>> Jeff, Wouldn't locating the resistor somewhere else lower the heat load on
>> the
heat-sink?
>> Dan Schaefer
If heat is a problem, which I'm not sure it really is, there is
another option when using LEDS.
There are basically two factors that limit how much power you can pour into a LED:
-Average power dissipation
-Peak Current
Average power dissipation is the total power applied over time. In the case of a LED with a series resistor, a
continuous and steady current, Ohm's law and P=IE get's you the number.
The other option often used to power LEDs is a modulated waveform. This is fancy talk for "sometimes it's ON,
sometimes it's OFF." In practice, the on time is typically much shorter than the off time. Further, the ON
current/power CAN be significantly greater than the steady state current/power as long as our two favorite parameters,
average power dissipation and peak current, are not exceeded. For a SIMPLE example, take a LED that can sustain an
average ON current of 10ma. One could apply current for a 1:4 duty cycle (ON for 1 period and OFF for 4
periods, 5 periods total) and apply 50ma during the ON period and achieve the same average power dissipation. That said,
the other "rule" Peak Current can not be exceeded. If the Peak Current is > 50ma this would work fine. In fact,
Peak Currents are often MUCH higher than the average power dissipation would lead one to belive.
Now why, might you ask, would one go to "all" this trouble? (It isn't really that much!) Well, several reasons:
- To make the LED seem brighter: Since the LED can be "Flashed" with MUCH greater power than the average AND they human eye
CAN'T tell it's actually flashing (retinal retention) the LED can SEEM to be producing more light output than it really is.
- To save power: If the LED doesn't need to be as bright, the duty cycle can be adjusted, within reason, to lower the
average power consumption thus lowering the brightness.
- To save/not waste power / generate heat: When a transistor is ON, its voltage drop is minimal. P=IE and if E is
minimal, so is P. When a transistor is OFF, I is minimal and again so is P. ONLY when a transistor is switching from
OFF to ON, ON to OFF, of if it remains "somewhere in the middle" does it's power dissipation become a major factor.
So, in this digital application, we switch from ON to OFF and back as FAST as possible to prevent the transistor from dissipating
power.
Using a transistor as a simple ON/OFF switch is an effective way to address controlling the power of LEDS. With this
approach there is no series element (resistor or transistor) that's dropping voltage/power and wasting energy in the form of
heat.