Can I drive a 5v LED without a current resistor?

[mackrackit]

I am sorry to have been so sensitive. Or Patriotic

[Bruce]

Can I drive a 5v LED direct from an I/O pin (without a current resistor) or do I have to have a current resistor to avoid problems?

Can't we simply answer the original question without all this I'm so much smarter than YOU are crap Or -I've sold ten thousand units breaking all the BASIC electronics rules, and got away with it thus far?

Was the original question asking about how smart you are, or asking if he could drive a single LED without a damn resistor?

I'm really glad that you managed to sell a TON of el-cheapo units without current limiting resistors. But the 100K that we have sold (with current limiting resistors) will still be ticking along even when the power-glitch causes a processor to HANG and hold that single LED ON until the user resets power...;o}

With our product, the user simply resets power, and it's all good. With your design, the user's forced to buy another unit. It's toast if it hangs when directly driving an LED. The customer that's buying your product doesn't give a damn how smart you think you are. They just want a reliable product that works well beyond whatever you state the warranty period will be, and that's exactly what we aim at. Our stuff works well beyond the warranty period.

We prefer to design-in protection against all failure options, and provide our customers with the little bit extra for a product that simply works

Of course, it would be in our best interest to sell them a new unit every six months or so, but, as a consumer myself, I think that approach sucks. I would rather sell our customers a unit that works forever. And if this means including an extra resistor or two, then we're totaly OK with that.

If it's got a warranty period of 2 months, we'll expect it to last 12 months. And, we would never consider calling anyone stupid, or laughing about anything someone had done in the past.

I think it's just plain arrogant of any real engineer to state anything like this. How do YOU or I know what level of learning anyone here has had?

And, if you're here to teach, then DO SO. Without the arrogant garbage.. I'm sure that all who read your postings here will be sure to recognise your importance...;}

[Melanie]

That's a little strong Bruce...

The question was can you run an LED without a current limiting Resistor. I answered YES you can with the proviso that you pulse the LED.

There was no rider to the question that stated "the circuit had to be failsafe".

I don't expect my designs to hang - is that arrogant of me? Maybe - you decide.

The thread then digressed into multi-segment displays. And the discssion was whether you run all simultaneously or pulse them indvidually. There was no discussion whether you did or didn't have current limiting Resistors at that point, but the argument was with the overal concept and the massive advantage of current consumption and reduced parts count.

If you rather I didn't express an opinion on this forum, then you only have to ask.. I'm more than happy to let you get on with it...

[Ioannis]

Hmm, I think its the first time that I feel Bruce a litle angry.

Anyway, I agree that pulsing a LED within its safe working area limits can be OK without resistor. Be careful of the Vf voltage. Usually on LEDs Vf is smaller than the one you may be used with other diodes.

About Melanies trick, it is of course doable 100%, with the assumption that LEDs are High Brightness to compensate for the Duty Cycle. Driving them for only a short time (1/30) at 10mA will reduce their brightness conciderably.

Ioannis

[Darrel Taylor]

I think there are two aspects to this question.

1. How much current can the LED handle.
2. How much current can the PIC deliver.

If you take 2 identical LEDs and place one from a PIC pin to ground without a resistor, and the other directly across VDD/VSS and turn them both on ...

The one directly across VDD/VSS will glow very brightly, often with a different color than intended, as it heats up and burns out within about 20-30 seconds.
But the one that's connected to the PIC will shine brightly and continuously without burning out at all. It's lifetime will be diminished by some factor no doubt, but then I'm talking continuous ON current. With a short pulse at a small dutycycle, there's no way it can burn out, ever.

The difference is in the PIC output, which when presented with an over current condition, simply drops it's output voltage.
The PIC's temperature will rise slightly after a LONG time, but it's nothing like the LED across VDD/VDD which gets very hot.

For instance, an amber LED with a normal forward voltage of around 2.1V will actually drop the PICs output voltage to 2.1V, but it will only be delivering ~29mA.

29mA is not that far off from the 25mA max, so you really have a hard time getting "too much" current from the PIC's pin because the more you draw, the lower the voltage drops. It's almost like a constant current driver. 30ma +/- depending on the LED color.
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[Fredrick]

I think my question is answered now.
Thanx!