Hello

I need some help with this simple question which I am having a brain cramp with. Using 4 basic commercial disposible AA batteries for a Total of 6v, what is there Total current? I plan on using a basic LED to indicate power on and the LED is a 20mA one so I would like to calculate what resistor I need to protect the LED. I looked at a AA battery and I can`t see clear enough what it`s current value is. Eyes need help. So do I.
Thanks
George

 

You don't need to worry about the current of the battery to calculate the current limiting resistor. All you need is the total of the battery, the forward current of the led and the forward voltage of the LED

The only time the ampere hours of the battery comes into play is calculating how long the LED stays lite before the battery is depleted.

 

https://www.sparkfun.com/tutorials/219

 

I plan on using a basic LED to indicate power on and the LED is a 20mA one so I would like to calculate what resistor I need to protect the LED.

You don't need to operate the LED at 20mA if it's just used as a power on indicator.

Use Ohm's Law to calculate the required resistor value. R = V/I.

Assuming a forward voltage of around 3V and a curent of 5mA, R = (6V - 3V)/5mA = 600 ohms. The nearest 5% values is 620 ohms.

P = I*I*R = (5mA)(5mA)(620 ohms) = 0.0155W; so power dissipation won't be an issue.

 

You don't need to operate the LED at 20mA if it's just used as a power on indicator.

Use Ohm's Law to calculate the required resistor value. R = V/I.

Assuming a forward voltage of around 3V and a curent of 5mA, R = (6V - 3V)/5mA = 600 ohms. The nearest 5% values is 620 ohms.

P = I*I*R = (5mA)(5mA)(620 ohms) = 0.0155W; so power dissipation won't be an issue.

Hello
Thanks for the help and tips.

 

AA battery current? A cheap Chinese "super heavy duty carbon zinc" produces almost no current and a name brand Energizer or Duracell alkaline provides more than 500mA as shown on their datasheets.

 

The question in principle not differs much of the question - How much Amperes is in the network mains. Measure it some day with a tester. Or better no, save the tester coins for the school. Because the answer is - in some cases even million Amps is not the physical limit for short period. Anyway there is inside much more than one may want and more than is wise to take out.
The same is with batteries, where only physical barrier in Your behalf is inner parasitic resistance.
That is why, George, Your head is spinning.
Set the limiting resistor in the circuit and Your brain will catch the clue immediately, according the Ohm`s law. By the way, LED connected to battery without of ballast resistor is equal the drunk blind man which dribbles on the 100th floor building roof very edge. One battery may happen to be a bad enough to have a rather large parasitic resistance, but other may happen be good battery, and then all Your LEDs immediately flies straight to the LED`s Paradise.

 

Many cheap LED torches use small batteries and rely on the internal resistances of the batteries and LED to limit the current. Bad practice, but is cheap = no resistor. Always use a resistor or current source to drive LEDs, because the current is not limited otherwise. Also the LED voltage reduces as it heats up so a resistor protects (a bit) against overheating (thermal run-away). A current source is better but not essential for low current LEDs.

 

You don't need to operate the LED at 20mA if it's just used as a power on indicator.

Use Ohm's Law to calculate the required resistor value. R = V/I.

Assuming a forward voltage of around 3V and a curent of 5mA, R = (6V - 3V)/5mA = 600 ohms. The nearest 5% values is 620 ohms.

P = I*I*R = (5mA)(5mA)(620 ohms) = 0.0155W; so power dissipation won't be an issue.

You can also get ultra efficient LEDs that work on about 2mA - very handy if there's not much mAh to spare.

 

A tutorial...

LEDs, 555s, Flashers, and Light Chasers

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