LED clock too dim

[prenavin]

I'm building a digital clock using LED's in a 7 segment pattern. Each segment has around 35 LEDs and the height of a complete display is around 12 inches. i'm using two displays each, for hours, minutes and seconds. A PIC 16f877 sets the number to be displayed and a 74HC238 demultiplexer selects each display. Only one display is on at any given time, to save power. Problem is the LEDs are not bright enough. I tried using transistors to increase the current but it doesn't help, because its switching between each display so fast. Any ideas???

 

[cbarn24050]

It's not a good idea to not use a resistor for current control. The leds will only be stable over a very small voltage range. If 1 should fail it will take out the rest in the line, as well as overloading the power supply.

 

[MacGyverS2000]

prenavin,

This is a VERY poor design practice... you need some manner of current control. The graphs shown on spec sheets are nominal values, not guaranteed... the graph may show a 20mA draw at 'X' voltage (assuming you can even read those graphs to any real degree of accuracy), but the chances of all of the LEDs matching those specs is beyond slim towards none.

Dan
Owner

http://www.Hi-TecDesigns.com

 

[IRstuff]

And don't forget that the current is proportional to e^Vd/kt

TTFN

 

[cbarn24050]

hey irstuff, how did you get that post that formula?

 

[IRstuff]

That's one of the TGML commands, using the brackets and sup and \sup

TTFN

 

[Myndex]

Prenavin - connecting your ammeter in series adds resistance, thus you really can't accept that as a remotely accurate reading. This is due to the burden resistor in the ammeter.

Either measure the voltage across your current limit resistor and use that to calculate the current, OR use a non intrusive (clamp on) ammeter.

B+K has a nice DC clamp on milliammeter.


Andy

 

[Myndex]

cbarn: When LEDs fail, they open - if all are in series, then you get zip. Using a resistor in series IS the proper design practice.

BAD design practice is using a single resistor to limit a plurality of LEDs when those LEDS are all in parallel. (one resistor for a bunch of parallel LEDs spells disaster as the variations in the LEDs will likely lead to one "running away" and frying, which will then open, causing the current to increase across the remaining and pow pow pow. Kinda like semiconductor dominoes....)


Macgyver: Yes indeed. - the rated specs are subject to the fluctuations inherent in all semiconductor technologies.

To improve reliability, derate by a factor of two - thus if the max average current is 20ma, design for 10.


As a matter of practicality though one can dispense with the current limiting resistor if one has the means for varying the duty cycle to limit current. What's ultimately important is keeping the junction temp within it's nominal limits.

I mention this as Prenavin's states that the LEDs are only on about 16% of the time. He could operate the LEDs at 60ma, and the average current would be only 10, yet appear plenty bright.





Andy

 

[prenavin]

Hi guys! I tried a new LED, its a L53LSRD made by KingBright. These babies are so cool, 8 mcd at only 2mA. They go all the way up to 30mA and the view angle is a whopping 60 degrees.
I was about to scrap the multiplexing idea and drive the LEDs on all the time, but the new LEDs have solved my problems. I hope they are cheap though, cause i need to buy over a thousand.
The old BL-B5134's that i was using could only handle 7mA max. at 5 mcd. That goes to show, DATASHEETS ARE VERY USFUL.
Back to my experiments !!!. I'll keep you posted on the latest developments.

 

[Myndex]

7ma? hmmm that's wimpy..


Check out these guys for CHEAP superbright LEDs:

http://www.hebeiltd.com.cn/?p=led.diode

Try 1800 mcd at 60 degrees.

less than 5 cents each in QTY of 1000.

http://www.hebeiltd.com.cn/?p=z.pricelist.led.diode

Andy Somers

 

[mstock]

Myndex: LEDs can fail high forward voltage or short (I have a few HPLMP4700's in my drawer with this failure mode). LEDs are not guarenteed to fail open circuit. For refrence, perhaps take a look at Luxeon LED failure mode RD25.pdf
We have used current control for our LED signage fault regulation (but it wasn't cheap!)

 

[Myndex]

mstock: Interesting - I've never had one fail like that, and I've been working with LEDs awhile (I design LED light up toys). Can you post a link for the PDFF file? I'd like to read it.


Cheers


Andy

 

[Myndex]

Nevermind I found it:

http://www.lumileds.com/pdfs/RD25.PDF

That's a great document, thanks for the insight.


ANdy