The table below summarizes the electrical characteristics of the RGB MR-16 circuit:
PA R A M E T E R
M I N I M U M
N O M I N A L
M A X I M U M
V
b
(Supply Voltage)
3 V
f
– 6 V
f
See text.
12V
18.5 – 21.5V
See text.
I
b
(Total Current)
18 LEDs
—
—
120 mA
I
b
(Total Current)
9 LEDs
—
—
60 mA
I
b
(Total Current)
18 LEDs, Red in Series
—
—
100 mA
Total Power
0.72 W (9 LEDs)
1.44 W (18 LEDs)
2.15 W @ 21.5V
Total Light Intensity
(RGB LED kit)
78,000 mcd
(9 LEDs)
—
156,000 mcd
(18 LEDs)
Viewing Angle
(RGB LED kit)
160°
—
180°
Total Light Intensity
(White LED kit)
234,000 mcd
(9 LEDs)
—
468,000 mcd
(18 LEDs)
Viewing Angle
(White LED kit)
—
15°
—
The maximum allowable supply voltage, V
max
, is given by:
where I
f
is the LED forward current (usually 20 mA), P
r
is the resistor’s maximum power rating (
¼
W), and V
f
is the
LED’s forward voltage. Usually this equation is dominated by the red LEDs’ current-limiting resistance since they
have the lowest V
f
and consequently the highest voltage drop across the series resistor. It is possible, however, to
place the two red LED strings in series by installing jumper J4 instead of J3. The V
max
is then 21.5V instead of 18.5V
with the normally-connected red LEDs. A desirable side effect of the red LEDs being connected in series is that the
total supply current is reduced to 100 mA with no loss in brightness. The minimum voltage, however, will be 6V
f
or
about 12V. Generally, there is no advantage to using supply voltages higher than 12V since the extra power will be
wasted in the current-limiting resistors.
V
max
=
P
r
I
f
+
3
V
f
C o m p u t e r I n s p i r a t i o n s
!
A s s e m b l y & Te c h n i c a l M a n u a l
6