When input AC is applied to the circuit, C1 drops the input current down to a lower level which can be considered to be safe for operating the involved electronic circuit.
The diodes rectify the low current AC and feeds to the next current sensing stage consisting of T1 and T2.
Initially T1 is biased through R1 and conducts fully illuminating the entire array of LEDs.
As long as the current delivered by T1 or rather current drawn by the LEDs is within the specified safe limit, T2 remains in a non-conducting state, however of the current drawn by the LEDs begins to cross the safe limit, the voltage across the limiting resistor R2 begins to develop a small voltage across it.
When this voltage exceeds 0.6, T2 begins to leak through its collector emitter pin outs.
Since the collector of T2 is connected to the base of T1, the biasing current to T1 now starts leaking to ground.
This inhibits T1 from conducting fully and its collector current stops rising any further. Since the LEDs form the collector load of T1, the current through the LEDs also gets restricted and the devices are safeguarded from the rising current intake.
Ths above rise in the current takes place when the input AC rises, producing an equivalent increase in the LED current consumption, but the inclusion of T1 and T2, ensures that anything that's dangerous to the LEDs is effectively controlled and curbed.
Parts List for the proposed current controlled LED tube light circuit
T1 and T2 = KST42
R1, R2 = To be calculated.
R3 = 1 M, 1/4 W
Diodes = 1N4007,
C1 = 2 uF / 400 V,