Re: Resistor on LED necessary?
- From: whit3rd@xxxxxxxxx
- Date: 6 Dec 2006 12:50:06 -0800
realexander wrote:
Of course, everywhere I see a circuit with an LED, there's a current
limiting resistor. But I'm wondering if, in my case, it's necessary.
Your design can work according to (your) plan, or can fail.
To the extent that it is planned, it's your responsibility to see to
any necessary design decisions.
In the general case, a LED has a characteristic curve (the I versus V
curve that you see on a curve tracer, or more accurately a family
of such curves for different temperatures and for diodes from the
same manufactured batch). And your drive provisions (power supply
and transistors and resistors and such) have a characteristic curve,
too, called
the load line. Where these two curves cross, is the operating point
for the LED. If that operating point is too high, it can waste power
or overheat the LED. If it is too low, the lamp will be too dim.
So, the problem is to get the operating point in the 'good enough'
region
under all conditions of temperature, power supply voltage, and
semiconductor
characteristics that are expected for your design. Some LED
flashlights
just connect the LED to a battery; they work, but only because the
battery
has significant internal resistance (do you know the load line for your
battery?).
Better LED flashlights, operating close to the high end of acceptable
LED
current, benefit from resistors or more complex current controls.
You have just come to a complex question of design tradeoffs, requiring
lots of the data sheets for your components and significant thought.
Welcome to engineering.
.
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- From: realexander
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