Re: Driving LED's with PWM
I don't know your exact setup, but I'd put a transistor after the PIC and use a pull-down resistor. If the PIC fries, transistor goes off. If PIC fails ON, not much you can do besides safety features you mentionned.
Robert
Re: Driving LED's with PWM
The ULN2003 is a transistor (darlington pair).
Years ago I learned (the hard way) that having a hardware backup to protect the device (i.e. After all of the controlling hardware/software) is worth a mountain of gold. Fuses are under-appreciated and have a thankless job.
Re: Driving LED's with PWM
(for what my "limitted knowledge" is worth)
PWN is intermittent DC, I would use DC mode to measure current.
As to make LED brighter without using more current, I think that strays into 'free energy' subject. I don't think you can.
Do a test, DC with varying voltage, adjust to desired brightness, measure current.
Repeat using PWM, adjust to similar brightness and measure current.
I think you won't find a way around 2+2=4. I hope you can but I couldn't a few years back.
Robert
Re: Driving LED's with PWM
Agreed about fuses, but they are slow to burn even if "fast" model. Crowbar, TVS, zener, etc offer much faster response for delicate devices.
I'd still shove a fuse in there regardless, super cheap.
Robert
Re: Driving LED's with PWM
Quote:
Originally Posted by
Demon
Do a test, DC with varying voltage, adjust to desired brightness, measure current.
Repeat using PWM, adjust to similar brightness and measure current.
Robert
I did that. It's hard to tell if the brightness is different but that kind of answers my own question. For me it looks the same whether the current is 125mA DC or 360 mA DC. It's technically an AC sine wave, which would make sense given the fact that I measured 300mA AC, but I don't know for sure.
I know I have some photocells at home and that's about the best I can do to measure the amount of light.
Re: Driving LED's with PWM
Wouldn't AC mode measure current above and below neutral, basically 2 readings and then adding them?
That might explain why your reading is twice as high on AC.
(total guess)
Robert
Re: Driving LED's with PWM
Rapidly switching DC on & off (PWM) is a square wave. It's still DC.
If you vary the duty cycle from 50% it will look different, but we still call that a square wave.
You can measure the current with LEDs running constantly for a single row like it sounds like you're doing.
If it's a POV display where you cycle rows, then no more than one row (or column depending on how you look at it)
is going to be powered at one time, so you can assume the total of the switching transistor and LEDs for a
single row is the maximum current that your display will draw.
Be careful trying to measure PWM with a digital multimeter.
The digital multimeter is a microcontoller too, and can only take samples at a certain rate that is not synced with your PWM.
I believe the best insurance against software failure is your watchdog timer.
The best place to clear it is in your display routine that cycles the rows.
Depending on complexity, I suppose some insurance against hardware failure is also prudent.
http://www.lizarum.com/assignments/p...pwm_wave_1.gif
Re: Driving LED's with PWM
Quote:
Originally Posted by
Art
You can measure the current with LEDs running constantly for a single row like it sounds like you're doing.
If it's a POV display where you cycle rows, then no more than one row (or column depending on how you look at it)
is going to be powered at one time, so you can assume the total of the switching transistor and LEDs for a
single row is the maximum current that your display will draw.
The PCB has 12 LED's in parallel. Not sure I completely follow your comment but I'm trying to validate how much current I'm saving by decreasing the duty cycle/frequency. I have an oscilliscope if there's a good way to measure the current with it.
Quote:
Originally Posted by
Art
I believe the best insurance against software failure is your watchdog timer.
Depending on complexity, I suppose some insurance against hardware failure is also prudent.
I've never used the watchdog timer but assume it checks the continuous loop that's running inside the PIC. If so, what happens if the ULN2003 fails or my power supply fails (overvoltage or overcurrent)? The LED's aren't expensive to replace but they are a PITA to access. I want to avoid replacing them at any cost.
Re: Driving LED's with PWM
That's the benefit of hardware failsafe. Something could break to leave an LED on,
but the watchdog timer is still the cheapest and most effective on the software side.
If your display routine fails to cycle, the watchdog should be on an independent timer,
and fire even if, for example the chip's oscillator shorted.
If you have rows of 12 LEDs parallel, that means they can't be independently controlled
for graphics, etc. You are just cycling whole rows of LEDs?
The current can still be measured at 100% duty cycle, and as you lower the duty cycle,
the average current should lower approximately with the duty cycle.
Transistors have cut off slopes, so it's not extremely precise. Someone else may have to chime in about that.
However, just because you use say 50% duty cycle, the fact remains that the row of LEDs and hardware to drive
them are using the same current for half of the time, and no current for the other half of the time.
That's if you negate a little error for the actual switching on and off, but I don't know exactly what the cost is there.
Re: Driving LED's with PWM
Hello
I am working with LED and trying to make them as bright as possible.
What I have read, I can provide higher current through the LED with pulses. I am now running a 30 ma LED with 90 ma and pulses. 1khz with 1/10 duty cycle.
I was trying to measure the light intensity, but there is no difference between a normal none wave setup wtih 30 ma, compared to my PWM setup.
The measurements was done with a photodiode and I measured the voltage after. Can this be a bad way to measure the light and therefore I cant trust the outcome, cause maybe it shows more the average light intensity/voltage.
Re: Driving LED's with PWM
Quote:
Originally Posted by
Art
If you have rows of 12 LEDs parallel, that means they can't be independently controlled
for graphics, etc. You are just cycling whole rows of LEDs?
It's not for graphics. I am testing a number and the 12 LED's are one segment of the number.
Quote:
Originally Posted by
Art
The current can still be measured at 100% duty cycle, and as you lower the duty cycle,
the average current should lower approximately with the duty cycle.
Transistors have cut off slopes, so it's not extremely precise. Someone else may have to chime in about that.
However, just because you use say 50% duty cycle, the fact remains that the row of LEDs and hardware to drive
them are using the same current for half of the time, and no current for the other half of the time.
That's if you negate a little error for the actual switching on and off, but I don't know exactly what the cost is there.
I did some testing tonight but things still aren't clear for me. I will post the results below and maybe someone can derive something useful from this information. Brightness (how the user views it) is the ultimate test so I put a photocell on top of one LED and wrapped it in black electrical tape. I took random measurements with a bunch of tests using only PWM and the other using straight DC with a resistor. For clarification, the lower the resistance the higher the brightness.
Code:
LED'S ON ON TIME OFF TIME CDS Resistance DC Current AC Current Resistor
1--------- 5uS 740 uS 92.2 Ohms 36.4 mA XXXXXXXXX No XXXXXXXXX
2---------- 5uS 740 uS 94.2 Ohms 41.3 mA XXXXXXXXX No XXXXXXXXX
3---------- 5uS 740 uS 98.3 Ohms 52.6 mA XXXXXXXXX No XXXXXXXXX
1+12 LED's 5uS 740 uS 150.4 Ohms 49.9 mA 189.3 mA No XXXXXXXXX
1+12 LED's 5uS 640 uS 141.2 Ohms 57.2 mA 214.2 mA No XXXXXXXXX
1---------- 5 uS 940 uS 118.4 Ohms 27.2 mA XXXXXXXXX No XXXXXXXXX
1---------- 5 uS 1140 uS 128.6 Ohms 23.5 mA XXXXXXXXX No XXXXXXXXX
2---------- 5 uS 1140 uS 133.5 Ohms 29.6 mA XXXXXXXXX No XXXXXXXXX
3---------- 5 uS 1140 uS 133.6 Ohms 37.2 mA XXXXXXXXX No XXXXXXXXX
1+12 LED's 5 uS 1140 uS 194.8 Ohms 33.4 mA 159.7 mA No XXXXXXXXX
1+12 LED's 5 uS 1240 uS 203 Ohms 31.3 mA 154.6 mA No XXXXXXXXX
1---------- 5 uS 1240 uS 127 Ohms 21.5 mA XXXXXXXXX No XXXXXXXXX
1+12 LED'S 100 0--------- 109.2 Ohms 94.2 mA XXXXXXXXX Yes 29.6 Ohms
1+12 LED'S 100 0--------- 117.3 Ohms 79.9 mA XXXXXXXXX Yes 35.6 Ohms
1+12 LED'S 100 0--------- 136.1 Ohms 64.7 mA XXXXXXXXX Yes 44.5 Ohms
1+12 LED'S 100 0--------- 159.4 Ohms 49.3 mA XXXXXXXXX Yes 59.3 Ohms
1+12 LED'S 100 0--------- 201.6 Ohms 33.5 mA XXXXXXXXX Yes 89 Ohms
1+12 LED'S 100 0--------- 310.2 Ohms 17.1 mA XXXXXXXXX Yes 178 Ohms
I don't see much of a difference between PWM and using a resistor.
Re: Driving LED's with PWM
Pretty much the same thing I observed.
Brightness requires current, I don't think you can get around that.
Maybe if you start tinkering with transformers?
Robert
Re: Driving LED's with PWM
Is this statement correct?
Even though the currents are similar, using PWM without a resistor must be more efficient since a resistor isn't being heated.
Another thing that doesn't make sense is the non-linear curve in current when adding diodes onto the PCB. One LED consumes 30mA but two doesn't equal 60.
Re: Driving LED's with PWM
An LED cannot have more than its forward bias voltage across it. For an old red LED that voltage is about 1.6 volts. Sacrifice one by driving one with a variable power supply. It will start lighting at a little over 1.5 v. It will be brightest at 1.6v. Only when it burns out will the voltage go much higher than that.
In a circuit, the current is limited by the impedance (resistance) of the circuit driving it. If you drive the LED directly from a i/o pin, the current will be limited by the impedance of the output circuit. The output transistor or FET will act as the resistor and limit the current to some value.
There are two different limits on the current through the LED, the maximum instantaneous current and the max average current. If you exceed the first, the LED will fail immediately. If you exceed the second, it will overheat and die more slowly. There should be a resistor to limit the current to below the max instantaneous current. You can then use PWM to stay below the max average current.
The instantaneous current through the LED can be calculated using i = e / r where r equals the sum of the impedance of the driver plus any external resistor. Heat will be lost by the voltage drop across the driver transistor and the external resistor.
Re: Driving LED's with PWM
Quote:
Originally Posted by
Christopher4187
...Another thing that doesn't make sense is the non-linear curve in current when adding diodes onto the PCB. One LED consumes 30mA but two doesn't equal 60.
I have a similar problem. I'm designing panels for my residential alarm system and I'm kinda overwhelmed by the possible maximum current. A LOT of LEDs could potentially be on at the same time (I'm using solid strand 24 AWG telephone wire with 12V 7Ah battery backup).
I started with LCDs, but these panels are not only for the house where it's warm, it's also for the garage (worried about cold and humidity).
Robert
http://i991.photobucket.com/albums/a...psa30cdcfc.jpg
Re: Driving LED's with PWM
Re: Driving LED's with PWM
Quote:
Originally Posted by
Demon
I have a similar problem. I'm designing panels for my residential alarm system and I'm kinda overwhelmed by the possible maximum current. A LOT of LEDs could potentially be on at the same time (I'm using solid strand 24 AWG telephone wire with 12V 7Ah battery backup).
I started with LCDs, but these panels are not only for the house where it's warm, it's also for the garage (worried about cold and humidity).
Robert
http://i991.photobucket.com/albums/a...psa30cdcfc.jpg
If it's a panel inside then you could supply 1-3mA per LED. There's no reason to supply the maximum amount of current since you won't have an audience to view them and they are inside. I recently used Screenmaster LED's and at 1mA they were really bright. I wish I had your problem. My LED scoreboard is outside and the sunlight hits it. Needless to say the LED's have to be really bright.
Re: Driving LED's with PWM
Thanks!
Just went through the 1st, great stuff!
Didn't download the dice yet. I want to see what I can do on my own first.
Robert The Very Happy Camper
:)
Re: Driving LED's with PWM
Quote:
Originally Posted by
Christopher4187
If it's a panel inside then you could supply 1-3mA per LED. There's no reason to supply the maximum amount of current since you won't have an audience to view them and they are inside. ...
I'll be using surface-mount LEDs that I have on hand (got some greens, yellows and reds). I think they require somewhere between 15-20mA to be reasonably visible in daylight inside the house (some panels could be in full sunlight).
I'll be making my own light chimneys with clear liquid plastic from Smooth-On.
Finished preliminary testing today, very low current is achievable, I just have to improve code logic and efficiency:
http://www.picbasic.co.uk/forum/showthread.php?t=18906
Of course none of the colours I have are of the same brightness so some tweaking of the resistors will be necessary.
Robert
:)
