Datasheet
such as the ACULED must never be used directly to a voltage source like batteries or voltage
supply source.
Calculation of current limiting resistor
The resistance R of a circuit as shown in figure 3 is given by Ohm’s law
R =
V
I
(1)
where I is the current in the circuit that is equal to the forward current I
F
that we want to adjust, and R
is the overall resistance of the circuit given by
R = R
RL
+ R
LED
(2)
with the internal LED’s resistance R
LED
and the wanted current limiting series resistor R
RL
. In
practice usually the LED resistance is very small with R
LED
<< R
RL
so as good approximation we
can assume R
≈
R
RL
. Since in serial circuits the voltage drops at each component are added, V
in equation 1 is the sum of the voltage V
RL
at the resistor and the forward voltage V
F
at the LED:
V = V
RL
+ V
F
(3)
V in this case represents the voltage of the source V
S
. According to equation 1 this leads us to
the wanted resistance R
RL
:
R
RL
=
V
−
V
(1a)
I
F
As an example let us assume driving the yellow chip from the RGYB ACULED VHL at 350 mA
with
a 9 V battery utilizing a resistor. According to the datasheet (only the yellow chip!) the typical
forward voltage is V
F
= 2.2 V. According to equation 1a the resistance of the current
limiting
resistor needs to be 20
Ω
or more. However, due to variations in the forward voltages of different
dies, the same resistor will lead to different forward currents and therefore to different light intensities.
Additionally, according to
P = I · V (4)
any current limiting resistor consumes power, in our example 2.4 W which is three times more than
the yellow chip itself! Therefore the easiest way is to use a constant current source like most
commercially available LED drivers are made of. Excelitas provides LED drivers for the ACULED;
please contact your local sales representative for more information.
Figure 3
Battery
Principle of LED-circuit using a
battery and utilizing a current
limiting series resistor.
LED Resistor
www.excelitas.com
Driving the ACULED® VHL™
5