PWM vs. variable DC?

I am trying to understand which drive circuit is more efficient - PWM or a variable DC. Looking at some of the more recent CREE data, this is how i understand it now...



This is the chart that Newbie very generously created. On another Newbie graph (lumens vs. current), you'll find that at:

• 160lm, you will be driving this LED at about 925mA, getting 52lm/W
• 80lm, you will be driving it at 350mA, getting 74lm/W

This is obviously for the variable DC case.

Now let's say you are driving the LED at 100% PWM duty cycle at 925mA, and you want to drop the light by half, does this mean that your efficiency doubles (104lm/w) since you are only driving it at 50% duty cycle? So PWM is more efficient than DC, right?

Thanks, (and sorry for the beginner question)

LMI welcome to CPF.

Actually, current dimming is more efficient at the LED than PWM dimming. Recall that for PWM dimming, during the duty cycle, you are always getting the LED efficiency for that period. With reduced current dimming, you will be getting the LED efficiency at the lower drive level, which is higher (at least until you hit around 20mA).

Here is the thread you want to read. Cheers,

Paul

chimo said:

Recall that for PWM dimming, during the duty cycle, you are always getting the LED efficiency for that period.

Click to expand...

Thanks for the welcome and that link, Paul - fascinating read. The part i still don't get, though, is where the effiency is added in due to the LED not being on during PWM. I guess i didn't see how that factored into the (as always) excellent data provided by Newbie. I can totally see how CC is more efficient than PWM during the, say, 50% of the duty cycle that it is on, but over an hour, it is only stealing energy from the battery for half the time... I guess this is where my understanding breaks down and your linked thread didn't help fix.



I definitely appreciate the reply, though.

With PWM, a 50% duty cycle means it's only half as bright. People keep forgetting (or not acknowledging) this part. Your eyes don't see the light being as bright as a single pulse, they average out the brightness of the on pulese and off time.

evan9162 said:

With PWM, a 50% duty cycle means it's only half as bright.

Click to expand...

I thought i mentioned that in my original question? The crux of it was, if you are driving at 50% duty cycle for half the brightness, does your total run time double since you are only using power for half the time? It seems that if you look at the lm/W curve above, your lumens/watt metric does not double (if you reduce the current enough to halve the light output).

So, i'm not completely talking about lumens/watt, but rather which drive methodology will give you the longest burn time for a given battery.

-Damon

Damon, let's assume that the 50% PWM output gives 50% brightness. Since the duty cycle is 50%, the average current is 50% as well.

To get 50% brightness with current dimming the actual current draw requirement will be less than 50% of full current because the LED becomes more efficient at the lower drive current. Since the CC driver has a 100% duty cycle, it is also the average current (<50%) so it draws a lower average current than PWM dimming.

Keep in mind that this is a simplified solution, you also have to consider power supply and battery efficiency for the big picture.

Oh and yes, your batteries should last about twice as long at 50% PWM duty cycle.

Paul

Thanks...

The sad thing is that i understood this a while ago, and then confused myself later on. I was oversimplifying how i was thinking about this.

-Damon