Need help designing a power saving circuit for two strings of mixed LEDs. (I don't have a lot of experience designing circuits)

BobTPH

Joined Jun 5, 2013
2,255
That assertion is incorrect.
Evidence? I agree that it might not be exactly the same. In fact, LEDs do become less efficient at higher currents, so doubling the current will not quite double the light output. But, I doubt that you could tell the difference in brightness of two identical LEDs, one run at 20mA 50% duty cycle and one run at 10mA continuous.

Bob
 

crutschow

Joined Mar 14, 2008
24,325
That assertion is incorrect. That is how LEDs are dimmed in most modern efficient applications.
I know that's how LEDs are often dimmed.
That doesn't mean it's more efficient than just changing the resistance in series with the LED to give the same average LED current, which was my assertion.
What's your evidence for saying that's incorrect?
 

MisterBill2

Joined Jan 23, 2018
5,186
Has anybody's calculations considered persistence of vision?
Absolutely, Spidey! Persistence of vision is the only reason that PWM in lighting is acceptable. Otherwise that flashing would bother some folks terribly. That is why we can see the whole other side scene while going past a fence with only narrow gaps between the boards. Persistence of vision is the wonderful proof that the eyes are very fast, even if the brain is not as fast. And that assertion that average current is not changed by reducing the duty cycle is a bit confused. PWM is the way to change the power, which handily is a useful way of reducing the effective intensity. I am not in a position to explain the persistence of vision, but it is both very real and quite useful.
AND, the whole purpose of PWM, at least in many applications, is to avoid wasting power as heat. There are alternative methods of reducing delivered power, but they are all more complex, require more space, and usually cost more to implement. Thus PWM is the better method.
 

MisterBill2

Joined Jan 23, 2018
5,186
I know that's how LEDs are often dimmed.
That doesn't mean it's more efficient than just changing the resistance in series with the LED to give the same average LED current, which was my assertion.
What's your evidence for saying that's incorrect?
Every bit of power dissipated in an external resistance is wasted as heat. That should be evident by power=E x I, and power not delivered to the selected load device is wasted.
Of course there is no simpler or cheaper way of reducing LED effective intensity, but sometimes efficiency is more important than the initial cost of a system. That is typically the case in battery powered systems.
Unfortunately, in many consumer electronics devices the primary design goal is minimum build cost, and so efficiency, performance, battery life, and reliability are all secondary, if they are considered at all.
 

crutschow

Joined Mar 14, 2008
24,325
Every bit of power dissipated in an external resistance is wasted as heat. That should be evident by power=E x I, and power not delivered to the selected load device is wasted.
Of course there is no simpler or cheaper way of reducing LED effective intensity, but sometimes efficiency is more important than the initial cost of a system. That is typically the case in battery powered systems.
Unfortunately, in many consumer electronics devices the primary design goal is minimum build cost, and so efficiency, performance, battery life, and reliability are all secondary, if they are considered at all.
None of that explains why you stated my assertion is incorrect.
 

MisterBill2

Joined Jan 23, 2018
5,186
None of that explains why you stated my assertion is incorrect.
LEDs are extremely non-linear, and a quite small increase in current, from some specific point, will certainly double the light output, but with a quite small increase in voltage, and thus a fairly SMALL increase in power input. Doubling the power input would probably destroy the LED if it had been near the design current level. So with a pulsed power input the brightness can be increased a lot with less power because the intensity can be a lot higher while the average power is not increased, because of the off-time with zero power dissipation.
 
Voltage linearity of an LED has nothing to do with its intensity vs current. If you double the current then the intensity is double if the current is not near its maximum allowed current. But if you double the current then of course the power is doubled.
Since the sensitivity of our vision is logarithmic then double the intensity is just a little brighter.

PWM works to dim an LED because our vision is not a fast peak detector. Pulse durations above about 30ms (33Hz) produce a seen intensity that is the average of the pulse widths.
 

crutschow

Joined Mar 14, 2008
24,325
LEDs are extremely non-linear,
The current with voltage is non-linear but not the output light intensity with current, and the PWM regulates current not voltage in an LED circuit.
So my assertion is still true, that for given average LED current (and equivalent brightness) it makes no difference whether that is determined by the PWM duty-cycle or a change in series resistance.
 

ci139

Joined Jul 11, 2016
1,178
made some test - that for now hint the (LinCMOS TLC)555 for the low power design might be a bad choice - both - because of it's pins' function set - and not being oriented to µ-power designs . . .
TLC555 - App - v1 - hp-1.png TLC555 - App - v1 - hp-0.gif
 

MisterBill2

Joined Jan 23, 2018
5,186
T.I. (Teas Instruments) has a large selection of IC devices aimed at low power applications. They also have a whole lot of stuff on their website to help folks know how to use those devices. So you might find an application note covering exactly what you need to do, and it may already be optimized for an application such as yours, but in a cell phone, which may have a few LEDs needing intensity control and minimum battery drain. AND THEIR NOTES COME FROM ENGINEERS who are experienced in that area. So all that is missing there is guesses.
 

ci139

Joined Jul 11, 2016
1,178
we wait the #3

it's not about gizmos you can buy but what you can build from what you got
. . . there might be good and simple Lo-PWR CMOS 555 current control circuit (i just don't build those daily . . .)

there is an option to build one with no feedback chain - relaying on hope your battery voltages wont exceed those you 1-st adjusted your circuit to max.20 mA . . .

 
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