Lou Schneider
Site Team
- Joined
- Mar 14, 2005
- Posts
- 13,269
I think you need to check your math, John.
Most white LEDs have a forward voltage drop of around 3 to 3.5 volts.
Voltage dropped across a resistor is wasted power - it dissapates as heat, not light.
Using a theoretical 0.2 volt LED on a 12 volt source, 1.6% of the power will go to the LED. The remaining 97.6% of the power will be wasted heating up the series resistor.
Using a more practical 3 volt LED, 25% of the available power goes to the LED. 75% disappears as heat.
To make a reasonably efficient light, you want as little voltage (and power) lost across the resistor as is possible. The way to do this is to stack LEDs in series until their total voltage drop approaches the source voltage, then insert enough series resistance to create the proper amount of current through the LEDs.
The problem with this approach is since there is relatively little voltage across the series resistor, a small change in source voltage will cause a dramatic change in current through the LEDs. Which is why you need a constant current source (essentially a self-varying resistance) instead of a fixed resistor to limit the current.
The LED itself drops around 0.2 volts when glowing,
Most white LEDs have a forward voltage drop of around 3 to 3.5 volts.
So you may consider the entire 13.6 to 16 volts as dropped across the resistor
Voltage dropped across a resistor is wasted power - it dissapates as heat, not light.
Using a theoretical 0.2 volt LED on a 12 volt source, 1.6% of the power will go to the LED. The remaining 97.6% of the power will be wasted heating up the series resistor.
Using a more practical 3 volt LED, 25% of the available power goes to the LED. 75% disappears as heat.
To make a reasonably efficient light, you want as little voltage (and power) lost across the resistor as is possible. The way to do this is to stack LEDs in series until their total voltage drop approaches the source voltage, then insert enough series resistance to create the proper amount of current through the LEDs.
The problem with this approach is since there is relatively little voltage across the series resistor, a small change in source voltage will cause a dramatic change in current through the LEDs. Which is why you need a constant current source (essentially a self-varying resistance) instead of a fixed resistor to limit the current.