Can somone double check this for me?

[Andy1845c]

Thanks philba. I will do some more measuring tomarrow and see what I come up with.

 

[cadstarsucks]

PWM is only useful when dim the LED or use an LED that's most efficient when pulsed at a high current at a low duty cycle, if your LED is most efficient at the normal continious current rating then PWM is a waste of time.

That is not accurate. The human eye acts as a differentiator and an integrator. That means that that it detects an intensity peak and then drifts down.

The end result is that even though the optical power is lower, the apparent intensity can be higher.

D.

 

[Sceadwian]

I've heard refrences on other sites of comparative measurements that say the integration time of the human eye is as high as 15 seconds under exceptionally dark conditions with no light pollution.

 

[frenzee]

This will drive up to 20 leds in series with regulated current up to 100mA from a 12V supply:

https://www.electro-tech-online.com/custompdfs/2007/02/HV9911DB2.pdf

 

[frenzee]

That is not accurate. The human eye acts as a differentiator and an integrator. That means that that it detects an intensity peak and then drifts down.

The end result is that even though the optical power is lower, the apparent intensity can be higher.

D.

That is incorrect. The human eye sees the avaerage brightness, not the peak brightness:

https://members.misty.com/don/ledp.html

 

[audioguru]

Yes, Our vision detects average brightness. When a 4-digit display is multiplexed it appears dimmed. It needs 4 times the current of one that is continuous to look the same brightness.

 

[cadstarsucks]

That is incorrect. The human eye sees the avaerage brightness, not the peak brightness:

https://members.misty.com/don/ledp.html

I did not say it sees peak... I said it registers a peak and then drifts down. the apparent brightness is the average of the result.

I can't find a REAL source off hand so I will have to rely on Maxim blech!

**broken link removed**

"An advantage of pulse drive is that the human eye behaves as a partially integrating and partially peak-reading photometer. As a result, the eye perceives rapidly pulsed light somewhere between the peak and the average brightness. This means that a low-duty-cycle, high-intensity pulse of light looks brighter than a DC signal equal to the average of the pulsed signal. An advantage of multiplexed operation, therefore, is an improvement in display intensity for a given average power consumption."

D.

 

[Hero999]

I did not say it sees peak... I said it registers a peak and then drifts down. the apparent brightness is the average of the result.

Still, pulsing an LED at twice the rated current at half the dutycycle will still look as bright as an LED run continuously at the rated current. The power dissipation in the resistor will be exactly the same so you've gained absolutely nothing in efficiency. The only time you will gain is if the LED is more effiecient at higer currents but this only applies to some red and inrfared LEDs, you loose with blue and white LEDs because they're more efficient at low currents and the PWM circuitary uses power.

 

[audioguru]

Maxim says that a pulsed LED looks brighter than the same LED operating continuously at the same average current. But since the brightness response of our vision is logarithmic then I think the difference in brightness will be very small so I won't even try it.

 

[frenzee]

I'm glad we agree on that point, however, the article you referenced seems to have multiple technical (not to mention grammatical) errors so I can't give much weight to it. These are some examples:

1) "The efficiency of an LED typically rises with forward current..." This is complete and utter.....bull (pardon my French). An LED's efficiency significantly drops with higher current. This is a well understood and discussed phenomenon and cited in various forums and technical papers. But as an anecdotal reference consider **broken link removed**. You will notice that a white Lux III puts out 65 lm @ 700mA vs. 80 lm. @ 1000mA. So a 43% increase in current (and an even higher % increase in power due to higher Vf) gets only a 23% increase in light output. And it gets worse from there. LEDs are at their most efficient when driven far below spec.

2) The intensity vs. current chart supposedly showing an upward and increasing positive slope is logarithmic on the current scale and linear on intensity (LOL). This is misleading. If you linearly plot power consumption vs. light output, you will see a positive but declining slope curve. (see https://candlepowerforums.com/vb/showthread.php?t=146219)

3) "Multiplying can often provide 1.5 times the light output ...". I think the author meant to say "Multiplexing", but it still doesn't make any sense.

4) "...multiplexed operation, therefore, is an improvement in display intensity for a given average power consumption..." I completely disagree. Here is one source:

**broken link removed** ("Temporal Source" paragraph) " Quote: "...At high flicker rates, and beyond the CFF (critical flicker frequency), the brightness is the same as the average brightness of the fluctuating light."

Don's page which I referenced earlier also disagree with that assumptions and the subject has also been extensively discussed on forums such as this:

**broken link removed**

So, to sum it up, PWM is great for dimming, but not so great for efficiency or for boosting the apparent or actual brightness. When comparing a continuous-current-driven vs. a PWM'd LED both at the same average power consumption levels (neglecting for now converter losses, etc.) there are two factors at work which contribute to the PWM'd LED appearing LESS bright than the CC one:

1) During the on cycle, the LED doesn't output light linearly, So driving say a 50% duty cycle circuit at twice the power level doesn't yeild twice the brightness (notice I didn't say twice the current level) and when ageraged out, will output less light than an equivalent CC circuit.

2) Assuming you could somehow solve problem #I and find an ideal LED that does output light linearly, again during the on cycle - let's take the previous example - the human eye would not perceive twice the brightness. It would be some figure less than that - due to the quasi-logarithmic sensitivity of the human eye - and averaged out over the full cycle, it would add up to be something less than what the constant-on LED puts out.

I know I've bored you and everyone to death by now, so I'll sign off for tonight.

I did not say it sees peak... I said it registers a peak and then drifts down. the apparent brightness is the average of the result.

I can't find a REAL source off hand so I will have to rely on Maxim blech!

**broken link removed**

"An advantage of pulse drive is that the human eye behaves as a partially integrating and partially peak-reading photometer. As a result, the eye perceives rapidly pulsed light somewhere between the peak and the average brightness. This means that a low-duty-cycle, high-intensity pulse of light looks brighter than a DC signal equal to the average of the pulsed signal. An advantage of multiplexed operation, therefore, is an improvement in display intensity for a given average power consumption."

D.