45
d) Connecting the LED Strips/LEDs
The combiboard offers the possibility of connecting up to eight LED strips with an operating voltage of 12 V and a
maximum power intake of 1 A/per strip. Even if the MOSFET switching outputs (FDS6890A) can resist up to 7.5 A, it
is recommended not to put more than 1 A on the individual outputs for board layout and cooling reasons.
If you want to connect individual LEDs with a lower operational voltage of only 2 – 3 V, a suitable dropping resistor (Rv)
must be calculated. For calculation of the dropping resistor, you should use an operational voltage of 12 V, since a
freshly charged 3-cell LiPo battery may have far more than 12 V instead of 11.1 V.
Example calculation:
Assuming that the light diode used has a supply voltage of 2.6 V and a power intake of 20 mA, first the dropping
resistor voltage must be calculated:
12 V – 2.6 V = 9.4 V
If you want to switch two corresponding light diodes in sequence, 2 x 2.6 V = 5.2 V must be subtracted from the
operating voltage of 12 V (12 V – 5.2 V = 6.8 V)
For three same diodes, this is 3 x 2.6 V = 7.8V to be subtracted from the operating voltage of 12 V (12 V - 7.8 V =
4.2 V).
Now use Ohm’s law R = U : I to calculate the dropping resistance Rv:
Rv = 9.4 V : 0.02 A = 470 Ohm
Use the formula P = U x I to also calculate the output the dropping resistor must be able to withstand:
9.4 V x 0.02 A = 0.188 W
A resistor with an output of ¼ W (0.25 W) would therefore be sufficient. The respective upper connection contacts of
the plugs “X3” (“L1” to “L4”) and “X4” (“L5” to “L8”) are intended for the LEDs or LED strip connection. The lower
connection contacts are connected to the plus connection (+) of the drive battery (11.1 V of the LiPo battery).
Connection pattern
Figure 22 shows the scheme for the connection of
a LED with dropping resistor at the output “L1” and
the connection of a 12 V light strip at the output
“L7”.
Important!
When connecting LEDs and LED light
strips, observe correct polarity of the
connection lines.
In practice, pre-manufactured BEC sockets with
open cable-ends have proven well suitable for LED
connection.
R2
T4
X9
IC2
T5
T2
T3
Q1
X4
X3
C3
C2
X10
C16
C1
1
C10
C9
C8
C13
C12
C15
C14
R23
R2
T4
T5
T2
T3
Q1
X10
C16
C1
1
C10
C9
C8
C13
C12
C15
C14
R23
Rv
LED
+
-
12 V
+
-
L4
L3
L2
L1
L8
L7
L5 L6
Figure 22
