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Author Topic: Looking for a helpful friend to carry out an experiment  (Read 2225 times)
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zuisti
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« on: February 04, 2011, 04:37:59 16:37 »

Hi my dear friends;

I have long been workin on some LED matrix MMD circuits, but only in Proteus.
In relation to the implementation always arises the required peak current issue:

- how much average current should be to get a good LED brightness, in case of the
  multiplexed method?


In my case 16 rows are multiplexed, all with 96 columns (LEDs), so the duty factor
is equal to 1/16 (6.25%).

Details are in my prev. thread:
http://www.sonsivri.com/forum/index.php?topic=33957.msg107490#msg107490

Does it possible significantly reduce the power consumption?
The worst case peak current can  be 3-5 A,  and not the theoretic (horrible) 15 A ?
 
Really bothered me this so I'm looking for a very long time on the net for a solution...

I've read several times this statement:

"The human eye functions as both a peak detector and an integrator;
therefore, the eye perceives a pulsed LEDís brightness somewhere (?)
between the peak and the average brightness"

 
Finally found a site where practical experience and test results released:
http://www.dr-iguana.com/prj_LEDactus/LEDactus_pwm_intensity.html
 
Using the published curve, the 1/16-os multiplexing (6.25% duty-factor) causes
only approx. 70% brightness-decreasing, instead of the expected (theoretic) 93.75%.
This means that it is sufficient  to increase the LED's current (the nominal 10 mA)
only three to fourfold (not 16x).
 
Also a much smaller secondary power supply is needed ...
 
However, this would be good to try with the used LEDs too.

And now (after a long description of the above), my request is as follows:

Do you build the simple circuit below to try out this?

I can not do this anymore (because of my bad eyes), I'm too old (66)
unfortunately,is no longer going to soldering.
 
A pulse generator (for example a simple two-transistor astable multivibrator
or a 555)  produces a 120-140 Hz signal with a 6-7% duty factor. This drives
one of the two identical LED, through a variable resistor. The second LED gets
10 (max 20) mA DC what you need to get the correct brightness.

Best would be two LEDS of a cheap 5x7 or 8x8 LED matrix are used.

Then the same subjective brightness is set with the variable resistor to find
out how much peak current need to use.
 
And ...does anybody have concrete practical experiences in this matter?

This would be a great help for the construction of the system.

Please help me!

Thanks: zuisti
« Last Edit: February 04, 2011, 04:42:16 16:42 by zuisti » Logged
solutions
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« Reply #1 on: February 04, 2011, 09:18:48 21:18 »

Need to know more about your application.  What is the ambient light level?  How big is this matrix?  What does the data sheet say (i.e. what device can you get for the price you want to pay)? What color does this need to be? Is the display in motion?  Why aren't you using a current source instead of variable resistor, or an LCD, if you are concerned about power, etc.  Most LEDs can be driven much harder than the continuous current rating of 10-20mA. 

As far as soldering goes, you don't need to - use one of these: http://electronic-circuit.org/wp-content/uploads/2010/12/breadboard-circuits.jpg

Despite your long post, believe it or not, for me there's not enough info to go on.
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Froggy
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« Reply #2 on: February 05, 2011, 09:24:44 09:24 »

Most LEDs can be driven much harder than the continuous current rating of 10-20mA.
Hi

i'm not an electronician but i know that in PWM you can drive them just below the Ipeak of the leds so it can be near 70mA (you have to read the datasheet of your leds or burn some of them to know it)

i'm right am i?
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zuisti
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« Reply #3 on: February 05, 2011, 01:02:52 13:02 »

...Despite your long post, believe it or not, for me there's not enough info to go on.
Hi;

Thank you for your very valuable and helpful response  what I got from you again.
Of course, since "Nomen est omen", right, Solutions?

In any event, I learned some things:

-- should not be much to write (like now :-), because will not read ...

-- my poor English knowledge is not enough to explain what I want
(probably misunderstood), although I also use the Google translator

-- I must not hope that somebody is looking at in linked topic, for the details,
although I hope so, that you looked at the uploaded video (MyAvi.rar) or the picture at
http://www.sonsivri.com/forum/index.php?topic=33957   

-- seems remains the PM as the best method, although I naively thought that others
might be interested in this thing



> Why aren't you using a current source instead of variable resistor, or an LCD ...
Excuse me, this potmeter need only for the experimental circuit what I asked,
if it was not clear enough.

> Most LEDs can be driven much harder than the continuous current rating of 10-20mA.
Well, special thanks, once again I learned something. Seems, although I worked more than forty years
as an electronic engineer, this was not enough ...

"Believe it or not", I also know the breadboard, but already I cannot use this neither, unfortunately


"Believe it or not", I do not want anymore of these "helpful" answers.
This topic is pointless and redundant, I'll delete it soon.

Yours sincerely,
zuisti, an old (66, unfortunately :-) electronic engineer
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engamor
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« Reply #4 on: February 07, 2011, 11:05:31 11:05 »

Hi, Zuisti! I have done some experiments times ago. Unfortunately the results are rather subjective and may not be representative of what you can get with modern LEDs. At that time (5 years ago) I had a 1/8 duty cycle and I found satisfactory to set the current at about 3 times the nominal current. That was done on 7 segment NOT on matrix led displays, but it does work in the field. As such the result confirms that 3 times the nominal current is satisfactory. I cannot set up a new experiment now, sorry, but I would be confident that you can continue your design  at 4 times the power peak instead than 16 times (on your 1/16 duty). Hope this helps.
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shams_iqbal
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« Reply #5 on: February 16, 2011, 03:05:55 15:05 »

 Smiley hi dear plz note that i have already posted a reply on ur other post and here i am saying same  that there is problem with ur hardware so plz provide the schemitic then i will help u to sort out this trouble and plz provide the display data that what it says about per segment of led current rating ie single led wil consume 30 mA current and nominal volts of led and dont burn that led to trying to feed them extra current this is not the proper way of slution and plz use npn transister not the mosfets bs170
       a genral number is 2sc1383 npn transister wich will give u better result or any other npn transister but mosfet wil not give better result due to its switiching speed wich u r ignoring just replace ur bs170 with any npn transister and tell me what is the result now
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zuisti
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« Reply #6 on: February 16, 2011, 05:57:22 17:57 »

Hi "shams_iqbal";

Thanks for your reply, but it seems I was misunderstood. I just wanted to ask
a friend to perform a simple experiment, to determine that a 6.25 % duty cycle
signal powered LED of how much current will be applied to that the same
subjective brightness is obtained, as in the case of the 10 mA DC powering.

Just a simple pulse generator should be (for example, a 555), the two LEDs
and two potmeter. And I very much hope that this experiment will demonstrate
that it is sufficient to apply max. four times the nominal 10 mA current, although
the average current is reduced to a sixteenth.

I say again, that's all I asked.
And it is precisely because the simulation does not show the real magnitude of brightness : - ).

There was no question or request in relation to my circuit, and I don't want to alter it.

Otherwise, the system works very well, the HW and the SW too.
This is not the first MMD circuit I designed, and what my friends are implemented.

In this case I'm using row scanning and have 16 rows and 96 LEDs/row.
It follows that at worst case 96 LEDs (one full row) lit simultaneously ...

The displaying time of a row is 500 uSec, so the whole refresh rate is better than 120 Hz
(at 8 MHz PIC18 clock).

BTW, what's the problem about the BS170?
It's fast (switching time on/off better than 10 nSec), it's cheap, it has small Rds-on (typ. 1.2 Ohm),
directly driven (without resistance) ...

Anyway, thanks again for your reply ... and welcome to the board!
zuisti
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engamor
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« Reply #7 on: February 17, 2011, 09:16:18 09:16 »

Hi Zuisti, You seem not have seen my previous post! I am not in a mood to re-do the experiment, but I did share with you my previous results. YES 3-4 times the nominal current is enough at 1/16 the duty cycle. Or was I not clear enough? Regards!
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zuisti
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« Reply #8 on: February 17, 2011, 11:10:48 11:10 »

Hi Zuisti, You seem not have seen my previous post! I am not in a mood to re-do the
experiment, but I did share with you my previous results. YES 3-4 times the nominal
current is enough at 1/16 the duty cycle. Or was I not clear enough? Regards!

Dear engamor;
Of course, I read your  valuable comment: see the date of my "thankyou" sign. It was clear.
But many people here have misunderstood my request ...
Thanks and welcome
zuisti
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