Driving LED's with PWM

[Christopher4187]

I'm confused.

The project: I'm making an outdoor sign that has 500 LED's.

The problem: It draws way too much current.

The current solution: Use PWM with a ULN2003 to drive the LED's.

The secondary problem: What if my PIC gets "stuck" with the PWM in the on position?

I researched my question and found some information that helps. However, I need a better understanding. I know PWM works because it's been running these LED's for two days now. A lot of information on the internet states that you shouldn't use PWM without a current limiting resistor. I understand why but if there was a hardware backup (crowbar, TVS, zener, etc) are there any other concerns? I know uneven current is one of them but if the brightest LED is kept below the maximum then the rest should be okay? I guess it may boil down to looks at that point.

My other confusion is about current measurement. I set up the LED's (I'm testing 12 in parallel at the moment) to display maximum sustained current, which the datasheet states is 30mA. I get about 352 mA and this is a lot of current and heat to dissipate. I set up PWM (240uS on / 5uS off) with the same configuration of LED's. My measurement shows 125mA but should this be measured in DC or AC amps since it is PWM? When I switch to AC amps the current measurement is 290 mA.

I'm trying to figure out two things. One, can I make the LED's more efficient and two, can I make the LED's look bright without having to use more current.

[Demon]

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

[Christopher4187]

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.

[Demon]

(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

[Demon]

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

[Christopher4187]

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.

[Demon]

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

[Art]

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.

[Christopher4187]

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.

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.

[Grefe21]

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.