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View Full Version : Uhm, so about those "Absolute Maximum Ratings" stated in "Electrical Characteri



Nak
12-16-2009, 05:53 PM
I was thumbing through the Dec. issue of "Nuts and Volts" this morning and found an interesting article on how to drive multiple 7-segment LED displays from some random microcontroller. The part that caught my eye was the circuit the author used in the article, and how the common cathode pin of each display was switched through a transistor rather than being connected directly to a pin on the microcontroller. Sure enough, he explains that the transistors are necessary b/c the cc pin carries the combined amperage of all the lit segments of the display, and that can exceed the amperage allowed on a microcontroller I/O pin.

So... I'm reading this in the electronics store while across town the Propeller chip in my dev board is driving a cute little animation across the six *sixteen*-segment LED displays included with the board... And I'm thinking to myself, "I wonder if the Propeller has summa them there maximum current limitations?..."

Well, it does. And so now I calculate that I was exceeding them pretty handily. http://forums.parallax.com/images/smilies/nono.gif

And that little demo has been scrolling for about three days now. http://forums.parallax.com/images/smilies/sad.gif

So, if my Propeller still seems to be functioning properly, can I assume that thanks to robust and generous engineering on Chip's part I've done it no harm? Or is it likely to be damaged in some subtle way that I just haven't noticed yet?

Leon
12-16-2009, 07:39 PM
It's OK if you are just doing something for your own amusement as a hobbyist, but you shouldn't exceed the maximum ratings on anything that is going into production. I'd keep well below them, in fact.

Leon

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Amateur radio callsign: G1HSM

Clock Loop
12-16-2009, 08:48 PM
Nak said...


So... I'm reading this in the electronics store while across town the Propeller chip in my dev board is driving a cute little animation across the six *sixteen*-segment LED displays included with the board... And I'm thinking to myself, "I wonder if the Propeller has summa them there maximum current limitations?..."

Well, it does. And so now I calculate that I was exceeding them pretty handily. http://forums.parallax.com/images/smilies/nono.gif



By the looks of the skeematic:
www.parallax.com/Portals/0/Downloads/docs/prod/schem/PPDB_A_Schematic.pdf (http://www.parallax.com/Portals/0/Downloads/docs/prod/schem/PPDB_A_Schematic.pdf)

The only way you can pull lots of current through the prop is by turning all segments on at once.

The only way to make an animation or make it so each display doesn't show the same thing, is by flashing each display in sequence.
Why would anyone just burn all 6 displays on with every segment on?

Perhaps to test, and yes, that might do damage.

But when you use it the way its supposed to be used, by flashing each display in sequence, at high speed then its proper.

I notice the 150ohm resistors for the segments also. This makes sense because when you pulse each display individually you need to do it very fast so you don't see flicker. So a smaller led resistor is used.

Definetly don't turn all segments on at once.

CassLan
12-16-2009, 09:30 PM
Nak said...
I was thumbing through the Dec. issue of "Nuts and Volts" this morning and found an interesting article on how to drive multiple 7-segment LED displays from some random microcontroller. The part that caught my eye was the circuit the author used in the article, and how the common cathode pin of each display was switched through a transistor rather than being connected directly to a pin on the microcontroller. Sure enough, he explains that the transistors are necessary b/c the cc pin carries the combined amperage of all the lit segments of the display, and that can exceed the amperage allowed on a microcontroller I/O pin.

So... I'm reading this in the electronics store while across town the Propeller chip in my dev board is driving a cute little animation across the six *sixteen*-segment LED displays included with the board... And I'm thinking to myself, "I wonder if the Propeller has summa them there maximum current limitations?..."
Thats a pretty nice story, I'm glad it has a happy ending http://forums.parallax.com/images/smilies/smile.gif

As Clock Loop said, the most common way to use these displays without anĚLED Driver IC is to only have one segment on at a time, pulsed faster than the human eye can see. It was a little tricky for me to do with a 5x7 LED Matrix, but I got it. I'm sure you will too.


Nak said...
So, if my Propeller still seems to be functioning properly, can I assume that thanks to robust and generous engineering on Chip's part I've done it no harm? Or is it likely to be damaged in some subtle way that I just haven't noticed yet?
IMO its likely just fine http://forums.parallax.com/images/smilies/smile.gif, but now that you know...the electronics gods won't let you get away with it again http://forums.parallax.com/images/smilies/lol.gif.

Happy Propelling!

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Nak
12-17-2009, 07:11 AM
Thanks for the insightful replies.

Clock Loop: I did indeed start out by driving all 17 LEDs in all 6 segments on, but happily I didn't leave it in that state long. (I have a driver IC on order but I just wanted to play with them / make sure they all worked.)

CassLan: I think I'll remember this now for some time to come. :)

I built a proper circuit last night using 4 NPN transistors (and leaving the last 2 segments off), and it's all still working fine... Knock on wood!