[simpits-tech] LED Resistors

[Roy Coates]

On Fri, 4 Apr 2003, Joseph Fagner wrote:

> Now, let's all have a group hug and we can talk about which resistors one
> needs to power LED's.
>

ok, now file this away in the "useful info" folder.

Generally, I personally now use 5v or 12v LED's which have the required
resistor built-in. This makes for an easier and neater job. Anyway, here's
the info you need.

To operate an LED from a given voltage a resistor needs to be placed in
series with it. The correct resistor value in ohms is calculated using:

  R = (Vs - Vi) / If

Where:

  Vs is the supply voltage
  Vi is the forward voltage drop across the LED
  If is the forward current through the LED (in Amps)

The values of Vi and If should be specified by the LED manufacturer.

For a typical 20mA LED with a forward voltage of 2.5v operating from a 12v
supply,

   R = (12 - 2.5) / 0.02  =  475 ohms


If powering LED's from an AC supply, connect a diode (1N4148 or similar)
in inverse parallel with the LED, and halve the value of the resistor
calculated using the above formula.


Roy.