LED Fluorescent Tube Replacement

 The once ubiquitous fluorescent light tubes are gradually being replaced by the LED light.  The once popular compact fluorescent bulbs are already near extinct.  Two of my 3' fluorescent tubes are not working right: they do not light up fully.  They are the F30T12 rapid start type (30W, 1.5" diameter).   The ballast is rated at 120V 0.65A and has eight wires: white and black for neural and live AC input, two tubes share one pair of yellow wires and one pair of red and blue go to the other end of the tubes with the so-call "tombstone" holders.  While I'm not sure if the tubes are or the electronic ballast are bad, replacing with LED tubes is less expensive than getting new ballast or fluorescent tubes.  

An 18W T8 (1" diameter) LED tube costs about $7; it can be installed plug-and-play or with the ballast removed.  It is rated at 100-277V 0.18A.  It claims 45W equivalent and 2520lm 6000K, which would imply 140lm/W, a little high (as a comparison, a Philip 18W puts out 2000lm 6500K). The tube is made of two rows of LEDs, total 120 individual LEDs (laid out on a PCB with groups of 5).  The back side is aluminum, where the LED PCB is mounted on, and is covered by a plastic on the front.  The two pins at each end are shorted.  The tubes work nicely when plugged into the existing fixture without modification, but I could notice some flickering and humming of the ballast.  There was a slight delay for the LEDs to turn on when starting cold; once warmed up, it started instantly.  When I measured the input current, I was surprised that it read 0.9A (that is over 100W), not only it exceeded the LEDs current by a lot (should be less than 0.4A) and but also more than the ballast rating.  I definitely need to try bypassing the ballast.  The LED tube itself draws only 0.135A, 16W; it is much better without the ballast: instant start, no humming and lower power (reduction of more than 50% with brighter light).

The common electronic ballast is a resonant half bridge with a capacitor bypass.  Initially the current follows through this capacitor and heats up the filament; a higher strike voltage is generated to start arc discharge through the tube.  Afterwards the current through the bypass capacitor and the filament is reduced and the voltage across the tube is also lowered. A transformer feedback sustains the oscillation, which is over 10KHz. Some include power factor correction. One day, I'll open the ballast to see what is inside.  The remaining question is how the LED tube is able to tolerate high voltage.  One possibility is that it has a low-pass filter to attenuate the high-frequency voltage.