Below is a dissertation by Jim Betz of the Yahoo Soundtraxx group concerning LEDs and lamps. Some of you might want to copy and paste the message into a word document and print it out. Could make easier reading.
When you are thinking about these two very different devices for
producing light the following will help you to understand their
behavior better.
Bulbs are, essentially, “voltage devices” - if you vary the voltage
you vary the amount of light coming out of them. And
changing the current will not, significantly, change
the amount of light. More importantly it is having too
much voltage that causes them to burn out.
In addition, a bulb ‘consumes’ its element in order to
produce light. And the higher the voltage the quicker it
consumes it. And even a brief amount of too much voltage
will cause them to blow.
When changing a circuit in order to ‘control’ a bulb
you should pay attention to the voltage produced - and
the bulb will last a -lot- longer if you use lower
voltages (within the range of voltages that work for that
bulb). And that’s why we refer to the resistor in the
circuit as a “voltage dropping resistor”.
LEDs are, essentially, “current devices” - if you vary the current
you vary the amount of light coming out of them. And
changing only the voltage will not, significantly, change
the amount of light. And it is too much current that
causes LEDs to burn out.
In addition, an LED has a much slower ‘consumption’
of its element in order to produce light. But the higher
the current the faster it consumes. And even a brief amount
of too much current will cause them to blow - in a rather
spectacular one bright pulse of light.
We refer to the resistor in a circuit for an LED as a
“current limiting resistor”. If you use lower current
values (within the range of cur