tams
elektronik
LC modules
English
Examples for serial connection of LEDs:
Nominal voltage
transformer (~)*
1
Operating
voltage *
1
appr.
Forward voltage
of the LED*
2
Max. number of
LEDs*
3
per output
12 V
17 V
2 V
7
4 V
3
16 V
22 V
2 V
10
4 V
4
18 V
25 V
2 V
11
4 V
5
*
1
Nominal voltage and operating voltage: The operating voltage with a.c. transformers is
approx. 1,4 times the nominal voltage given on the transformer. With d.c. power packs the
operating voltage corresponds to the given nominal voltage.
*2
The forward voltage of white and blue LEDs is approx. 4 V, differing coloured LEDs approx. 2 V.
*
3
Tolerance and / or voltage fluctuations in practice often cause the calculated operating
voltage not being available. For that reason it is recommended to connect one LED less than
possible in theory.
Calculate the necessary series resistor as follows:
nec. R
V
[Ohm] = ( U
B
[V] – U
F
[V] x n
LED
) / (I
F
[mA] x 0,001)
U
B
= operating voltage| U
F
= forward voltage of the LED
n
LED
= number of LEDs | I
F
= current with max. luminance
Example: Connection of 2 blue and 3 red LEDs with using a 18 V a.c.
transformer. Designated current: 20 mA:
nec. R
V
= (25 V – 4 V x 2 – 2 V x 3) / (20 mA x 0,001) = 550 Ohm
We choose the next resistor´s value which is provided as a standard,
which means a resistor of 560 Ohm. If using a resistor with a higher
value the current of the LEDs would be reduced and the LEDs would
light less brightly.
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