Re: Resistor on LED necessary?
- From: "Paul E. Schoen" <pstech@xxxxxxxxx>
- Date: Wed, 6 Dec 2006 19:09:40 -0500
"realexander" <realexander@xxxxxxxxxxxxx> wrote in message
news:1165372664.088851.134890@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Hi,
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.
I have an eight digit multiplexed display, using 14 segment LEDs. A PIC
selects the digit and turns on its common anode through an NPN
transistor (and I also don't have a resistor between the PIC and the
transistor's base). The transistor's collector is at +5 with no current
limiting resistor. The PIC selects the individual segments within a
digit by having its I/O lines tied to the cathodes and setting the
output to 0 to turn on the segment. There's no resistor on the cathodes
or the anodes.
Everything seems to work great; the LEDs light at a reasonable
brightness (though there's some variation in the brightness - I learned
too late why I should've bought common cathode LEDs instead of common
anode). If I put in current limiting resistors of 220 ohms, the LEDs
are dimmer than I'd like (remember that the display is multiplexed, so
each digit is off 7/8 of the time, or more if I use PWM to dim the LEDs
in a dark room).
Is this a "safe" configuration? Can I get away with no resistors since
each LED is off most of the time? What about the PIC - the LEDs might
be off most of the time, but some of the PIC's I/O lines are
potentially sinking current all the time. Am I going to eventually fry
my LEDs, PIC, or both?
Thanks,
Bob Alexander
PICs have some (most?) outputs designed to drive LED loads, and maximum
current is specified as 25 mA. The low output is typically about 0.5 V at
this current, and the high voltage is about 2.5 V, so if the LED segments
drop about 2 volts, you will not exceed the rating of the PIC or the LED.
You can drive the LED segments with up to about 100 mA in a multiplexed
display, so the average current is only 100/8 or 12 mA. Using resistors and
higher current driver transistors for the digit and segments will give you
a brighter and more consistent display. Without resistors the segment
current will vary depending on temperature of the PIC's MOSFET outputs as
well as the tempco of the LED itself, and also how many segments are on for
each digit. What you have is probably OK (safe) for a hobby application but
not for any product you want to produce.
Paul
.
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