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Dave Martindale

writes:

For the sake of my education let me ask. Doesn't an LED have a
predictable voltage drop across the junction? I seem to recall 1.5
volts is that correct? If it is I only need as many LEDs in series as
will equal the voltage and they will need no external resistance. Is my
memory wrong?

The voltage-current characteristic of an LED has a pretty sharp "knee"
characteristic of most diodes. Very little current flows as voltage
increases up to the turnon voltage, and then current rises very rapidly
with very little change in voltage. The actual voltage depends on the
type of LED (different colours use different chemistry and have
different voltage) and temperature.

LEDs are easy to drive from a constant-current power source. You set
the current you want, and the power supply adjusts its voltage to
whatever is needed to make the LED draw that amount of current. Put a
bunch in series, and all get the same current.

But most power sources are closer to constant-voltage instead of
constant-current. It's nearly impossible to set the voltage needed for
a particular current, since a small voltage change results in a large
current change, and the voltage needed depends on temperature (which
depends on current among other things!).

So you generally use a resistor in series with the LED, or string of
LEDs. The power supply voltage is equal to the sum of the voltages of
the LEDs in the string, plus a few extra volts. The resistor is sized
to give the desired LED current with the "extra" voltage applied across
it. This gives a system where small changes in LED turnon voltage
produce small changes in the voltage across the resistor, which produce
small changes in current through the LEDs. Without the resistor, you'd
get large and unwanted changes in current.

As others pointed out, it is possible to build a system using only a LED
and a battery *if* the battery's own internal resistance is enough to
stabilize the system.

Dave