Re: Driving a led without a series resistor (PWM technique)

On Tue, 29 Apr 2008 23:52:38 -0700 (PDT), zigbee@xxxxxxxxx wrote:

I'm designing a board where the microcontroller is supposed to drive
10 red leds and 4 optocouplers.

I would like to drive the leds and the opto's without a series
resistor using the PWM technique. I made some experiments and all
seems to work fine in the lab.

There are two related issues here:
1) Total GPIO output current
2) current limiting and heat dissipation

Most CPUs have a maximum current per pin, and a maximum
total current per port and chip. Driving 14 LEDs total
is likely to exceed this.

Regardless of whether you are using PWM or not, driving
the LEDs without a resistor will cause the GPIO pin to
current limit, the voltage will then not be Vcc or Vss
and there will be increased heat dissipation in the CPU.
The limiting current is not well defined.

Resistors and simple transistors are cheap, and will
improve reliability.

Stephen

--
Stephen Pelc, stephenXXX@xxxxxxxxxxxx
MicroProcessor Engineering Ltd - More Real, Less Time
133 Hill Lane, Southampton SO15 5AF, England
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