Simple Current Controlled LED Tube Light Circuit Diagram

The given circuit of a current controlled LED tube light uses a couple of high voltage transistors which works on a very basic principle for implementing the required current control operation. Resistor R2 is placed for converting the rising current to voltage across itself.
This voltage is sensed by R2 which immediately conducts and grounds T1s base rendering it inactive, the instantaneous process initiates a switching effect, producing the desired current control and safeguarding of the LEDs. Each channel consists of 50 white LEDs in series. R2 is calculated with the following formula:

R = 0.7 / I, where I = Total safe current consumed by the LEDs.

The whole circuit of the current controlled LED tube light may be understood in this manner:

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,

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  1. can i use this circuit in 110Vac?

    1. same circuit or there is a modification for 110v?

    2. The LED series cannot be more than 25nos per channel, the 2uF cap may be rated at 250V for reducing the cost and space.

  2. Dear Mr.Swagatam,
    I'm Niro
    # Can you please give 01 or 02 example for calculating R1 and R2.
    # Can I use only one or two 50 white LED in series?


    1. R1 = (Supply V - Total LED fwd V) / LED required Current.

      R2 = 0.7 / LED required current

  3. Hi Swagatam,

    What will happen under the the following conditions.

    1. one of the LEDs in a string short circuits?
    2. one of the LEDs in a string open circuits?


    1. Hi D,

      Normally such conditions will never happen, under short circuit other LEDs will remain lit, under open, all LEDs in the string will shut off.

  4. hi swagatam
    can i use BC548, 547 or 549 (easily available parts)?

    1. Hi Max,

      The transistors should be able to handle in excess of 220V, BC54X can handle up to 65V not suitable here.

      You may try MJE340...

  5. i am trying to design a greeting card with a circuit and LEDs in it, but it should be like when we open the card the LEDs should be on and when we close the card they should go off, can you pleas help me with it?

  6. i am trying to design a greeting card with a circuit and LEDs in it, but it should be like when we open the card the LEDs should be on and when we close the card they should go off, can you pleas help me with it?

    1. I've noted down your request, will try to design and post it soon...

  7. Dear Swagatam,

    I want to use 50 led in 2 series(total 100 led) with 50mA/string. As per my calculation R1 is 1.6 ohm and R2 is 6.8 ohm.pls help

    1. Dear Ganesh,

      How did you get the 50mA current, is it your LED spcification?

      Anyway with 50mA, R1 = (300 - 50*3)/0.05 = 3000 Ohms or 3K

      and R2 = 0.6/0.05 = 12Ohms

  8. what should be watts for R1 I placed 1.35 k 1/2 watt for 100 ma string it burned

  9. What should be watts of R1, I connected 1.35 k 1/2w for 100 ma LED string it burned. R1 was calculated as R1=(300-3.4*50)/.1

    1. R1 = (300 - 3.3x50)hfe/0.1
      hfe = gain of transistor

      example (300 - 3.3x50)200/0.1 = 270k

  10. Dear sir,
    May i know how many series LEDs can connect maximum in parallel.
    Is there any problem if I connect 50 LEDs(50mA each) in series of 4 sets parallel connection.
    If so what i have to change in the circuit. and please provide R1 & R2 resistor values with wattage.

    Thank you.

    1. Dear Hanish,

      You can connect the said number of LEDs.
      50mA each means, 4 channels would consume 200mA

      R2 formula is given in the article

      for R1 formula you can try this formula:

      R = (Ub - 0.6) × Hfe ÷ 0.2

      Ub = supply voltage
      hfe = 40

  11. sir, could six of 100 watt LEDs (in series) run with forwarding current 900ma? and what would be the values R1 and R2 work perfectly? help me against 220 VAC .. rather runs in paraller with 3 sets each means in two channels...what you suggest?

    1. Muhammad, 100 watt leds according to me need minimum 3amps of current to illuminate optimally, so I don't think 900mA would be sufficient ...

    2. thanks for reply majumdar. then please suggest 220 AC input transformer less simple circuit which runs six of them in series or parallel.

    3. here's a simple design you can try

      connect the six 100 watt leds in series and connect their free series ends across the output of the bridge.

      the 5uF capacitor may need to be doubled for optimum brightness.

      use large heatsink for the led modules and a cooling fan if possible.

      an MOV across the mains input should be also employed for maximum safety

    4. thanks for additional help& suggestions. but let me put question in other way.

      suppose i require minimum 700mA (forwarding current) to lit up the single LED and for maximum/peak light i require 1400mA (forwarding current) and require 32 to 36V. And on 220 AC how could it be optimal?

    5. for a single 32v LED you'll need an SMPS or a transformer supply rated at 700mA or 1400mA, the above circuit in the link cannot be used.

  12. Hi Mr. Expert, how to become adjustable brightness, is it possible to replace the R2 with resistor+ normal potesiometer, thanks for the time.

    1. Hi Ronald,

      you can replace R1 with a pot and add a 100 ohm series resistor for protection....this pot can be then used for adjusting the brightness of the LEDs


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