LED Drivers

Discussion in 'General Electronics Chat' started by RodneyB, May 22, 2014.

  1. RodneyB

    Thread Starter Active Member

    Apr 28, 2012
    633
    13
    Hi

    I am wanting to build a flood light for a rural hospital in Zimbabwe it is a project I have been working on for years in a maternity hospital after seeing a 6 hour baby with 60% of its body covered with third degree burns.

    I have attached a datasheet of some LED's that I have. Firstly the 3 Watt LED's I would like to put 4 into a housing to provide light for stitching patients. Currently they are using cell phone torches. I don't know anything about LED drivers or why they used. Usually I just use an online LED wizard and connect the LED's with series resistors.

    The power supply will be 12 Volt as we have a solar system charging batteries at the moment.

    I need to know a suitable driver for this preferably but not essential THM components. I also really need to understand why we use LED drivers and how to look for suitable drivers.

    The second LED is for some passage lights I want to use this surface mount LED have a housing already and have been using a series resistor combination. But seeing I am doing things correctly I would like to use a driver. I am not sure how many I can use.
     
  2. wayneh

    Expert

    Sep 9, 2010
    12,155
    3,061
    An LED needs a controlled current. Brightness is roughly proportional to current, so more is good, but an LED will be destroyed by a current beyond its rating. So it must be controlled.

    The series resistor you are familiar with is one way to control current. It's an inefficient and limited, but simple, method. It is not good for higher power LEDs, since it wastes power. Since your power supply is solar, efficiency matters. But it would work.

    A driver is a more efficient method than the series resistor, and may add the ability for dimming the LED, and may accept a wider range of input voltages without affecting the output (brightness).

    Be sure you follow recommendations for heat sinking your LEDs. Higher power LEDs get hot and require heat dissipation to ensure a long life.
     
  3. ronv

    AAC Fanatic!

    Nov 12, 2008
    3,293
    1,262
    RodneyB likes this.
  4. RodneyB

    Thread Starter Active Member

    Apr 28, 2012
    633
    13
    Thank you so much for this comprehensive explanation.

    If I understand correctly from my datasheet and the explanation above, I must source an LED Driver that limits the current to 350mA, As my power supply is 12 Volt I will only be able to run a maximum of 3 LED's in Series as the forward voltage of each LED is 3.2 Volts. ( I could possibly run 4 as the charged voltage of the battery is 13.8 Volts.

    As for the heat dissipation this cause a new challenge as I have been un able to find LED specific heat sink. So as a an option I am considering riveting my PCB onto a piece of finned heat sink then securing it to the lamp housing and then possibly putting a small fan to blow. I would imagine some kind of temperature controller would be required to improve efficacy.

    Thank you for your continued help.

    Rodney
     
  5. ronv

    AAC Fanatic!

    Nov 12, 2008
    3,293
    1,262
    RodneyB likes this.
  6. RodneyB

    Thread Starter Active Member

    Apr 28, 2012
    633
    13
    Thanks very much, ebay cant be used in Zimbabwe for some reason.

    The LED is from Ledman Optoelectronic

    Part number LPEH01RRWW3-MD0

    I will re check on the web for the data sheet
     
  7. RodneyB

    Thread Starter Active Member

    Apr 28, 2012
    633
    13
    Please find attached the datasheet it is a 3 Watt LED I must have somehow attached the wrong datasheet.

    If my understanding is correct I connect 3 x 3 watt LED's in series and use a 700mA Led driver
     
  8. RodneyB

    Thread Starter Active Member

    Apr 28, 2012
    633
    13
    I have found this LED driver from what I can see there are no additional components required?
     
  9. wayneh

    Expert

    Sep 9, 2010
    12,155
    3,061
    Looks pretty good, but I would not plan to run an LED at its absolute maximum current rating. It may not last long. The 600mA option should be ideal.

    Is your system voltage really 12V, or more like 13V? I'm a bit concerned you might have trouble lighting 3 LEDs in series if/when your system is at its lowest voltage. Like 11V on a long night after an overcast day. This driver is a step-down only converter, so it cannot raise the voltage.

    I'm not saying it won't work, just that it needs careful study or an adjustment (eg. to 2 LEDs in series) to eliminate the low-voltage concern. You could probably just try it to see if it works, if you're prepared to "upgrade" it later.
     
    RodneyB likes this.
  10. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
    4,771
    971
    Size the heatsink properly and you won't need to add a fan..
    3W LED's are fairly easy to cool via natural convection..
    Now 20W-100W+.. thats another story.
    I've been cooling single 50W multichip LEDs with an old/cheap Duron heatsink/fan combos and they do just fine.
     
    RodneyB likes this.
Loading...