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 ?

 

Try 1kΩ resistor in series.

 

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.

Ron

 

@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 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.

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.

Ron