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#1 2020-09-09 19:04:33

JosephFlage
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Registered: 2020-09-09
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Pingback: Advice About Over-Driving LEDs

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Over-driving LEDs for brightness
Most of the time when I’m driving an LED

I do it with a simple ballast resistor, and that’s all there is to it.
Lately.

However, I’ve been working on a few LED matrix projects

and while it’s certainly possible to use the same simple resistor approach, this leads to a very dim display.
( The good news is that there’s a straightforward solution to this problem.

LEDs have a rated forward current

but they also have a rated peak current, specified at a given pulse width and duty cycle.
For instance,  are rated for 30mA forward current and 185mA peak in 0.1 ms pulses at 10% duty cycle.
Why is this stat useful.
As long as you stick to the pulse width and duty cycle parameters, you can intermittently drive an LED at an excess current and get a brighter light without burning it out.             To prove that everything was working, .

I started out by connecting the steady current resistor and probed all the LEDs

Everything lit up fine. With the baseline established, the next step was to set up the PWM signal.
According to , the Arduino’s built-in analogWrite() PWM runs at about 500Hz, which is too slow for the target pulse width.
Since I didn’t need crazy precision or performance here, I decided to quickly write up an Arduino sketch that toggled the output pin manually and used delayMicros() to set the interval.
The sketch turned out to be pretty simple: [gist https://gist.github.com/bryanduxbury/7729613] To verify that this was giving the output I expected, I hooked up the output pin to my handy.
On the first try I ended up with a   The over-driven LED was the most complicated to set up.

I plugged the peak current number into an LED resistor calculator

and it told me I needed a 12 ohm,  resistor.
Yikes.
I certainly don’t have anything like that on hand.
Rather than putting a whole ton of resistors in parallel, I wired up a simple constant current LED driver circuit using some common transistors and resistors.
I’ll leave the discussion of that driver circuit for another time, but the cool thing about it is that it allows the transistors to do all the heavy wattage dissipation while the resistors see a tiny load.
Closeup of the final brightness test.
Hooray for DSLR cameras.
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Cellular Chronometer Part 2: The PCB     Simple constant current LED driver                   17 thoughts on “Over-driving LEDs for brightness ”.
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December 3.

2013 at 5:22 pm            ” LED resistor calculator

and it told me I needed a 12 ohm, 2 watt resistor” I guess that the calculator did not take in account the 10% duty-cycle.
So a 1/4 watt resistor was probably just fine.
Reply                   December 3, 2013 at 6:10 pm            I think you’re probably right.
I didn’t think to look in the datasheet for my resistors to see if they have a similar pulse behavior until after I’d built the constant current driver.
Reply                   December 4, 2013 at 11:02 am            Resisitors are simple devices, if you put in 2 Watt for 10% of the time, they see 0.2 Watt.
At least if the frequency high enough.
Don’t try it with 0.001 Hz ????.
Tytower December 3, .

2013 at 7:10 pm            I would place an arrow or mark of some sort on the LED Photo

I have no idea which LED you refer to or even what colour of each in the photo.
Perhaps a video might be better but definately say which LED is which            Reply                   December 3, 2013 at 10:34 pm            Good call.
I updated the image with some text and arrows.
Hope that helps.
Reply.
Telford Dorr December 4, 2013 at 2:48 am            The only problem with this technique is if the firmware gets lost and stops the multiplexing process, the display over-currents and is damaged.
Dealt with this years ago, driving VFD’s.
Added a hardware retriggerable one-shot (watchdog), driven by a data strobe, which enabled the output driver.
In case the firmware goes nuts, the one-shot times out, killing current to the display, preventing display damage.
Otherwise, .

A minor firmware bug could ruin a whole bunch of LEDs before you could hit the off switch

Reply                   December 5, 2013 at 8:48 pm            Wow, this is a pretty cool idea.
I’m not sure exactly how it will fit into the design I have in mind just yet, but I’ll think about it.
Reply.

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August 16, 2016 at 6:15 am            ok a little lost  you talk about over driving leds for brightness but in the results :I would say that the PWM-only LED is substantially dimmer, .

And the overdriven LED is almost but not quite as bright as the steady-on LED

This turned out to be exactly the sort of obvious difference I was looking for.
It seems well worth adding the additional circuitry to get a much brighter display.” you say the steady on is the brightest.
Reply.
cubmanky August 16, 2016 at 6:27 am            so you say over driving for brightness but then your results “I would say that the PWM-only LED is substantially dimmer, and the overdriven LED is almost but not quite as bright as the steady-on LED.
This turned out to be exactly the sort of obvious difference I was looking for.
It seems well worth adding the additional circuitry to get a much brighter display.” you say the steady on is brightest            Reply.
johny radio September 30, 2017 at 12:01 am            Great comparison, but I don’t get your conclusion.
If steady on is brightest, isn’t that the best choice.
Thx            Reply                   January 11, 2018 at 10:35 am            Yes, but in LED matrices, you usually strobe the grid and rely on persistence of vision, rather than powering each LED individually.
This decreases the overall power draw, but more critically reduces the number of microcontroller pins required to operate it.
So by over-driving the LEDs, you make them substantially brighter during the fraction of a second that they are on, making up for the fact that they’re operating at a substantially lower duty cycle.
Reply                  Graeme Smith (@GraemeS55376103) January 22, 2019 at 4:30 pm            I am a retired engineer but still keeping tabs on companies who advertise SMD LEDs at a brightness rating which I believe is a straight lie, or they are overclocking the LED.
If I use a chip with 600mcd Red + 1350mcd Green and 280mcd Blue the total is 2,230mcd or 2.23cd.
If the driver is working at 1/2 scan then that makes total 1.115cd.
10,000 chips per sq Mtr makes total lumens 11,150 or 11,150 NIts.
This would just be a bright unintelligent light.
To be part of a large format Video Display (say 100 Sq Mtrs) it would probably only ever show 50% or 5,575 Nits.
Am I correct to think this way.
January 23, 2019 at 9:40 am            Your math seems at least directionally right, but I’m not knowledgeable in the area of displays.
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