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I need more power!

Discussion in 'Electronic Design' started by [email protected], Mar 20, 2007.

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  1. Guest

    Hi!

    I am trying to design a printed circuit board with quite basic
    functionality. (My first one!) The main part of it is taking pulse
    width signals, passing it to buffers which then goes to LED's.

    The system voltage is 3.3v with a power supply of 3.7v

    I took the LED's to the lab and ran some tests on them. When you give
    them the appropriate voltage, around 3.5v they draw 20mA of current.
    So naturally I assumed at around 3.3v they would draw somewhat less,
    right? WRONG! they draw upwards of 40mA.

    I can't find a single buffer that will give me more then 100mA per
    chip, with a max of 50 mA per pin at 3.3v. There are ton of chips
    that would work if I could supply it with 4.5 source, but the max
    voltage from the battery is 3.7v.

    I've looked at using the buffers to power a transistor but with my
    limited math skills the transistor won't turn on because I need to
    give it more than 3.3v! everything seems futile without more
    voltage. There has to be a way to draw more power (current) from a
    source without increasing the voltage!

    Any suggestions at what I should look at?

    Thanks a ton!

    Matt
     
  2. Guest

    Before making a PCB, I suggest you learn more about LEDs. They are
    current devices, not voltage. You *did* put a resistor in series with
    the LED?
     
  3. Guest

    Yes and no, a resistor is in series with the red LED, though to
    calculate the resistor I looked at the voltage the data sheet
    required. Then using the expected current figured it out from
    there... I.E. for the red part it wants a voltage around 2.0V. So I
    put a 65 hm resitor in series which drops it 1.3v at 20mA. This
    seemed to work.

    For the blue in green they want a voltage of around 4.0v for a 20mA
    current. As I only have 3.3 I just connected that straight. That
    probably explains the jump to 40mA. How should I calculate the
    resistances for those?

    Thank you for the information on the fet's I will look into that!

    Matt
     
  4. Hawker

    Hawker Guest

    I assume a ULN2003 will give you what you want,
    however in a similar situation I would probably find me a pile of SOT-23
    logic level FETS instead. Go to Fairchild Semi and look around for
    SOT-23 logic level FETS. Finding good 3.3 ones is slightly hard. Best to
    find a 2.5v one and use that so you know it is fully on.

    Hawker
     
  5. Guest

    Alright, Thank you very much! I will go back and do some more tests.
    One last general question about Tri colored LED's. Is it normal for
    RED to be barely visible in comparison? To make white I have to
    basically not Pulse RED, and then BLUE and GREEN have to be nearly
    off. It is also possible that I damaged the RED by applying to much
    voltage at first.

    Thanks
    Matt
     
  6. Noway2

    Noway2 Guest

    You need to look at the part data sheet, which will show you an I-V
    (current - voltage) curve. The curve will show you what the forward
    voltage drop for the device at a given current level. So if for
    example, you have a 3.3V supply and want 10mA to flow through the diode,
    you find the 10mA line on the chart and the corresponding part on the
    curve, which will then tell you the forward voltage at that current
    level. To continue the example, say at this point the forward voltage
    is 1.3V. Then the resistance you need is (3.3V - 1.3V) / 10mA = 200 Ohms.

    Similary, a look at the datasheet should provide some insight as to what
    happened when you put 3.3V across the diode, assuming you didn't run
    into a physically limiting feature.

    With the diodes, you need to be sure you don't exceed the maximum
    forward current, least their life will be very short.
     
  7. ehsjr

    ehsjr Guest

    Since all you want to do is light LEDs at ~ 20 mA,
    you can use a small boost DC-DC converter chip, like
    a TPS61070. The datasheet shows you how to hook it up.

    Ed
     
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