AA batteries heat up, do I need a resistor?

Discussion in 'Power Electronics' started by jettam, Jun 18, 2017.

  1. jettam

    Thread Starter New Member

    Jun 18, 2017
    I'm really new to electronics, so please bear with me and this newbie question.

    I have a simple circuit, two AA batteries a tactile switch and a led. I have noticed the AA batteries heat up.
    So my question is. Is it common practice to add a resistor to a circuit? and if so, how do I know what size resistor I should use ?
  2. Sinus23

    Active Member

    Sep 7, 2013
    Try 1kΩ resistor in series.
  3. Reloadron

    Distinguished Member

    Jan 15, 2015
    Your LED should have a Forward Voltage and a Forward Current and using two AA batteries in series you have a Vsupply of about 3 Volts. The resistor value would be Vsupply - Vled/Iled so your 3 volt supply minus the forward LED voltage divided by the LED Current. For example if the LED Vfwd = 1.2 Volts and LED Ifwd = 20 mA you get 3 - 1.2 / .020 = 90 Ohms or about 90 Ohms you could use a 100 or 120 Ohm resistor.

    jettam likes this.
  4. djsfantasi

    AAC Fanatic!

    Apr 11, 2010
    @Reloadron and @jettam

    As written, the equation for the resistor value needed is wrong or at a minimum misleading. It should be:
    (Vsupply - Vled) / Iled​
    The parentheses are necessary to ensure the correct order of calculations are performed.
    Reloadron likes this.
  5. Reloadron

    Distinguished Member

    Jan 15, 2015
    Uh Oh, yeah, I screwed that up. To have the order of calculations correct it should be as you stated. Thank you for pointing that out.

    djsfantasi likes this.