Do LOR controllers make LEDs flicker more when dimming?

[kvnschmdt]

I hope this is not a stupid question...

From my research, I understand that all LED lights flicker - 60Hz for half wave lights and 120Hz for full wave lights.

I'm trying to understand how the LOR controllers make the LEDs dim...

My understanding is that they use triacs and send slices of the electrical wave to the lights.

Does this mean that they are making the LEDs flicker more than either 60Hz or 120Hz? Or are these slices smoothed out somehow?

Thanks in advance for any help in understanding this.

[plasmadrive]

If you mean the AC controllers, they do not make the LEDs flicker. The frequency of the lights remain the same, they just "chop up" (change duty cycle or "on time") the wave per half cycle. If you have full wave LEDs you will find it much less likely that you will see the flicker. The controllers don't do anything to smooth out any flicker. They do have dimming curves (G3) that allow for a more linear dimming perception with respect to the input signal.

The DC controllers are completely different.. PWM... Do some reading on the subject and you will understand.. look up "color temperature" and "CRI" with respect to LEDs.. then use info from the two to figure out if you even care about such things for Christmas lights. This is probably way more then you want to go into.. but the reference is there now..

[TonyD]

If you mean the AC controllers, they do not make the LEDs flicker. The frequency of the lights remain the same, they just "chop up" (change duty cycle or "on time") the wave per half cycle. If you have full wave LEDs you will find it much less likely that you will see the flicker. The controllers don't do anything to smooth out any flicker. They do have dimming curves (G3) that allow for a more linear dimming perception with respect to the input signal.

The DC controllers are completely different.. PWM... Do some reading on the subject and you will understand.. look up "color temperature" and "CRI" with respect to LEDs.. then use info from the two to figure out if you even care about such things for Christmas lights. This is probably way more then you want to go into.. but the reference is there now..

So to ask in other words, are the LEDs "on" at full intensity for a shorter amount of time and hence to our eyes appear dimmer? Or are the "on" at a lower intensity? Or, are they "on" for less time that it takes to reach full intensity and again to our eyes look dimmer?

Thanks, it's always interesting to know how this stuff really works.

[-klb-]

On at the same intensity, just extending the off times between the 120 hz on times, and correspondingly shortening the on times. With fewer total photons per second reaching the eye, we percieve it as dimmer.

[plasmadrive]

So to ask in other words, are the LEDs "on" at full intensity for a shorter amount of time and hence to our eyes appear dimmer? Or are the "on" at a lower intensity? Or, are they "on" for less time that it takes to reach full intensity and again to our eyes look dimmer?

Thanks, it's always interesting to know how this stuff really works.

Are you asking about the AC controllers or the DC controllers? The answer is different for the two.... kinda

With AC the perceived brightness has to do with the RMS value of the voltage out of the triac which will change the current drawn by the LEDs.. (mostly) With AC controllers the voltge is sinusoidal and therefore only has one full on peak per half cycle of the AC waveform unlike DC, which has no waveform per se`

With DC PWM, the peak voltage is for all intents and purposes, always the same. The PWM controller feed the same voltage to the LEDs current limiting device (resistor perhaps) when it is on regardless of when that happens. The brightness at that point is our perception of the ON vs. OFF time. The cool thing about PWM control is that the LED will always be at or about the same color temperature when on regardless of the on/off time ratio because the current to the LED when its on, is always the same.. (Almost). It will just appear dimmer or brighter. This is not necessarily the case with the AC controller because the actual current to the LED is always changing and therefore the color temp will change.

I hope this is the info you wanted.. it is a bit more technical then I wanted it to be but not sure how to explain it any better and be fairly accurate.

[TonyD]

Are you asking about the AC controllers or the DC controllers? The answer is different for the two.... kinda

With AC the perceived brightness has to do with the RMS value of the voltage out of the triac which will change the current drawn by the LEDs.. (mostly) With AC controllers the voltge is sinusoidal and therefore only has one full on peak per half cycle of the AC waveform unlike DC, which has no waveform per se`

With DC PWM, the peak voltage is for all intents and purposes, always the same. The PWM controller feed the same voltage to the LEDs current limiting device (resistor perhaps) when it is on regardless of when that happens. The brightness at that point is our perception of the ON vs. OFF time. The cool thing about PWM control is that the LED will always be at or about the same color temperature when on regardless of the on/off time ratio because the current to the LED when its on, is always the same.. (Almost). It will just appear dimmer or brighter. This is not necessarily the case with the AC controller because the actual current to the LED is always changing and therefore the color temp will change.

I hope this is the info you wanted.. it is a bit more technical then I wanted it to be but not sure how to explain it any better and be fairly accurate.

I didn't know there would be a difference between AC/DC controllers so i guess answers for both are good!

Is RMS kinda like an average? So since the voltage on AC is rising/falling in a sine wave it is really the "average" voltage that drives everything?

What is the PWM in DC PWM?

Why does the AC work differently than the DC? It would seem that you could achieve the same effect with the AC controller by having the LED "on" 100% but just limiting the amount of time it is on hence changing our perception of brightness.

Thanks but if these questions are too much just don't respond!

[plasmadrive]

I didn't know there would be a difference between AC/DC controllers so i guess answers for both are good!

Is RMS kinda like an average? So since the voltage on AC is rising/falling in a sine wave it is really the "average" voltage that drives everything? Kinda, yeah.. a bit differenet mathmatically but you have the right idea.

What is the PWM in DC PWM? Pulse Width Modulation You modulate the output in a form of square wave with varying duty cycle. The DC buss is constant as opposed to AC which varies constantly

Why does the AC work differently than the DC? It would seem that you could achieve the same effect with the AC controller by having the LED "on" 100% but just limiting the amount of time it is on hence changing our perception of brightness. The DC buss is constant as opposed to AC which varies constantly. DC has a 100% duty cycle, not so with AC.

As well from a manufacturing standpoint, triacs are very low cost and it is easy to make them work on AC. They will not work on DC unless you want them to be just a latching switch because the current will never drop below the hold on threshold of the triac until you turn the power supply off..

FETs (DC switching devices) are a bit more expensive for that high a voltage and will only work in one direction of the AC wave form. There are things that could be made to work, but too costly and more complicated.

Thanks but if these questions are too much just don't respond! Never too much.. all depends on the time I have to answer.

[-klb-]

RMS (Root Mean Square) This is because power goes up as the square of the voltage applied. So it is a weighted average, where the power/voltage closer to the peaks contributes more than that closer to zero. On incandescent lights, output should track RMS voltage pretty closely. But with the non linear nature of LED lights, this isn't really true, resulting in tails that fade much more gradually than the middle section.

PWM (Pulse Width Modulation) This is just switching the LED on and off at a given frequency (think it is about 400 Hz on the LOR cards) and varying how long you leave the light off before turning it back on.

The answer you get for why is AC different from DC depends on how you approach the question. The power itself is different. With DC, you never have the polarity switching. You have a fixed voltage, instead of a sine wave. AC is usually 10 times higher voltage than DC. Most designs use different components for AC & DC switching.

Also remember, that dimmable AC LED stings don't run at 100%. They are blinking on and off constantly. Half wave LEDs are on half as much time as full wave, and put out 60 pulses of light per second. Full wave go to zero output at the same places the off cycle for half wave starts and ends, but they also have a pulse of light during what is the dark time for half wave lights.

[plasmadrive]

The answer you get for why is AC different from DC depends on how you approach the question.

That is what I was trying to say.... Thank you klb

And I like your answer better..

Edited January 18, 2013 by plasmadrive

[plasmadrive]

Also remember, that dimmable AC LED stings don't run at 100%. They are blinking on and off constantly. Half wave LEDs are on half as much time as full wave, and put out 60 pulses of light per second. Full wave go to zero output at the same places the off cycle for half wave starts and ends, but they also have a pulse of light during what is the dark time for half wave lights.

As well the turn on voltage threshold of the string of series wired LEDs must be achieved before they even light at all vs incans that can start to produce light at very low voltages..

[-klb-]

Separately, when people talk about LED's on LOR controllers flickering, they are usually talking about cases where instead of fading, they flicker on/off much slower, like about 3 Hz. This is actually a case of the LED strings making the controller misbehave. As long as the LED strings only have parasitic capacitance from the wiring, adding some resistive load should solve the issue. But there are strings that you just can't dim correctly. Usually they add a capacitor either to increase the number of bulbs they can have in series, or to completely eliminate the blinking. Either of those designs can't be dimmed correctly.

[TonyD]

Plasma and klb, thanks for the responses. I'm amazed how anyone ever engineered some of this stuff. As usual, what appears to be simple in reality is not, although i suspect overall this is probably pretty simple stuff when you get into EE.

On a slight tangent, i introduced a wierd flickering problem with the LED lights i have in my hallway in my house (A19 style in "cans"). I added some icicles (LED) this year to my static display outside (i only have static). They were too bright so I bought an in-line dimmer (supposedly for LED/CFL). When i dimmed the icicles my inside lights would flicker when dimmed to about 50%. Fortunately they didn't flicker at the lowest setting which i usually keep them on during the night. Some wierd combo of the 2 dimmers caused them both to flicker at some point. Certainly was annoying.

[plasmadrive]

Don't you just hate it when that happens?