Misterious Watts value in led (diagram)

Discussion in 'General Electronics Chat' started by jordi 1234321, Mar 15, 2016.

  1. jordi 1234321

    Thread Starter New Member

    Mar 15, 2016
    4
    0
    I have a doubt about some numbers I'm getting in LiveWire... Can anybody explain origin of these values?

    I believed that watts come from Amperes x Volts, but these ciphers are driving me crazy...

    I get that
    [​IMG]
    P = I * V

    V = P / I (fixed)

    V = 5.46 / 6.63

    V = 0,8235 --- what is this number? It isn't V drop from de led, and is not V from batteries... What represents this number?

    Thanks in advance!


    Edit :


    It's possible that the value reflects led's resistance? I mean:

    P = IV

    V=P/I

    V = 5.46mW / 6.63mA

    V = 0,82V (V "efficiently" consumed by led)

    1.5V - 0.82V = 0.676V (V "wasted" by led)

    calculating resistance:

    V = RI

    R = V/I

    R = 0.676V / 6.63 mA

    R = 676mV / 6.63 mA

    R = 101,96 Ohm

    I't possible that nearly 102 ohm are the "resistance" coming from led? Or It's implicit in V drop from led, and I'm calculating it twice?

    Note 1: I know there must be a resistor, it's only a way to simplify diagram.

    Note 2: The value I can't understand is 5.46 mW, I don't know what is its origin...

    Any advice will be very appreciated,
    Thanks in advance!
     
    Last edited: Mar 15, 2016
  2. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    7,394
    1,606
    What seems to have happened is you confused your simulator with a bad circuit. The led would take a huge current if connected like that. Your modeling program must have too simple a model of the led to account for that.

    Try adding a series 100 ohm resistor and see if the numbers work out better.
     
    jordi 1234321 likes this.
  3. jordi 1234321

    Thread Starter New Member

    Mar 15, 2016
    4
    0
    Thanks for your comment ErnieM.
    Putting a resistor still produces a strange value. Placing a 100ohm changes led values to 3.41 mA and 2.75 mW, and resulting from these is 0.806V. Not V from battery, not consuming V because remaining is 0.341V
     
  4. dl324

    Distinguished Member

    Mar 30, 2015
    3,250
    626
    Why do you think the voltages/currents are strange? You are operating the LED in a manner where small changes in forward voltage give large(ish) changes in current.
    LED-iv.jpg
     
    jordi 1234321 likes this.
  5. crutschow

    Expert

    Mar 14, 2008
    13,052
    3,244
    Most circuit simulators require a ground connection.
    Where is yours?

    Note that, if that is an LED model and not just a standard diode, it requires more that 1.5V to conduct significant current. Try about 5V.
     
    jordi 1234321 and RichardO like this.
  6. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
    4,771
    971
    Change your source voltage to one higher than the forward voltage of the LED, include the series resistor and then the numbers will make perfect sense..
     
    jordi 1234321 likes this.
  7. andrewmm

    Active Member

    Feb 25, 2011
    30
    6
    Leds are not resistors,

    In a resistor circuit,voltage , current and ohms are connected as you say., then graph of current against voltage is a straight line.

    In a diode, then this is not so, as shown above by some one, the current flowing a diode measured against voltage is not a straight line, as you increase the voltage, current stays very low, then at a certain voltage, the current will start increasing significantly with voltage.

    A led will typicaly not conduct till a set voltage , dependent upon many things, but in the range of a volt or so.
    So for a led , battery circuit, if you apply a voltage below this , then the led wont turn on, above this, the led will light and drop an almost constant voltage . As you increase the voltage , the led will still drop the same voltage , just take more current.
     
    jordi 1234321 likes this.
  8. ian field

    Distinguished Member

    Oct 27, 2012
    4,415
    784
    AFAICR: Only IR LEDs have low enough Vf to do anything at 1.5V. Red is about 1.8V, Green is 2.0V and blue/white is about 3.4V.
     
    jordi 1234321 likes this.
  9. Veracohr

    Well-Known Member

    Jan 3, 2011
    552
    76
    The OP is asking why the simulator's power and current measurements for the LED work out to a strange voltage value.

    I don't know why but just ignore it. The voltage across the LED is TP1-TP2.
     
    jordi 1234321 likes this.
  10. jordi 1234321

    Thread Starter New Member

    Mar 15, 2016
    4
    0
    I started thinking that simulator has its own values, It does not seem realistic to me... Strange values are a constant whatever cell or resistance I try. Maybe livewire is kidding me hahaha

    I will move forward, maybe I find a solution in the future :)

    Thanks again and regards!
     
  11. dannyf

    Well-Known Member

    Sep 13, 2015
    1,828
    365
    The model may have assumed some internal resistance in the battery. Looks to be about 100ohm.
     
    jordi 1234321 likes this.
  12. dannyf

    Well-Known Member

    Sep 13, 2015
    1,828
    365
    Any diode, including an led, will always conduct at however small a forward voltage across it. It is that the current at such low voltage tends to be very low - aka the diodes appear to be equivalent of a resistor of very high value.
     
    jordi 1234321 likes this.
  13. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
    4,771
    971
    Just like any other simulation software..
    garbage in = garbage out
     
    jordi 1234321 likes this.
  14. ronv

    AAC Fanatic!

    Nov 12, 2008
    3,292
    1,255
    But cheaper to trouble shoot. :D
     
    jordi 1234321 likes this.
  15. ian field

    Distinguished Member

    Oct 27, 2012
    4,415
    784
    Not after the medications you end up having to buy..........................
     
    jordi 1234321 likes this.
  16. jordi 1234321

    Thread Starter New Member

    Mar 15, 2016
    4
    0
    Thank you everybody for your comments... Now I know that simulation software is not a magical tool... It has bugs and oddities too, reality is the best simulator!
    (just hope don't burn many expensive components trying hehe)

    Regards!
     
Loading...