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?
MCU: 18F2550
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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?
MCU: 18F2550
For a standard LED, use the series limiting resistor. If you're short on space, you
can find LEDs with built-in current limiting resistors.
If your program logic allows you to do, you can pulse it or pwm it at or below its operating voltage.
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I assume a 5v LED would have a built in resistor as standard LEDs are around 2v.
Just need to check the datasheet to make sure that the current flow of the LED is within the PIC limits (normally 25mA)
Even with a very short duty-cycle, you're still over-driving the PIC output pin, and the LEDQuote:
If your program logic allows you to do, you can pulse it or pwm it at or below its operating voltage.
during peak-pulse times. I for sure don't recommend you do this without the series resistor
when directly driving the LED with a PIC output pin.
With a transistor driving the LED, you may be able to get away with high peak-pulsed
current levels, for short durations, but I would never try this directly driving the LED from
the PIC output pin.
I wouldn't assume the LED had a built-in resistor. I would check the data sheet for the LEDQuote:
I assume a 5v LED would have a built in resistor as standard LEDs are around 2v.
first. If you have an LED with a forward voltage drop of 2V, and a MAX forward current of
10mA, then just subtract 2V from 5V for the working voltage, then /10mA to find the series
resistor value.
I.E. 5V - 2V = 3V / 10mA = 300-ohm series resistor you'll need to operate the LED at 5V
with 10mA current through the LED.
What if you need to operate this same LED with a 12V supply? Same thing applies. 12V -2V
= 10V. 10V / 10mA = 1K. So you just pop in a 1K resistor. Most LEDs don't care what the
voltage is, you just have to limit current through the LED at whatever voltage you're using.
Now, if you want to use PWM to dim or brighten the LED, your peak-pulse current levels will
not exceed the LED max DC current levels during the peak-pulse periods.
If you drive the LED with a 50-50 PWM duty cycle, your time average current will be
50% of 10mA, but your peak-pulse-current will still be the full 10mA.
If you exceed the MAX drive levels for the PIC or LED, even for brief periods, you can
pretty much count on failure at some point.
Seems pretty silly when you could have prevented total failure, or degredation over a
period of time with a 1-cent resistor...;o}
I have a design in which a PIC drives two seven-segment displays directly. No segment is ON more than 1mS in any 15mS period (allowing all 14 segments and the Decimal Point to be cycled across 15mS). I dispensed with the Common Anode Resistor in each display saving two Resistors on the Basis that having the LED ON for only 6% of the time was an acceptable risk (with the advantage of a brighter display) and the product was unlikely to fail within the warranty period. It wasn't the cost, but the labour in their insertion that was the consideration in a cheap $20 product. 25,000 sales across six years with zero returns - I can live with that.
Same technique used in this very versatile project; http://members.cox.net/berniekm/super.htmlQuote:
Originally Posted by Melanie
I have built this, and been using it for a couple of years with no problems either to the pic, or the display.
Regards,
Anand Dhuru
wouldnt having the resistor on the common anode cause you to get different brightness for different digits?Quote:
I dispensed with the Common Anode Resistor in each display saving two Resistors
ie 8 would be dimmer than 1 as the current would be split between 7 segments to display 8, and only 2 segments to display 1.
i normally put the resistors in series with each segment, and use a transistor on the common pin
In theory, yes, but in practice I find no discernible difference displaying a '1' or an '8'. For the purists, the URL I gave for the Superprobe drives *each* segment at a time.
Anand
No, you're not thinking this 7-segment thing through...
Only ONE segment is ON at any instant in time. Assign a letter to each segment, 'a' thru 'g'. Each of those segments is allowed to be ON only for 1mS in any period. Now my period time was 15mS, but let's say for examples sake you have a 7mS Period. The first segment ('a') will illuminate only for the first 1mS of any period. The second segment ('b') will illuminate only during the second mS of the 7mS period. The third segment ('c') is only allowed to illuminate during the third mS of the period... and so on. So each segment gets it's own single mS all to it's-self and is the ONLY segment that is switched ON during that particular mS.
So in a 7mS cycle divided into 1mS segment periods, a number one (which might have segments 'a' and 'b' illuminated) would have the cycle...
OnA-OnB-OffC-OffD-OffE-OffF-OffG
So in that 7mS cycle, only ONE segment is illuminated at any time (and then for only 1mS).
If you want a Resistor, then a single Common Anode (or Common Kathode) Resistor is fine. It only ever has to cope with ONE segment being illuminated and the brightness is constant.
Your eye will not distinguish the flicker and the display will appear constantly (and consistantly) illuminted regardless of the number of segments displayed - but if you film the display with a video camera, you will then notice the flicker because the refersh rate of the camera will not the the same as that of your display and you will observe a strobe effect.
There is a SECOND AND VERY BIG ADVANTAGE to this technique... if you want to display '1' or '8888' the current consumption will be the same (that for ONE segment - say 10mA). Go display '8888' on a quad 7-segment display using the usual amateurish sloppy techniques of switching all the segments ON that you want at the same time and tell me how much current you pull... (7 segments, multipled by the number of digits, multiplied by say 10mA for each segment...). 7 x 4 x 10mA = 280mA... I've had so many laughs at some of the schematics and coding that I've seen for 7-segment displays...
I get it now! completly missed the point that only 1 seg is lit.Quote:
Only ONE segment is ON at any instant in time
I must admit ive just built a project that uses 4 x 7seg displays, and have wired in a way that only 1 display is lit at a time for 1ms, but have all the required segments on that 1 display light at the same time. So my circuit drawing around 50mA when measured with a meter.Quote:
Go display '8888' on a quad 7-segment display using the usual amateurish sloppy techniques of switching all the segments ON that you want at the same time and tell me how much current you pull... (7 segments, multipled by the number of digits, multiplied by say 10mA for each segment...). 7 x 4 x 10mA = 280mA... I've had so many laughs at some of the schematics and coding that I've seen for 7-segment displays...
Now ive seen the light!
I am trying to understand this technique as a coder using "amateurish sloppy techniques of switching all the segments ON".Quote:
Originally Posted by Melanie
Suppose I have 10 digits and will show 8 on all of them.
I have to stop at each common pin and scan through all seven segments with a total of 7mS, and jump to the next common pin do it again ...and so on...
I get a total of 70mS at the end.
So displaying 8 on all 10 digits will take me 70ms to complete?
Do I get it right?
I wanna be a pro !
:)
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I've not tried beyond four digits and a decimal point (ie a typical Clock)... but altogether that accounts for 29 segments at 1mS each. Now I have a personal rule that I must accomplish everything within 30mS otherwise I'm running the risk of introducing a visible flicker on the display and that just about fits inside the specification. For that you need 12 PIC pins (1 pin for each of the 7-segments, 1 for the Decimal Point, and 4 for the Common Anodes). Advantge: Current consumption 10mA. Worst case conventional senario say displaying "00:00" time = 25 segments at 10mA each = 250mA. Your choice - 10mA or 250mA?
But in your case with 10 digits I'd need a PIC with more pins as I would consider driving the display as three independent sets of digits (two sets of three and one set of four). For that I would need to devote 34 pins from my PIC, but I'd still be able to scan all potential 80 segments (the Decimal Point also being considered a segment) giving each 1mS and all still within 30mS. Advantage: Current Consumption 30mA (maximum of 3 simultaneous segments at 10mA each). Worst case scenario displaying "888888888.8" = 71 segments at 10mA each = 710mA. Your choice 30mA or 710mA?
OK, you can chose to run 80 segments as one display devoting say 300uS to each rather than 1mS (and needing only 18 pic pins), and to compensate for the lower brightness you can chose a more expensive high-intensity LED. 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.
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.
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Originally Posted by Melanie;
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Originally Posted by Melanie;
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Originally Posted by Melanie;
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 :).Quote:
Originally Posted by Melanie;
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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...
And who walked on the moon???Quote:
Originally Posted by Melanie
And I will respectfully add, do not start with the American bashing. There may come a time when you need our help again.
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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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...
I am sorry to have been so sensitive. Or Patriotic :)
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?Quote:
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?
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...;}
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...
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
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.
<br>
I think my question is answered now.
Thanx!
Hi Melanie,Quote:
Originally Posted by Melanie
I think your calculations are a bit off. The multiplexed 4-digit display you mention above would only have one digit lighted at a time and so if you were driving each segment at 10-ma while displaying "8888" you would only be drawing 70-ma total current (70-ma while displaying digit 1, 70-ma while displaying digit 2, and so on) and not the 280-ma you mentioned.
You seem to be ignoring "duty cycle" in your explanations. While the "one-at-a-time" method you mention would provide 10-ma total current draw the average current per LED in a 4-digit (28-segment) display would be only 1/28th of 10-ma or approximately 0.357-ma. By comparison, driving each segment of a multiplexed display at 10-ma at a 1/4th duty cycle would provide 2.5-ma average current per LED. Now as you've pointed out, painting "8888" on a 4-digit multiplexed display would cost 70-ma compared to 10-ma with the "one-at-a-time" method but you forgot to mention that the average current per LED is seven times higher which translates into a much brighter display.
Suggesting that a "one-segment-at-a-time" method or design is better than a multiplexed design based solely on the total current used is silly but it did provide me and several associates with a few giggles.
Kind regards, Mike
I never argue in favour of anything that I haven't tried first Mike... the 10mA was an arbitrary figure, taking a typical LED segment with a current limiting Resistor (a lot of folks drive their LED's at a lot more than that), and I used that 10mA figure to show what would be the typical current draw when you have multiple segments illuminated simultaneously. Without a current limiting Resistor the LED is much brighter, with the actual brightness being dependent on how much time you devote to having the actual segment illuminated and the specifications of your chosen LED. I find that the time slice I devote to having my chosen Kingbright LED illuminated in my design, gives me an adequate brightness per segment for daylight viewing (about 10mA give or take). If you give it less or more time or a different specification of LED, you will have different results. If you do a google search for a CAD-1DS for example, you'll see the technique applied to a real-life product. You may laugh and giggle, but I am doing so all the way to the Bank every day... and since the purpose of Resistors is basically to heat-up the planet, I have an additional spring in my step for giving the world another eco-friendly product...
Whilst the subject has been reopened, there was also a counter argument (which at the time I didn't bother addressing) that if the PIC hung (for any unexplained reason - and you've also deliberately turned WDT and BOD off in your code) with any given segment illuminated permanently and without a current limiting Resistor it would damage the PIC. If you typically supply the PIC through a 78L05 or similar low current PSU, the excessive current actually crow-bars the PSU, +VDD dives through the floor and the PIC simply restarts. This is like having all your connected LED segments having a dual role and additionally behaving like your personal WDT. Generally, if you are designing a circuit that only requires say 20mA worst-case, wouldn't you design your PSU for not much more than your worst-case scenario rather than waste your money with anything larger?
Actually, if you want to give it a go, I have published on this forum an 'Electronic Dice' (shortly to be available as a kit of parts for those who may be interested) using the same kind of technique and using a 10F series PIC. Have fun.
I didn't mean to suggest that there was anything wrong with your "one-segment-at-a-time" design Melanie. You designed the product for a particular current budget and brightness level and that's fine. I simply wanted to mention that your statements might mislead other Forum members when you ignore "duty cycle" and its effect on average current and brightness in your statements.
I've tried this method and it works great but performance drops considerably as you add more LEDs and decrease duty cycle.
Kind regards, Mike
Yes, indeed, there is a 'cut-off point' beyond which you need to approach the problem with some additional solutions (some being discussed above when a hypothetical 10-digit display was mentioned).
If it's truly a 5V LED it will have a current limiting resistor built in like this one...Quote:
Originally Posted by Fredrick
Now this is getting interesting :DQuote:
Originally Posted by dhouston
mean while, back at the ranch...
What interests me more is WHO ON EARTH is willing to pay $1.25 for a single LED...
Show me these guilty people (so I know who I can deal with)...
Well, that was just a quick example I googled up. You can find them for less.Quote:
Originally Posted by Melanie
And the OP didn't ask whether they were cost efficient but only whether they needed an external resistor.
- http://www.mouser.com/Optoelectronic...5v+led&FS=True
http://www.maplin.co.uk/Module.aspx?...&ForceUpdate=Y
They are really handy for breadboarding - as long as you don't mix them with standard LEDs.