Hi there, Carl5219!
For a basic LED circuit, you will need a resistor to limit the amount
of current flowing through to prevent a short circuit.
I found an excellent web page describing this very well which contains
the following quotes:
"It turns out that normal wire has very low resistance. Often, its in
the neighborhood of .01 ohms, or even less. If you use a 6 volt
battery, and connect a plain wire across its terminals, then the
amount of theoretical current flow is Current = Voltage / Resistance,
or Current = 6 / .01 which equals 600 amps. Now, it turns out that
most batteries will only hold around 2 or 3 amps. This means the
battery will drain almost immediately, which is not good.
(...) you should never allow electricity to flow from one end of the
battery to the other without using some sort of resistance. If this
occurs, its called a short circuit, and will usually start to burn or
at least smoke badly."
So basically, the resistor is there to prevent the huge current
flowing straight from the positive terminal (+) on the battery to the
negative (-) which will cause a short circuit.
"(...) resistance is extremely important in our circuit. First, it
insures that we haven't created an electrical short. Second, it turns
out that the LED has some really small wires inside it, and cannot be
allowed to carry very much current. In fact, most LED's will burn up
if you allow more than 30mA to pass through them."
So the resistor will have two roles here.
One, to limit the current to prevent a short circuit and two, to
prevent the LED from overheating due to excessive current.
The LED has very little in the way of resistance by itself and because
the wire inside is so small, it will overheat in the same way as a
lightbulb and it will not lower the current going back to the negative
terminal on the battery.
Resistors are usually wired in series.
All quotes taken from:
A basic circuit can be seen here, taken from the above page:
There are several ways of calculating resistance and current (amps) in
Voltage = Current * Resistance
Current = Voltage / Resistance
Resistance = Voltage / Current
(For more on these, visit Ohmslaw.com: http://ohmslaw.com/ohmslaw.htm
So, for a 9 volt battery we can work out the following:
Current = 9 / 330 ohms which is .027 amps or 27mA.
So knowing that a typical LED will not handle more than 30mA, a 330
ohm resistor would be almost at the limit of a typical LED.
I also found a very handy resistor calculator for LEDs:
For example, have a look at the LED here:
http://www.allelectronics.com/spec/LED-94.pdf (PDF document)
It has a 2.4v max voltage with an average 30mA current rating (max
If we use the resistor calculator I mentioned above, we need a 120 ohm
But what if you only have a 1.5v battery?
Simple, using the Resistance method: 1.5v / .050 amps (50mA) = 30 ohms
In general, most LEDs will be rated for a max of 5v but you should
check first, if possible.
I hope this helps!
Related Google searches:
"led circuit guide"