The light emitting diode
(LED) is commonly used
as an indicator.
It can show when the
power is on, act as a
warning indicator, or be
part of trendy jewelry
etc.
It needs to be fed from
a DC supply, with the
anode positive and the
cathode negative, as
shown in the diagram. To calculate the value
of the series resistor
we need to know the
diode forward voltage
and current and its
connections.
The necessary data can
be obtained from a
catalogue or data book.
In our example it is 2
volts and 20mA (0.02
amps).
The cathode lead is the
one nearest
a "flat" on the body.
Since the voltage across
the diode is 2 volts and
the battery voltage is
12 volts, then the
voltage across the
resistor is 12-2 = 10
volts.
The diode is in series
with the resistor, so
the current through then
both is the same, 0.02
amps.
We now know the voltage
across, and the current
through the resistor.
From Ohm's Law we can
now calculate the value
of the resistor.
Resistance = Volts
divided by Amps = V/I =
10/0.02 =500 ohms.
Since this is not a
standard value we can
use a 470 or 560 ohm
resistor as this
application is not
critical of values. |