7 segment led question on PDB

Redd Dawg

So I'm working on experiment 10,·"A DIGITAL CLOCK",·in StampWorks and am concerned about the current draw when it's running.· Can't find the forward votage and forward current ratings for the 7 segment LED's in the manual or Parallax site.· I know there is a 470 infront of·each segment, but not sure if that's enough?· Looking at other LED specs, it looks like the draw would be more than 25mA per digit on the common cathode when all segments are lit.· Should I worry, or go for it.· ·I know only one digit is on at a time, but...· As a newbie, just trying to be cautious and to learn a bit more.· Thanks for your help in advance.

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Zoot

Say you have a voltage drop of 2.1v across your LEDs. Everything OVER that is what is calculated against the resistance to see what real current draw is:

5v - 2.1v = 2.9v "left over" from forward voltage of LED
2.9v / 470ohms = .00617amps (ohm's law)

So that's a little over 6 ma.

Say the LEDs have a greater forward voltage:

5v - 3v = 2v
2v / 470ohms = .00425 amps (4.25ma)

Or say they have lower forward voltage, which could give you bigger current draw:

5v - 1.7v = 3.3v
3.3v / 470ohms = .00702 amps (7ma)

So across a range of possible foward voltages, you are certainly not going to overpower your Stamp even with 7 segments + DP lit (presuming you are using an outboard regulator).

Here is a handy calculator that lets you figure resistances and current draws for various LED configurations, forward voltages, etc:

ledcalc.com/

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When the going gets weird, the weird turn pro. -- HST

1uffakind.com/robots/povBitMapBuilder.php
1uffakind.com/robots/resistorLadder.php

Redd Dawg

And since the LED's are blue on the PDB, I can assume a higher forward voltage along the lines of the page you gave me?· Might just explain why they put the blue led's on the PDB.· Thanks much!!!

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Redd Dawg

But then the brain is working, but maybe not quite right. If I go with the middle case, .00425 mA, then times 7 for the digit would mean the cathode pin would see 29.75 mA for sink, which would be just a bit close to max???

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erco

30 mA is pushing it in the case of showing digit 8, all seven segments on, but since the display digits are rapidly multiplexed, it's essentially on a 25% duty cycle. That doesn't mean effectively 7.5 mA, but certainly lower than 30.

A Stamp pin is rated to continuously sink 25 mA, so in this case it all works out.

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·"If you build it, they will come."

Redd Dawg

Thanks Erco, I was wondering if the multiplexing would kind of give you a fudge factor. Love this forum. You guys rock!

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