Re: LED Flicker

On Tue, 24 Jul 2007 15:40:09 GMT, "TKM" <nomail@xxxxxx>
wrote:

Working to mount a group of fixtures for an LED lighting fixture design
competition recently, I noticed what appears to be a wide variety of designs
for the LED power supplies and drivers. Some are 2-piece units (and rather
heavy -- transformers, I suppose); others are small, obviously electronic
and similar to computer and small electronic device drivers.

What's important though is that some of the drivers cause the LEDs to
flicker -- strobe is maybe a better word. There's a definite effect on the
quality of the LED light. Now that we've gotten rid of flicker from
fluorescent lighting, are we going to go through the same problems with
LEDs?

Who knows something about the types of driver circuits that are out there
and can reference or discuss what lighting specifiers can do to avoid
problems.

Terry - there are good reasons other than flicker reduction
to filter the rectified DC voltage in a fluorescent lamp
ballast. The key one is that fluorescent lamps will
operate at lower efficacy when they are run from a 120 Hz
modulated HF power source. (The efficacy loss is about 5%
for an F32T8 lamp.)

The 120 Hz modulation is eliminated or greatly reduced by
using a large electrolytic capacitor as an energy storage
device after the rectifier portion of the DC power supply of
the ballast. This capacitor introduces three problems:

1) It is often the largest component in the ballast.

2) It has the shortest life and sets the life of the
ballast.

3) It creates line current distortion that in turn creates
low input power factor. The user than has to either live
with this power factor or pay more money and give up perhaps
1% in efficacy to have a high power factor front end added
to the ballast.

Based on this issues it is no wonder that LED driver
manufacturers do not want to add the energy storage
capacitor unless it is necessary.

I suspect the only thing that light specifiers can do is to
require that the flicker index be below some value. Then
they need to measure the luminaires to make certain they
comply.

It would be interesting to see if Energy Star is going to
have a flicker requirement in the new LED luminaire spec. I
suspect not.

--
Vic Roberts
http://www.RobertsResearchInc.com
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