This approach may well shorten the lifetime of the string of LED's. The LED's are brighter because the duty cycle has increased from about .5 too near 1, the average current has increased by a factor of 2 and device (LED) heating has certainly increased. It appears in the video that one of the LED's in the demo string of white LED's has been replaced with a red LED. Was this due to failure of the white LED operating under the more stringent conditions?
Light emitting diodes are becomming very popular these days people are using them instead of other lights because they are cost effective .Because leds are connected to the AC Supply it produces ripples at the outer end which results in flickering.Any light source which is connected to AC supply results in flickers.
However flickering can be reduced with different methods one method is to use high frequency switching supplies, which helps to reduce the AC component of the main supply and reduces the flickers but not all flickers are avoidable .Remaining flickers can be removed by attenuating g the AC component at the output
The answer to just about any why question in this context is going to be money. Granted, the rectifier is quite inexpensive. The margin on such products is probably quite low - adding the rectifier while staying cost-competitive won't work.
There's also the cost of getting this now-new product tested by UL so a company can affix the all-important UL label.
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