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

[Mike, K8LH]

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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...

Hi 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

[Melanie]

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.

[Mike, K8LH]

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

[Melanie]

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).