Hi, I've been trying to google-educate myself but I want to make sure of what I'm doing and ask for some expert advice, if we've any in the audience. :) I have a battery case that takes two AA batteries (energizer if that matters), I've checked on my voltmeter and I believe it gives a DC current of 3.20v (if that's way off I may be using my voltmeter incorrectly) And the LEDs I have are 5mm Nexxtech LEDs, which from searching I've found this information on them: 5mm (T1 3/4): Forward Voltage 2.2 V, Forward Current 10 mA (max 15 mA).Now I want to hook up 4 LEDs to this battery pack, to be continuously illuminated. I have also got a pack of 1/4 watt 5% resistors. On the back of the pack it lists a whole slew of specs, apparently there are two of each type, but I'm not sure what the numbers mean, or how to differentiate which resistor is which. (see the summary of
The battery pack you have is +3.2 volts, which is correct since *unloaded* batteries are typically 1.65V and are 1.5V when loaded down. Now in order to hook up (4) LEDs with a (minimum) forward voltage of 2.2V, you will need 4 * 2.2V 8.8V (minimum) as a voltage source. Therefore you would need (3) dual AA battery packs that are connected in series to get 9.6V to work properly. Next you will need to control the current thru each of the LEDs that are also connected in series. The resistor would be need to be the voltage divided by the current from Ohm's Law as follows R V / I (9.6V - 8.8V) / 10mA 0.8V / 0.010A 80 ohms With this solution you can use (1) switch to turn off all the LEDs. An alternate way to power the LEDs is when each of the (4) LEDs are in parallel to the single 3.2V source. Since the voltage drop of each LED is 2.2V, the resistor for each LED would be R V / I (3.2V - 2.2V) / 10mA 1.0V / 0.010A 100 ohms Now you would need a switch to the batteries that is in series will all (4) LEDs that are in parallel to each other with their separate 100 ohm resistor. Notice that the batteries will not last as long with the LEDs in parallel as they would with the LEDs in series. The series combination is more efficient, but requires a higher voltage. LEDs do not give off heat, but the resistor might. However, the current of 10mA is not enough to create any danger.
