Author Topic: Approximately how long will an LED last with no resistor  (Read 2526 times)

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Offline airman00Topic starter

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Approximately how long will an LED last with no resistor
« on: February 02, 2009, 09:01:10 PM »
I soldered up a circuit and only now realized that I neglected to put a resistor between my ATmega168's output pin and the LED's power pin.
Approximately how long will the LED last without a resistor in between ?

I am using a 3.2V drop LED with 25 mA forward current.

Please do not reply that I should fix the circuit - I only need the circuit to run for about 50 hours in total use and if its not absolutely necessary for me to add a resistor, I won't go through the hassle.
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Offline Webbot

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Re: Approximately how long will an LED last with no resistor
« Reply #1 on: February 02, 2009, 09:16:03 PM »
Wow!
If its on a PWM pin thats on a duty cycle of 3.3v/5v ie 66% or less then you could be ok.

But if its on an I/O pin, or on the power supply, and so is always fed by 5v - in which case I'd give it about one-third of a second before it goes to meet its maker. And the surge MAY blow your 168.
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Offline airman00Topic starter

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Re: Approximately how long will an LED last with no resistor
« Reply #2 on: February 02, 2009, 09:27:03 PM »
The LED is on a PWM pin so thats an option.
One thing I dont understand is the 3.3V/5V you said. PWM means that its at 5V 66% of the time and its at 0V for the rest of the time. So I'm not really supplying the LED with 3.3V , rather I am supplying it with 5V at a not-constant rate. If the LED gets 5V for 66% of the time, will it still burn out in under 50 hours.

The main goal here is to see if I could possibly run an LED for 50 hours or more before it burning out.

One thing to note: I've connected LEDs to straight 5V before without a resistor ( by accident) and they have never burned out in a second nor have they killed the microcontroller. I've never left an LED on for an extended amount of time so I don't know how that will go.
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Offline Webbot

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Re: Approximately how long will an LED last with no resistor
« Reply #3 on: February 02, 2009, 09:51:03 PM »
The LED is on a PWM pin so thats an option.
One thing I dont understand is the 3.3V/5V you said. PWM means that its at 5V 66% of the time and its at 0V for the rest of the time. So I'm not really supplying the LED with 3.3V , rather I am supplying it with 5V at a not-constant rate. If the LED gets 5V for 66% of the time, will it still burn out in under 50 hours.

It depends really on the PWM frequency. To think of extremes: if your PWM cycle was every 24 hours and a 50% duty cycle it would have 5v for 12 hours and 0v for 12 hours. In which case the average over 24 hours is 2.5v but with peaks of 5v for 12 hours. If the frequency is milliseconds ( or faster than the load acts ) then it becomes a D/A converter. So if you pulse 5v at high frequency with a 66% duty cycle then, as far as the load is concerned, it becomes equivalent to 66% of 5v = 3.3v. Of course the peak is still 5v, but only for a very short period, so as long as the LED can cope with 5v for this short period then you are ok.

Quote
One thing to note: I've connected LEDs to straight 5V before without a resistor ( by accident) and they have never burned out in a second nor have they killed the microcontroller. I've never left an LED on for an extended amount of time so I don't know how that will go.
I always tend to check the polarity of LEDs before I solder them and I tend to do it with a 9v batt and a resistor. All balanced in one hand!! There are times when I've accidentally put the 9v across the small red LED without a resistor and, believe me, they blow very fast. You're using 5v and maybe an LED with a greater forward voltage. I know you don't want a 're-design' answer but I'm afraid that is the only successful answer. I may be able to drive my ATMega168 at a slightly higher voltage: will it survive - maybe. For how long: hopefully long enough for me to unplug stuff. Would I drive it that way: no.
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Offline Soeren

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Re: Approximately how long will an LED last with no resistor
« Reply #4 on: February 03, 2009, 05:04:31 PM »
PIC µ-controller I/O is current limited. I haven't studied that end of AVR's lately, but I'd imagine that it's what saved your LEDs.
However, be smart and don't strain you controller (some breakage will show later than when they took the hit).
Never mind the LED, it's cheap enough to hurt a few in the name of getting experience, but for anything you need to be reliable, use sensible design - LEDs are current controlled device (emphasis on controlled) and how hard is it to add a resistor?
Even if you forgot initially, cut a trace and add a 0805 or smaller, practically invisible (well, sort of) :)
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