LEDs - general information

[Digsy]

***MODS *** Please feel to move to general if this is in the wrong section.

 

 

 

 

There seems to have been a lot of questions regarding wiring up LEDs recently, so I thought I'd type this up.

 

The LED is a semiconductor which emits visible light when a forward current is passed through it. Like a normal diode, an LED will also block current in the reverse direction (it will also emit no light when connected in this way).

 

The light from an LED is created directly as radiation from the semiconductor chip, to there is no need to have a coloured lens over the LED to make, say, green light. Although LEDs with coloured lenses are widely available, many have water-clear lenses which illuminate the proper colour.

 

As an aside, IIRC the exception to this rule are blue LEDs, which are actually green LEDs with a very small yellow filter over the chip, but I may be wrong.

 

LEDs run cold unless the maximum constant forward current (If) specifcation is exceeded, in which case they will quickly burn out. For this reason, an external current limiting resistor is required, the value of which is calculated as below:

 

R = ( Vs - Vf ) / If

 

Where:

Vs = supply voltage

Vf = forward voltage drop across the LED (from LED specs)

If = LED forward current (from LED specs)

 

The equation is written for one LED. If you want to wire up several LEDs into a series array, then the equation is re-written as:

 

R = (Vs - ( n x Vf) ) ) / If

 

Where n = the number of LEDs in the array. It is important that all the LEDs in the array have the same Vf and If values.

 

Two interesting things to note:

1) If the sum of the Vf = Vs then you do not need a current limiting resistor.

2) If you try to wire up so many LEDs that the sum of Vf > Vs then the LEDs will only light dimly.

3) The current load is the same (If) no matter how many LEDs are in the series array.

 

So how do you wire up, say, 15 LEDs with Vf = 2V from a 12V supply? This can be done by wiring up several sets of LEDs in series so that ( n x Vf )

 

In this arrangement, each series set will draw If amps, so the more series sets you wire together, the heavier the load will be.

 

One final thing to check for is the power in the resistor, which can be found thus:

 

P = ( Vs - ( n x Vf ) ) x If

 

Although in practise If is usually so small (0.03A) that you are unlikely to exceed the power limitation of the resistor (0.25 - 0.6W).

 

Some LEDs already come with an internal current limiting resistor which will make them ready to wire directly to, say, a 12V supply. This makes them easy to use but also restricts them to one supply voltage, and means that if you want to run several together they must be connected in parallel rather than in series, which in turn means they will draw more current than a similar array using several series sets.

 

Hope this is of use to someone

[lightwave]

Yes. Thanks!

 

I have some LEDs I'm going to wire up to beam down onto my central dash area - I now know to wire them up in parallel.