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Biasing a transistor to drive LEDs

Discussion in 'Electronic Basics' started by Wong, May 27, 2004.

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  1. Wong

    Wong Guest

    I would like to bias a 2N3904 transistor as a switch to drive 4 LEDs
    in parallel. So I have this configuration but I am not sure whether
    the cct will work.
    The connection is from 5V source to 47 ohms resistor and then 4 LEDs
    in parallel, then to Collector. Emitter is directly to ground.
    For the Base, it is connect by a 1K current limiting resistor and
    have a 5V, 0V ON/OFF control. I assume LED will consume 20 mA and
    require 1.5 V.
    Is this cct going to work ? If yes, how to justify by calculation ?
    Thanks in advance.
  2. Tim Wescott

    Tim Wescott Guest

    Everything looks more or less OK, except that PN junctions tend to vary
    from one to another.

    So with the four diodes in parallel you may see quite a bit of
    brightness variation -- if I were doing this I'd use four resistors, one
    for each diode. You may also want to consider using the diodes in
    series pairs, with one resistor for each pair. Assuming a regulated 5V
    supply and a fairly constant diode temperature this will leave you with
    about 1.8V for the current limit resistor.
  3. Tam/WB2TT

    Tam/WB2TT Guest

    Better to use a separate resistor for each LED.

  4. It works but each LED should have its own individual resistor. Each
    resistors should be about 150 ohms. Use a 2N4401 instead if the
    collector voltage is above 0.2V when it's turned on.
  5. Wong

    Wong Guest

    I am concerning more to the transistor actually. I think the
    calculation on the LEDs should be OK for me but I just not so sure
    with the biasing of transistor. In another words, is the Base current
    setup correctly to operate ?
  7. Tim Wescott

    Tim Wescott Guest

    Oh, _that_.

    You're set up for about 70-100mA on the collector, and you're giving the
    base a hair more than 4mA. This means that you need an hFE of about 25
    for it to work.

    Now, I'm too damn lazy to go look, but 100mA and hFE of 25 sounds
    plausible for a 3904, but it may be asking for too much collector
    current. It's your design, so _you_ should get on someone's website,
    get a data sheet and check to make sure.
  8. My choice would be 12V power supply, and the LEDs in series.

    One of my college dorm projects was to make a big counter/display
    with four LED's for each segment. A 7447 will directly drive
    such four LED's, I think to 30ma. A 12V power supply and appropriate
    resistor are needed.

    -- glen
  9. Wong

    Wong Guest

    Yes, I was designed this circuit based on the following:
    absolute maximum of Ic is 200mA, min hFE of 30 at Ic=100mA, Vce=1V
    maximum dissipation=625mW
    So I think it should be OK, isn't it ?
  10. You want to saturate the transistor, by forcing the beta to be 10. THe
    Ic is 10 times the Ib. The voltage at the collector should be only a
    few tenths of a volt when it's on. If it's higher, then you need more
    base current.
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