12V DC & 24V DC LED lightsLED Lights that can work off a Standard Car battery is an attractive proposition. We have created a few designs on 12V as well as 24V DC.
It is important that LEDs are driven with a constant current. An LED is a current driven device. That is it will light up to its given intensity when the rated current flows through it. Its colour rendition will also be specified at that current.
Anything above or below the rated current two things will happen.
- The intensity will change, as will be the efficacy i.e. lumens/watt
- The Colour temperature will shift and you may not get the white light that you expect.
Once you ensure a constant current (we will see how we can achieve that later), we can then measure the voltage across the terminals of the LED. This voltage should be in a range specified by the manufacturer. For White LEDs it is usually in the range of 2.9 to 3.5 Volts. Whatever the current you deliver this voltage will remain more or less close to these values.
That is precisely the reason that you should not directly apply a voltage source to an LED. Let us assume your LED voltage (Called the forward voltage) is 3,2 Volts, and you apply say 13.2 Volts (a normal lead acid battery voltage) to it. What will happen?
As noted the voltage across the LED terminals cannot increase significantly. Let us assume it remains at 3.2 volts.
Now 13.2 - 3.2 =10.5 is the voltage that should be dropped somewhere else in the loop.
This is what will be dropped by the internal resistance of the battery. Since the value of such a resistance is very small the resultant current which is 10.2/R will be huge. This will burn out the LED.
Of course, this is an extreme case. However, in reality for even a 4 volts put directly across an LED, excess current will flow through the LED and shorten its life.
Hence: The requirement of a constant current source for the LEDs
Below is a simple circuit which achieves this in a DC circuit.
And here is the board if you want to fabricate this PCB