LED clock too dim
LED clock too dim
(OP)
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???





RE: LED clock too dim
RE: LED clock too dim
Multiplexing and scanning the digits can reduce wiring, but it does not save power because the average current sets the brightness.
RE: LED clock too dim
In other words, if you are running the LEDs such that they are only on 10% of the time (10% duty cycle) you should be able to run them at a much higher current. While the rules here have certain variations, with a 10% duty cycle you *should* be able to operate the LEDs at 100 ma with no problems.
NOTE: Your mileage may vary
Read this:
Quoted from:
From: http://www.gilway.com/html/appl-ledlamps.htm
Andy
RE: LED clock too dim
RE: LED clock too dim
What about super bright LEDs?
Tofflemire
RE: LED clock too dim
TTFN
RE: LED clock too dim
A 25% duty cycle at 100Hz should appear as bright as 25% at 1000 Hz.
On to the issue at hand, looking more closely at the application as stated at the beginning of the thread:
If I understand, you have a total of 6 digits, and each digit is 7 segments, and each segment is composed of 35 LEDs, which we'll assume are common LEDs with a 1.7 v drop each.
VE1BLL makes two very good points. One is that for a given brightness, you're not going to save power by multiplexing. If you multiplex you will have to increase the current to each segment to get the same brightness. And running the 35 of each segment in series would be the best design for a host of reasons.
Let's talk about duty cycle again. With 6 digits, and at least one ON and the other 5 OFF at all times, and when on each one is on for the same period, then you have a duty cycle of 16.67%. So if you are happy with the brightness of an LED at 10ma constant current, in this configuration, multiply that by 6 and operate the LEDS at 60ma. Since they are only on for 1/6th of the time, they will still only consume an average of 10ma.
Second, in running the 35 leds of each segment in series, you lower your parts count considerably (when running LEDs in parallel you should use a separate limiting resistor for each).
Now, with 35 in series, and assuming a 1.7v drop per LED then you have a total drop of 59.5 volts. Your voltage into the LEDs needs to be at least 59.5 volts, but will need to be much more if we are to get the 60ma per LED we want for good brightness.
35 LEDs running at 60ma pulse is 10ma average, accounting for the 16.67% duty cycle. If all 7 segments are lit, that's a total peak of 420ma, or 70ma average. With 6 digits, and all segments lit (assuming that each digit is on one at a time, with a full 16.67% duty cycle, as you indicated) then your max average current draw is 420ma for the display. (It's useful to note that if you have all digits and segments on (with a 100% duty cycle) but set the current for each segment at 10ma (instead of 60ma), you'd still only be drawing 420ma total). You could vary the pulse WIDTH (i.e. shorten the duty cycle) to eliminate the need for a series current limiting resistor keeping the average current in bounds for the LEDs you're using, depending on the voltage you'll be driving them at.
Regards,
Andy
RE: LED clock too dim
Forget 35 LEDs x 7 segments x 6 displays = 1470, I just want to get ONE LED to work correctly for now. I powered up the new circuit with the BD's, no difference. It seems like these transistors don't make a difference. I'm using a 7805 regulator (1A type) for power. 1A should be enough to drive ONE multiplexed LED.
Could it be a software problem?? Highly unlikely, cause my normal 0.5 inch 7 segments work perfectly. Should i revamp the entire circuit ? More importantly would this solve the problem ? thanks guys I really appreciate your advice
RE: LED clock too dim
Or, use the transistors to set the current and forget about the extra resistor (assuming the transistors can handle the current draw and power dissipation).
You haven't specified if the LEDs are wired in series or in parallel, but from your verbiage it sounds like they're wired in parallel. If they're in series, the 5V regulator isn't going to give you enough voltage to drive that many LEDs in series. The whole thing is confusing, as you mention a 10 ohm resistor limiting current to 0.5A, but that makes me think you have a single resistor in series with a number of paralleled LEDs... that's bad.
Post a schematic... a pic is worth a thousand words.
Dan
Owner
http://www.Hi-TecDesigns.com
RE: LED clock too dim
Andy
RE: LED clock too dim
I swapped the NPN's and PNP's around. I added BC 547's from the 74HC238 to drive the bases of the PNP's. I've only used about 15 LEDs so far cause my power supply goes up to 30V. I'm sure if I add another 15 LEDs or so with the correct voltage, it will be bright enough. Strangely though, if one looks closely, the LEDs are illuminated slightly when they supposed to be off. I'm sure its a software problem, an error i probably made when i swapped the BD's.
I initially wanted to wire the LED's in parallel and use a PC power supply ( 30A on the 5V line). However, I now can see the benefits of wiring it in series. Which brings me to the problem of power supply. I need around 70V DC with 4A at least. I have 220V A.C. from the wall and i haven't worked with A.C. alot, so PWM seems challenging. I was thinking of stepping up a DC voltage, but then current becomes a problem. Is this a good idea ? I drew a schmatic, but i don't know how to post it. YAY !!!! it works. he he he.
RE: LED clock too dim
2) Wow - 220 - well, simplest is to use a step down transformer. You ought to be able to find one with a secondary of 56V AC, which full wave rectified and filtered with a big cap would give you around 72V DC.
http://w
A
RE: LED clock too dim
RE: LED clock too dim
RE: LED clock too dim
TTFN
RE: LED clock too dim
Measure the drop and see what it is.
I've generally found that red and yellow superbrights have a drop of around 2.1 V. Blue, green and white around 3V to 3.2V (remember that white is just a blue LED with phosphor on top).
Based on your 20ma, I'll venture the average drop is about 2.3V. If that's the case, you can pretty much toss the resistor.
Also, how are you measuring the current? The drop across the 100 ohm resistor?
A
RE: LED clock too dim
RE: LED clock too dim
Try measuring the true resistance of the 100R resistor when it is out of circuit, then measure the volt drop across it when it is in circuit and apply Ohm's Law. By measuring the voltage across the resistor, your measurement makes virtually no difference to the operating conditions of the circuit. Post the results.
----------------------------------
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I'm getting a great education!
RE: LED clock too dim
Firstly, if i had 10 LEDs in series, that would create a volt drop of 17V. If i supplied 17V to these LEDs, there would be no need for a current limiting resistor, as these LEDs would bias themselves. If i wanted to increase the current through them, i would have to INCREASE THE VOLTAGE. Aha! At 25V, 10mA should flow through them, with no current limiting resistor. More voltage = more current. Of course using a resistor is a good design practice when high voltage allows more than 20mA through the LEDs.
Sometimes i get caught up in the complex stuff and tend to forget the basics.
Is there any formula to calculate voltage, for the required current.
RE: LED clock too dim
RE: LED clock too dim
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
RE: LED clock too dim
TTFN
RE: LED clock too dim
RE: LED clock too dim
TTFN
RE: LED clock too dim
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
RE: LED clock too dim
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
RE: LED clock too dim
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.
RE: LED clock too dim
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.
h
Andy Somers
RE: LED clock too dim
We have used current control for our LED signage fault regulation (but it wasn't cheap!)
RE: LED clock too dim
Cheers
Andy
RE: LED clock too dim
http://www.lumileds.com/pdfs/RD25.PDF
That's a great document, thanks for the insight.
ANdy