The PRIZM Sound Board User's Manual
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In most cases a resistor is required on each die, especially with the rise of modern
LEDs with low forward voltage. In certain special cases, only a drive adjustment is
requested to limit the current in the LED.
High Power LED resistor calculations
There are many online resistor calculators but here are the calculations for both
resistor and wattage. They aren't difficult and it's important for the user to understand
and memorize them.
Stunt Upgrade
If the user is adding sound to an existing saber and is already using a resistor, it can
be used as it is. Beware though, this applies only if the battery solution is already
Nano Biscotte compliant (single cell, < 5.5v). If the saber was made with a 2 cell
pack, the resistor needs to be changed, see calculations below.
Resistor calculation
The Ohm's Law gives us R = U / I. R is the resistor we're looking for, U is the voltage
across the resistor (unknown for now) and I is the current at which you want to drive
the LED.
The Voltage across the resistor is simply the difference between the supply voltage
(the cell voltage) and the LED voltage (also called forward voltage or Vf):
R = (Vsupply – Vled) / MaxLedCurrent
The LED voltage is often mentioned in the datasheet or in the product page. The LED
current is chosen by the user, depending on the battery solution and the LED ratings.
For instance, even if a LED can take up to 1.5A, a regular 14500 cell will not be able
to deliver such a current due to its limited maximum discharge current. A 18650 cell
will be able to deliver such a current.
Let's take for instance a red LED with a 2.9V forward voltage, driven at 1.5A with a
18650 cell (3.7V)
R = (3.7 - 2.9) / 1.5 = 0.53 ohm (the closest resistor might be 0.56 ohm)
Wattage calculation
The Ohm's Law gives us P = U . I with again U being the voltage across the resistor
and I the current in the circuit, and we now know both of them.
P = (Vsupply – Vled) * LedCurrent
In the previous example:
