Driving LED's with PWM

[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.

[Art]

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.

[Christopher4187]

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.

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.

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.