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driving an LED with a 7406 question

Discussion in 'Electronic Design' started by panfilero, Feb 13, 2007.

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  1. panfilero

    panfilero Guest

    Hello, I have a question about driving an LED with a 7406. Basically
    I have an MCU going into a 7406 inverter going into an LED going to a
    resistor R and that going to a 5 V power supply. All in series, and
    I'm trying to find the value for R that would make this thing work....
    here's a sketch of what I'm talking about (this is my first time
    skecthing on notepad)

    o 5 Volts
    \ R
    \/ LED MV5353
    ------- |
    | | |
    | | |\ |
    | | | \ |
    | MCU |-----------| \-----------------|
    | | | /
    | | | /
    | | |/
    | |
    ------- 7406

    This is how I began to tackle this problem: for MCU output = high I
    looked on the spec sheet for the LED it said that the LED uses a
    continuous forward current of 20 mA at a voltage drop of 3 Volts.
    Then I went to the 7406 specs sheetand looked up IOL = 40 mA. So I
    think to myself.... great, it can sink 40mA and I only need 20mA. so R
    = (5-3)/20mA = 100 Ohms.... now I just have to check R with MCU out =

    I start to get stuck here, cause I begin to think.... should I include
    VOL as a voltage drop when finding R? shold R = (5-3-VOL)/20mA ? Cause
    I'm thinking VOL is the voltage drop for the 7406.... but I look on
    the spec sheet and there's 2 values for VOL = 0.7....

    so R = (5-3-0.7)/20mA = 65 Ohms

    so.... when MCU output = low.... I'm not sure what to do here.... I
    look up VOH.... VOH = 30 Volts.... whoa! hmmm..... I look up IOH, IOH
    = 0.25 mA....... and then i think, i've got 25mA going towards that
    LED, but wait.... it's going the wrong way, the LED will stop it...
    but I wonder how much current the LED can stop?

    at this point I'm totally confused and am not sure if I'm looking at
    things the right way anymore, the VOH,IOH,IOL,IOH stuff is confusing
    to me..... can anyone tell me if I'm on the right track?

    thank you

    so for MCU = low: I guess I need to find IOH? That's IOH = 0.25
    mA.... I'm not sure what to do next, should I look up VOH
  2. Genome

    Genome Guest

    Your 7406 has an open collector output so its output is a transistor... with
    the emitter grounded, the base going into the guts of the IC and its
    collector connected to the output pin.

    It can only sink current, not source it.

    The transistor has a maximum rating of 30V when off so you could, with an
    appropriate limiting resistor connect your LED to 30V. You could connect a
    number of LEDs in series along with an appropriate resistor and light them
    all from one output.

    The IOH of 0.25mA is probably the maximum specified 'leakage' current that
    the transistor will sink when it is off with 30V applied to it. It's still
    sinking it so it won't hurt your LED.

    [I'm guessing, I haven't read the dirty sheet]

  3. Anirban

    Anirban Guest

    It's not that (5V-3V)/R = 20mA.
    You need to account for the 0.7V drop across the output transistor in
    the 7406.
    So it should be (5V-3V-0.7V)/R=20mA
    Otherwise it is fine.

    Aside, if the MCU can directly drive an LED, why use a 7406. Most MCUs
    these days can. Tie the anode of the LED to Vcc=5V and the cathode to
    the MCU pin. If needed, throw a 100ohm resistor in series. The cathode
    should be tied to the MCU because most MCUs can sink more current than
    they can source.

    --- Anirban
  4. panfilero

    panfilero Guest

    I'm confused about this part, if the MCU = Low, then my output of the
    7406 is high..... does this mean that IOH = 25 mA is coming out of the
    7406 and headed towards the LED? and what is the VOH = 30 V about?
    That seems really high? I thought I could treat the VOH like a
    voltage drop inside the 7406, (like i did for the VOL = 0.7 V part)
    but.... I guess I ignore the 30 V, cause there's no where for that to
    be coming from.... so, why is the LED off now? I don't understand
    how the LED is off, there's still that 5 Volts and that resistor....
    aren't they pushing a current through the LED? does the lead from the
    LED now connect to another reistance and voltage source equivalent
    thing inside the 7406?

  5. Eeyore

    Eeyore Guest

    What on earth are you using a 7406 for ?

  6. Eeyore

    Eeyore Guest

    Yes, it's an inverting buffer.
    No. In any case, the LED would be the wrong way round for that to work, if it
    was the case, which it isn't.

    The maximum voltage the open collector output can withstand.
    It seems your confusion relates to not understanding what the data sheet's
    telling you. And some problems understanding basic electricity and how circuits
    work too !

  7. Thad Smith

    Thad Smith Guest

    That is the right approach, although Vol probably will depend on the
    actual current.

    One thing to note is that 20 mA will be very bright, brighter than what
    is typically needed for an indicator. The best answer is probably to
    try different values. You are in the right ball park.
    Assuming the driver is powered by 5 V, the output will simply go to that
    voltage. Since the other side of the LED is 5 V, there will be no
    output current.
    Not needed. With no significant potential difference the current will
    be insignificant.
  8. panfilero

    panfilero Guest

    ok, thanks that does clear it up quite a bit... i was confused as how
    we treat the VOL as a voltage drop but not the VOH.... what if I were
    to remove the LED and put a transistor there in it's place with a load
    in the emitter going to ground and the collector going to a 5 Volt
    source.... like this:

    o 5 Volts
    \ R
    | o 5 Volts
    | |
    | |
    ________ | |
    | | | ----|
    | | |\ | /
    | | | \ | |/
    | MCU |-----------| \---------|---------|
    | | | / |\
    | | | / \
    | | |/ \/
    | | |
    ------- 7406 |
    | |
    | |
    | Load |
    | |
    | |

    If my VBE = 1 [V].... and I needed 750 mA to get my load to turn on,
    and the load was small, like 4 Ohms.....could I do this? If I try to
    figure it out like the LED one, then my 7406 has a voltage drop of 0.7
    Volts. but that's in parallel now wtih the 1 V from the VBE drop and
    the VLoad = (4)(.750) = 3 V drop from the load.... if my transistor
    has a gain of hfe = 40, and the 7406 sinks IOL = 40 mA, then do I need
    my base current to be..... IB = hfe*IC + 40mA ? that way the 40mA will
    get sunk but the rest will excape in order to turn on my transistor
    and turn on my load? I think this is correct, but then I don't know
    what voltages to use in order to figure out what size resistor R I
    would need to do this?

    I don't know whether to use the VBE and VLoad.... or the VOL from the
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  10. Tom Lucas

    Tom Lucas Guest

    Bit presumptuous?

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  11. John Fields

    John Fields Guest

  12. Eeyore

    Eeyore Guest

    There is nothing in the original post to indicate a reason for its use thank you
    very much Mr Allegedly Polite !

    Does anyone even still make a standard TTL 7406 ? It seems unlikely to me.

  13. John Fields

    John Fields Guest

  14. panfilero

    panfilero Guest

    Who cares? It's what _he's_ using.

    Thank you JF. I appreciates everyones concern over the components I'm
    using, but I'm not looking for a more efficient design to this simple
    problem, I was looking for some understanding to these kind of simple
    circuits. This problem is just for helping myself understand things
    not for efficiency or pratical use in any specific application.

    Thank you
  15. Eeyore

    Eeyore Guest

    Well ? So ?

    Do you never think to offer advice about component selection ?

  16. panfilero

    panfilero Guest

    When the 7406 is off, can I treat it as a 0.7 Voltage load with 40 mA
    running through it to ground?
  17. tsest(1)

    tsest(1) Guest

  18. g

    g Guest

    The first thing I sometimes do, is take the LED and see how much
    it needs to be bright. I'm not talking about lighting up the room
    I usually use between 5 an 10 ma. max for indicators. Back in the old
    you needed the current for a little brightness.

  19. rickman

    rickman Guest

    It can be hard enough to explain this sort of design in person, much
    less by group postings, but I'll give it a try.

    The 7406 or any other switching device has two sets of ratings; one
    for input and one for output. The input specs tell you what is
    required to drive the inputs to a valid state. The output specs tell
    you what sort of load and voltage can be driven by the output.

    The LED requires about 20 mA at about 2.2 volts to drive it. If you
    use a 5 volt supply and use a 74LS06 as the control (I can't find a
    data sheet on the 7406) you need to subtract the LED voltage and the
    74LS06 output voltage ([email protected] mA = 0.4V) from the power supply to find
    the voltage on the current limiting resistor. 5 - 2.2 -0.4 = 2.1V.
    At 20 mA this will give you 2.4V / 20 mA = 120 ohms.

    Since this is an open collector device, it will not source any current
    to the LED, so you don't need to worry about that. The IOH of this
    device is not given because it does not drive current when the output
    is high. Regardless, IOH and IOL are maximum values or conditions for
    measuring VOL and VOH. The outputs of digital devices are just
    transistors connected to ground and power; they are not current
    sources. They provide a voltage and the current drawn from the output
    is determined by the load.

    In the above example I only needed to know the current into the 74LS06
    output because there is some resistance in the transistor connection
    to ground so that the output voltage of the 74LS06 depends somewhat on
    the current flowing. At 24 mA the output voltage is 0.4 V, at 48 mA
    the output voltage is 0.5 V. You can see this is not a large
    difference. Even a microAmps of current, the output voltage will
    likely be around 0.2 volts. For most circuits you can estimate the
    output low voltage to be about 0.4 volts for TTL type devices. CMOS
    devices are much more like resistors on the output so the resulting
    voltage varies directly with the current.

    Does that help?
  20. redbelly

    redbelly Guest

    One approach is to try the larger resistor value (100 ohms) and
    measure the current. If you're wrong, then you'll get too little
    current. But it WON'T fry any of the components, and you can then get
    a better idea what the correct resistor should be.

    Sometimes a little experimenting helps more with learning electronics
    than spending a lot of time getting the calculation correct.


    p.s. to measure the current, I'd measure the resistance before putting
    the resistor into the circuit, and then measure the voltage drop
    across the resistor while running the LED. But that's just my
    preference, I tend to avoid using ammeters directly.
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