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Powering LEDs

Discussion in 'd.i.y.' started by PaulMB, Nov 1, 2019.

  1. PaulMB

    PaulMB pfm Member

    I've just ordered some 3W LEDs, each on a roughly round shaped printed circuit board, diameter about 20mm.

    The specs say they need "forward voltage" of 2.4 - 2.9 volts and 800 mA max.

    Does this mean I could power 3 of these in series with an old 'phone charger marked 5.7 volts and 150mA?

    Or alternatively 5 in series with another charger marked 12 volts and 1 A (1000 mA)?

    Or would it be better to buy a specific "driver"?

    Thanks for any help.
  2. PaulMB

    PaulMB pfm Member

    Sorry! The first charger says "50/60 150mA" but then below it says "Output 5.7 volts 800 mA.
  3. martin clark

    martin clark pinko bodger

    That's not going to work directly - LEDs are a thing that have a defined forward-voltage to turn-on, and for lighting , an optimal current for rated illumination.

    Neither of your two scenarios work, and while you can fiddle such things - i.e to run one such LED from say a 5v wall wart ( a low-impedance) supply with a power resistor or similar, it's inefficient. By the time you want to run three, messy and hot.

    A suitable LED driver actually works in constant-current mode - it will be a switch-mode supply, designed to be supplying current - say your 800mA - and the voltage across the LED string will float/vary to ensure this constant current. That's good from the POV of constant illuminance (which depends on maintaining a constant current) and also, because the driver is designed to behave in this way, considerably more efficient overall - in terms of power supplied vs power only reqd by LEDs.

  4. davidsrsb

    davidsrsb pfm Member

    Four in series, together with a 2.2 Ohm 2.5W wirewound resistor off a 12V@1A regulated power brick should work fairly efficiently
    martin clark likes this.
  5. PaulMB

    PaulMB pfm Member

    Thanks to you both. I think I'll buy a suitable driver from the same place I bought the LEDs rather that messing about with a soldering iron. Would I be right in thinking that if I add a resistor in series from the driver I can make the LEDs dimmer? Or if instead of 2 LEDs in series I have 3 each will be a bit dimmer? Or if I use a supply that gives less than the maximum 800 mA they will be dimmer? I ask because these are red LEDs for a photographic darkroom.
  6. davidsrsb

    davidsrsb pfm Member

    If you use a proper current drive power supply putting LEDs in series will not affect the brightness.
    As these are red, I would expect a lower forward voltage. A red led is about 1.5V plus any drop due to resistance. This comes from the physics of generating light
  7. russel

    russel ./_dazed_and_confused

    Higher power ones usually have a combination of series and parallel LEDS, to reduce Resistive losses.
  8. PaulMB

    PaulMB pfm Member

    Thanks, everyone. With all due respect for Martin's technical explanation, which from many years' experience I am sure is perfectly accurate, but which is over my head, I'll probably try something like David's suggestion, even if there is a waste of power. All the dedicated LED drivers I've seen in shops have a range of voltages, e.g. 8 - 15, and it has been explained to me that it is the LEDs themselves that choose their appropriate voltage. And that with a bigger or lower value resistor, if I use one at all, I can make them a little dimmer or a little brighter. Just to be clear, my red LEDs are rated 2.4 - 2.9 volts. If it is of any significance, I plan to use them to make a safelight for a darkroom.
  9. davidsrsb

    davidsrsb pfm Member

    True, you find series strings for 12V car usage, but the forward voltage is in steps of 1.5V for red at low currents
  10. PaulMB

    PaulMB pfm Member

    OK, I've just bought a little plastic thing the size and shape of a matchbox. It says: For LEDs, Constant current, Output 7 - 13 volts, 500mA. Will this work with 3 or 4 3watt LEDs, rated 2.4 - 2.9 volts, in series?
  11. martin clark

    martin clark pinko bodger

    Yes it should work fine; they’ll each run at about 1.5watts, so should be plenty bright.

    - basically it will drive 500mA through the series string, and the voltage across them will be whatever is dictated by the LEDs running at that current.
  12. PaulMB

    PaulMB pfm Member

    Bless you, Martin! I knew I could count on you for precision and clarity!


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