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

[sayzer]

...using the usual amateurish sloppy techniques of switching all the segments ON...

...There's lots of work-arounds, but I've just told you the way I would go - doesn't mean to say it's path you would take.

......I've had so many laughs at some of the schematics and coding that I've seen for 7-segment displays... .

......You're the engineer - which way would you chose? The answer (for most professional engineers) is probably revealed in their pay packet at the end of the month.

So it seems that this would not always be the most prefered way of doing it. This techniue is case sensitive with many factors in it; and the way people make their choices should be "amateurish" for those with personal rules .

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[Melanie]

Oooohhhh... I've touched a raw nerve here...

If you read my replies you will understand there is more than one way of doing anything - AND that INCLUDES multiple ways of scanning a segment at a time rather than switching all the segments on simultaneously which (and I will repeat it again) is sloppy and amateurish for the reasons I have already stated - namely massive power consumption and the almost certain need for additional circuitry to either latch the display on, or to handle the excessive driving power requirements. They haven't made a PIC yet to handle 710mA worth of LED's - and that's me being conservative and driving them at 10mA and not at 25mA! Why have a 6VA PSU when you can get away with 1.5VA? Why have a 7805 and great lump of metal for a Heatsink when you can get away with a 10 cent 78L05?

My 'personal rule' for scanning/refreshing a display within 30mS is based on good scientific principles formed from the reaction and persistance times of the Rods and Cones in your Retina (Human Physiology and Performance not covered in your University Engineering Course?). If you think you can scan an LED in a slower time and still get away without flicker - go for it - you might end up with a patentable idea and make yourself a great deal of money.

You know there is no 'correct' way of doing anything, but there sure are crazy ways of doing things which may still achieve the same goal. If you and a competitor both chose to make a 10-digit Frequency Counter, and your competitor came up with a design that was 25% cheaper to manufacture, used 50% fewer/smaller parts and tumpeted the fact that his used 80% less engery when working than yours - you'd be pretty upset about it!

It's like the old engineering joke, but based on fact... the Americans spent a million Dollars developing a pen that could write upside-down and in zero gravity. The Russians used a Pencil...

Allow me to laugh at the crazy designs/ideas...

[mackrackit]

It's like the old engineering joke, but based on fact... the Americans spent a million Dollars developing a pen that could write upside-down and in zero gravity. The Russians used a Pencil...

And who walked on the moon???

And I will respectfully add, do not start with the American bashing. There may come a time when you need our help again.

[sayzer]

Melanie,

I am well aware that I am an amateur, and also I know that there is a lot for me to learn from professionals like you in whom I know there is a lot to give and share willingly and greatfully.

I respect and admire the knowledge you have and some others on this forum have .

So there is nothing personal here,no offense either.

Given my thoughts, I can now learn more from you.

Your technique may seem better in some respect. But in a case with 10 digits more i/o needed. Thus, a more expensive pic or an addition of an i/o extender is in need. This way, creating more expensive and also more complex design, conflicts with your idea.

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[Melanie]

I didn't bash the Americans at all... I'm just siting a single example of crazy engineering (the British have the Billion Dollar Millenium Dome as an example of crass stupidity) - mind you Pentel Corporation did very well out if it...

Don't be offended Guys... a lot of this is "tongue-in-cheek" but based on facts too...

[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...