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09-30-2016, 03:33 AM   #16
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QuoteOriginally posted by phoebus Quote
Yes they do, if they are high output (ie any used for lighting) - the pulsing is generated 'on-chip' to reduce current consumption and over-heating.
On chip, never heard of that. Do you have any examples?

09-30-2016, 05:46 PM   #17
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QuoteOriginally posted by Gimbal Quote
On chip, never heard of that. Do you have any examples?
By using the the ' ' around 'on-chip' I was implying effect rather than actual. If you buy an LED lighting module the function is generally pacakaged with the actual LED chip.

For instance the Cree website provides datasheets for their LED chips, all of which have something along the line of: .Pulse width ≤0.1 msec, duty ≤1/10
This would imply at 1000Hz the LED would be powered for 0.1ms, or 10% of the time. That is a driver requirement that would be between the supply and the LED. Give the chip pure DC and it will be toast.

Does that make sense now?

;-)
10-01-2016, 01:25 AM   #18
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QuoteOriginally posted by phoebus Quote
By using the the ' ' around 'on-chip' I was implying effect rather than actual. If you buy an LED lighting module the function is generally pacakaged with the actual LED chip.

For instance the Cree website provides datasheets for their LED chips, all of which have something along the line of: .Pulse width ≤0.1 msec, duty ≤1/10
This would imply at 1000Hz the LED would be powered for 0.1ms, or 10% of the time. That is a driver requirement that would be between the supply and the LED. Give the chip pure DC and it will be toast.

Does that make sense now?

;-)
Not really.
That LED is an RGB led, where you can drive each of the red, green and blue colors independently to get different colored light. And according to the data sheet you feed it 25mA to the red channel and 17mA to the green and blue respectively, that is constant DC if you want white light. Each channel can take 65mW (not a power LED apparently). As a user you must mount the LED with sufficient cooling, not a big problem in this case though.

If you want different colored light (using an RGB LED), or dim the light you should use pulse with modulation (PWM). Since merely lowering the DC current will (if we use a single white power LED) change the temperature of the light. And thus we use PWM to drive the LED at the specified current so that the color temperature stays right but the total amount of light will be less.

So usually PWM is used to dim LEDs, one can of course drive more current through the LED as long as the total wattage stays within limits. But as far as I know that is not the normal way of driving a LED used for normal lighting.
10-01-2016, 01:41 AM   #19
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You can use the following free app to check on the flicker level of your LED:
Flicker tester for iPhone | Viso Systems

10-01-2016, 02:48 AM   #20
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It's absolutely the case that PWM is used to reduce power consumption and heat in a large number of LED lighting applications as standard - including at full rated illumination levels (e.g undimmed). The exact implementation varies between bulbs.

Therefore (as with fluorescent lights, or TV screens) photographers will need to be careful around these illumination sources.
10-01-2016, 01:37 PM   #21
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QuoteOriginally posted by Gimbal Quote
Not really.
...If you want different colored light (using an RGB LED), or dim the light you should use pulse with modulation (PWM). Since merely lowering the DC current will (if we use a single white power LED) change the temperature of the light. And thus we use PWM to drive the LED at the specified current so that the color temperature stays right but the total amount of light will be less. ...
Gimbal, you seem to completely misunderstand how LEDs produce light. They can only produce one wavelength, regardless of power. To produce a 'white' LED, there are two options.
  1. Have a triplet of red, green and blue LEDs of suitable intensity to simulate white
  2. Have a single UV LED with a suitable phosphor coating

Option 2 is the most common solution, with the higher energy UV light photons from the LED stimulating the emission of lower frequency photons from the phosphor mix. The white balance of the output is purely dependant on the mix of phosphorescent materials in the phospor - just like a fluorescent tube. The light pulsing is not completely on/off, as the phosphor exhibits some afterglow.

Option 1 gives control of the white balance through the intensity of the different coloured LEDs.

In both cases, the white light is an incomplete spectrum, unlike an incandescent lamp (or the Sun).
10-02-2016, 07:31 AM   #22
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QuoteOriginally posted by phoebus Quote
Gimbal, you seem to completely misunderstand how LEDs produce light. They can only produce one wavelength, regardless of power. To produce a 'white' LED, there are two options.
  1. Have a triplet of red, green and blue LEDs of suitable intensity to simulate white
  2. Have a single UV LED with a suitable phosphor coating

Option 2 is the most common solution, with the higher energy UV light photons from the LED stimulating the emission of lower frequency photons from the phosphor mix. The white balance of the output is purely dependant on the mix of phosphorescent materials in the phospor - just like a fluorescent tube. The light pulsing is not completely on/off, as the phosphor exhibits some afterglow.

Option 1 gives control of the white balance through the intensity of the different coloured LEDs.

In both cases, the white light is an incomplete spectrum, unlike an incandescent lamp (or the Sun).

And the link to the cree data sheet was to an RGB LED (option 1 in your list), and as I tried to describe you feed it DC current to all three LED's to get white light. If you want another color or dim the light you use PWM instead of lowering the DC current. You either dim all three colors or only one or two to create different color mixes. You CAN lower the DC-current as well but if you connect several RGB LEDs in parallell chansens are that the output from the different LEDs will be uneven compared to when using PWM.

Anyhow, cheap LED bulbs use cheap drivers, a simple rectifier with a capacitor that will produce "DC" with a lot of ripple depending on the size of the capacitor (typical for some of the filament led lights) and some of them flicker a lot. Other ones have more advanced DC drivers and produce a steady light.
10-02-2016, 07:56 AM   #23
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A couple of things that could be causing this noticeable flicker
Agreed LED lights operate off low voltage DC but unless the rectifier and condensing filters in the power supply is very clean the lights will flicker and even with today's technology producing a clean flicker free power supply would be cost prohibitive. The smoothness of the DC produced in the light power supply is also affected by transient frequencies passed through the Neutral Power line from just about every other thing, especially as motors, plugged in the building.
I also noticed that it wasn't mentioned to verify that the light Flicker Reduction frequency is set to 60HZ.

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