Dimming LEDs: Analog or PWM

tom66

Joined May 9, 2009
2,595
It really depends what they mean by analog dimming but I'd generally discourage it because of the nonlinear characteristic of an LED. I run one LED at 4V and one at 6V (they are 5V LEDs, with series resistors) - the change in voltage is 50% but the brightness change is almost unnoticeable (maybe 10%), and the 6V one gets warmer. The LED buck controller is an example of using analog dimming, but it is current controlled, not voltage controlled, unless the writers mean current control.
 

Thread Starter

jpanhalt

Joined Jan 18, 2008
9,681
The article will come out soon on-line, and I was reluctant to show too much of it here. According to the article,
It is a known fact that all LEDs are more efficient when driven with less forward current.<a figure derived from a datasheet is included to substantiate that statement>
The argument is basically that dimming with PWM uses multiple pulses of higher current than is used in analog methods. Hence, it is less efficient.

Obviously, one needs to generate the lower currents efficiently, and the article give ways to do that.

John
 

SgtWookie

Joined Jul 17, 2007
22,210
Just put a low-pass filter after your PWM, if the efficiency of the LEDs really matters.
A low-pass filter as in an inductor in series, and a cap across the LED(s), so you have a switching supply, which will be more efficient than either the PWM method or the analog method.

And yes, efficiency really does matter.
 

Thread Starter

jpanhalt

Joined Jan 18, 2008
9,681
Of course, that is one of the approaches used in the article (check Figure 1, when available). A resistor and capacitor can also be used.

John
 

SgtWookie

Joined Jul 17, 2007
22,210
Of course, that is one of the approaches used in the article (check Figure 1, when available). A resistor and capacitor can also be used.
If you throw a series resistor in the mix, efficiency will go right out the window. The resistor will dissipate power. An ideal inductor will dissipate no power. Real-world inductors have parasitic resistance and capacitance, but are still vastly more efficient than resistors in this case.
 

Thread Starter

jpanhalt

Joined Jan 18, 2008
9,681
It's the flicker from PWM'd LEDs that bother me, not the efficiency. I had a 1990 Buick Regal that had PWM'd LEDs in its radio stack. It was so bad, I made some plastic covers for the lights to drive at night. Some of us are more sensitive than others, I guess. Same problem with early CRT monitors.

John
 

tom66

Joined May 9, 2009
2,595
It's the flicker from PWM'd LEDs that bother me, not the efficiency. I had a 1990 Buick Regal that had PWM'd LEDs in its radio stack. It was so bad, I made some plastic covers for the lights to drive at night. Some of us are more sensitive than others, I guess. Same problem with early CRT monitors.

John
Probably due to low frequency. If the frequency is in the kilohertz range, most people do not notice it, even in their peripheral vision. (Ever try and look at a CRT in the corner of you eye - it looks like it's flickering because your peripheral vision is much more sensitive to movement but less sensitive to detail.)
 

Thread Starter

jpanhalt

Joined Jan 18, 2008
9,681
Yes, I am well aware of the sensitivity of peripheral vision to flicker. That was what was so annoying about the Buick. It is probably a survival adaptation from thousands of years ago or more.

John
 

tom66

Joined May 9, 2009
2,595
Yes, I am well aware of the sensitivity of peripheral vision to flicker. That was what was so annoying about the Buick. It is probably a survival adaptation from thousands of years ago or more.

John
Think about a predator trying to pounce, the extreme sensitivity of peripheral vision would make it possible to avoid the attack.

It would seem no-one actually drove the Buick before they started selling it or the problem would have been obvious. Unless it was a limitation of hardware, maybe they were using an MCU and PWM rates were limited.
 

Wendy

Joined Mar 24, 2008
22,145
So why not boost the frequency up there?

I've designed both sides for noobs. It is easy enough. I don't have the ability to see flicker well, so what frequency do you think you can't see the flicker.
 

Thread Starter

jpanhalt

Joined Jan 18, 2008
9,681
So why not boost the frequency up there?
On a Buick Regal? Is that allowed to be discussed here?

More seriously, there can be a strobe or beat effect when there is more than one source of flicker. Fluorescent lighting accentuates that, but so can other factors. Anything that can interrupt the ambient light (e.g., propellers, machine chucks, fans, etc.) can cause the same effect.

John
 
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