BuckPucks that are available:
    More categories on this page:
    Last edited: May 19, 2011
  5. Cr_rydah02

    Thread Starter New Member

    Apr 18, 2011
    thanks so much guys.. i appreciate your time!

    Im actually going to be running some cree Q5's, they can be run as high as 1000mA (228 lumens) but ill probably keep them closer to 600-700mA at 3.3V, so it sounds like i need to take your advice and use something similar to a buckpuck. My last question is this, if im running 4 of these Q5's in series, can i just use one buckpuck? or is this a bad idea? should i be running them in parallel instead? My reasoning for wanting to run them in series rather than parallel, is just simply to pull less current in my circuit.

    thanks again !
  6. ke5nnt

    Active Member

    Mar 1, 2009
    Generally speaking, it is a matter of plowing over datasheets. Running in series will keep the current much lower like you said, but the required voltage will be diode Vf * n where "n" is the number of LEDs in the string. 3.3Vf * all 4 in series would need 13.2V but only require the 600-700mA.

    If you find a driver you like, but the datasheet says max current is 700mA, I would continue looking for a driver that can handle higher current. Some people say that if your current requirement is 700mA, that you should choose components based on doubling that number, as in 1.4A. Though how much higher you go from what you need is totally up to you.

    Finally, don't overlook max power dissipation either. The datasheets will show you the formulas for determining this (usually). Datasheets are your friend.
  7. Audioguru

    New Member

    Dec 20, 2007
    Don't connect too many if any LEDs in series if you want the light to still work when one part of it fails. 4 LEDs in series and in series witrh a current source will ALL turn off if something fails.
    But LED traffic lights frequently show a couple of LEDs not working, not 4 or more in a row.