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Discussion in 'General Electronics Discussion' started by Luis Costa, Mar 13, 2015.

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  1. Luis Costa

    Luis Costa

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    3
    Mar 13, 2015
    Hi guys... I'm new to the forum. I've got a lot of questions on electronics but for now i've got simple issues... I mean: all components work at specific voltages but it's hard to know what resistors and components to use. For example: a regular LED has max. voltage of about 2.0v. if you were to use a 6v power source we'll need a 330R resistor. Except that measuring what comes out of the resistor using that same battery doesn't get me a 4v voltage drop. How does that really work? How does one calculate the equivalent resistance/capacitance and make ohm's law calculations work for purposes of a real application?
    Thanks in advance. Please feel free to bug me about machine language. i'm keen on it.
    Later
     
  2. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    Welcome to electronicspoint.
    Depends on the LED. A LED has a varying voltage drop that has a non-linear relation to current. Therefore the "resistance" of a LED varies with current.
    Measure the voltage drop across the resistor and the drop across the LED and they will sum up to the voltage of the battery.Read more about driving LEDs in our ressource.
     
    Last edited by a moderator: Mar 13, 2015
  3. Luis Costa

    Luis Costa

    30
    3
    Mar 13, 2015
    QUOTE="Harald Kapp, post: 1645441, member: 22009"]Welcome to electronicspoint.

    Depends on the LED. A LED has a varying voltage drop that has a non-linear relation to current. Therefore the "resistance" of a LED varies with current.
    Measure the voltage drop across the resistor and the drop across the LED and they will sum up to the voltage of the battery.Read more about driving LEDs in our ressource.[/QUOTE]
    thanks... and pretty good... so... there's also resistance across base-emitter of a transistor? i was trying to make this 80mV bias and was around 18 MOhm. some burned up.
     
    Last edited by a moderator: Mar 13, 2015
  4. Harald Kapp

    Harald Kapp Moderator Moderator

    11,443
    2,627
    Nov 17, 2011
    Don't think of semiconductors as resistors. They behave differently. Look at the V-I characteristics of e.g. a diode: it is highly non-linear.
    It is customary to never apply a voltage to an PN-junction (diode, base-emitter etc.) of a semiconductor without some means of current limiting. A typical current limiter is a resistor inserted in series with the PN junction (sometimes you will see circuits that use an LED with a battery without resistor - these circuits "rely" on the batteries internal resistance and are not very reliable).
    Again, this is explained for LEDs in our ressource but applies in a similar manner to other PN junctions as well (within limits, as each type of semiconductor device has different characteristics).
     
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