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Forward voltage help!

Discussion in 'General Electronics Discussion' started by Sbimbon, Apr 8, 2014.

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  1. Sbimbon


    Apr 8, 2014
    Hello Forum,

    I'm new here, and am new to electronics, so please forgive me for my lack of knowledge!:eek:

    I've bought an led driver:

    input AC100- 240v
    output DC8-12v, 3w

    I'm about to buy some leds and I've come across the phrase forward voltage, which for the 3w high powered led is 3.2 - 3.6v.

    Is the driver I've bought suitable for this? I've had a look at various pages etc and so far I've ascertained that the forward voltage is minimum voltage for the led to work, but I can't seem to find out whether 8-12v would be too much...

    i realise that this is probably a stupid question, but I really am unsure!!

    Thanks for your help!

  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    We'd need to see the specs, but I suggest that your device is designed for powering three 1W LEDs in series. So it will be a 300mA current source (or similar).

    In most cases, connecting a single LED to this will result in it working correctly at 300mA. Your 3W LED may require 700mA or more for full rated brightness.

    A link to the actual device or more complete specs would assist in letting us give you more accurate information. (oh, same for the LED you have).

    The LED will require heatsinking (probably).
  3. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    Nov 28, 2011
    Hello Simon :) Welcome to Electronics Point.

    In addition to what Steve has said...

    The forward voltage for an LED is the voltage that will be across it when it is running with a particular current through it (current loosely relates to brightness). Usually, the recommended or nominal current.

    When you connect LEDs in series, these voltages add together. (The current is the same, because the same current flows through all components when they are connected in series.)

    Power can be calculated as P = V I, where P is power (in watts), V is voltage (in volts), and I is current (in amps). Be careful to get your units correct - LED currents are often specified in mA (milliamps, 1/1000th of an amp). For example, 200 mA is 0.2 amps.

    The P = V I formula rearranges to I = P / V. Applying that to the LEDs you're looking at, P = 3W and V = 3.4V (half way between the limits, since you didn't give a typical value), so
    I = P / V
    = 3 / 3.4
    = 0.882 amps
    = 882 mA

    So to illuminate one of those LEDs at the rated wattage, your LED driver needs to supply about 880 mA. Each LED will have about 3.4V across it. If you have more than one LED, connect them in series. Three LEDs will add up to about 10.2V which is within the range of the driver

    But it's not clear what the driver's output current will be. You've quoted an output of 3W, which corresponds to only about 300 mA if the output voltage is 10.2V. Generally, current-regulated LED drivers are specified for a particular output current (which is regulated by the driver) and a range of voltages, which determines how many LEDs can be connected in series.

    So we really need more detail about your LED driver. And data on the LED would be useful too.
  4. Sbimbon


    Apr 8, 2014
    Hi Kris/ Steve,

    thank you for your responses, I can;t tell you how much i appreciate the help!

    here's the led's i've bought...
    Here's the drivers too...

    both bought from ebay:confused: I bought he driver based on size as well as I need it to fit inside a restricted space. I'm making a desk lamp and I'd like everything to be as compact as possible so that I can fit them in the main body of the lamp. I plan on making of number of lamps designs using LED's to make them lightweight neat.

    Anyway i recognised some of the info you were talking about from school, but that was best part 15 years ago! so its good to have someone to ask!

    Thanks again
  5. BobK


    Jan 5, 2010
    It is not clear at all what those drivers do. It says constant voltage and constant current, which is impossible to do at the same time, and it does not appear to be configurable, so I have no idea what it will do.

  6. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    "Constant voltage and constant current" typically means it's a voltage regulator with a current limit.

    My concern is that while the current may be limited to 300mA (say) between 8 and 15V (or whatever they say), what happens if the current limit requires the voltage dropped lower?

    You could try and find out. Just beware that these things are hardly what I would call safe. You can bet that they have inadequate separation between the mains voltage and the output. They may have a live output or be a single small fault away from one. (Don't kill yourself)
  7. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    Nov 28, 2011
    OK, well, the LEDs at least seem to have reasonable specifications. They're designed to operate at 750 mA. If you connect three of them in series, which is the best option, you need to feed 750 mA through the series string. The forward voltages add together, so the total voltage across the string will be between 9.6V and 10.8V. The power supply will need to be able to provide this much voltage and current. The LEDs will need heatsinking.

    The power required will be P = V I = 10.8 * 0.75 = 8.1 watts. So that power supply is probably not suitable. It's very hard to say because the specifications are very vague. I also agree with Steve that it's unlikely to be safe. I would add that it's unlikely to be reliable, and it may not even work properly except over a narrow range of load conditions.

    If you want to find out about the power supply you have, try measuring the output voltage when running into a 33 ohm resistor. If the voltage is about 10V then it's likely that it's a current regulated output of around 300 mA which is not suitable.

    You need a power supply that has a 750 mA constant current or current-limited output and can provide up to 10.8V.
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