tams elektronik FD-R Basic.3 Manual page 21

FD-R Basic.3
In order to determine the necessary series resistor for a serial LED´s
connection first add the forward voltages of all LEDs. The forward
voltages depend on the lighting colour and should be given in the
technical specifications. In case there is no manufacturer information
available, you can take as a basis 4 V for white and blue LEDs and 2 V
for yellow, orange, red and green LEDs.
The remaining voltage has to be "eliminated" by a resistor. The formula
for the calculation of the resistor is:
required R [Ohm] = ( U
V
U = operating voltage (peak value) | ∑ U
B
I = current with max. luminosity
F
Parallel connection of LEDs
Alternatively, you can connect several LEDs in parallel, each via a series
resistor of its own. The current consumption is max. 20 mA for all LEDs,
depending on the series resistor´s value. The maximum number of LEDs to
be connected in parallel results from
maximum current at the output
- sum of the current consumption of all LEDs
> 0
Advantageous with this solution is that the LEDs already lighten when
their forward voltage has been reached (2 to 4 V, depending on the
fluorescent colour), which makes this solution suitable for analogue
mode. Disadvantageous is the high current consumption.
The formula for the calculation of the resistor is:
required R [Ohm] = ( U
V
U = operating voltage (peak value) | U
B
I = current with max. luminosity
F
In order to save current, you can limit the LEDs´current consumption to
10 mA, which normally does not cause a visible loss of luminance.
[V] – ∑ U [V] ) / (I
B F
= sum of the forward voltages of all LEDs
F
[V] – U [V] ) / (I
B F
= forward voltage of the LED
F
[mA] x 0.001)
F
[mA] x 0.001)
F
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Page 21