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Basic voltage drop question across LED

Discussion in 'LEDs and Optoelectronics' started by onpiopah, Jul 10, 2013.

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  1. onpiopah


    Jan 18, 2012
    Hi I am just starting out so please bear with me as I realise this is probably a very basic problem I have. I am currently learning about Voltage and have made a simple circuits with a 6volt battery, resistor and LED.
    • The voltage drop across the battery is 6volts.
    • Across the resistor is 4.5v and across the Led 1.5 v which all adds to 6volts. This makes perfect sense to me.
    • If I take out the LED the drop across the resistor is 6v and the battery 6v – again it makes sense.
    • But If I just use the LED the drop across the battery is just over 2 v and across the LED is just over 2v.
    • And If I try the experiment in software such as “Circuit Wizard” the voltage drops over both remains at 6v – can someone explain what is happening – I thought it should always be 6v?

    thnaks for your reply.
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    A battery is not a perfect device (Circuit wizard assumes it is).

    Assume a battery always has a small resistance in series with it (it is called the "equivalent series resistance" or the "internal resistance")

    So when you have just the LED attached, you read the "perfect" battery voltage less the drop across the ESR of the battery.

    Also, you would note that the LED gets very bright and depending on the battery you have damaged it slightly or a lot.
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