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Mains 20 Watt Electronic Ballast Circuit

Mains 20 Watt Electronic Ballast Circuit

The post explains a simple 20 watt fluorescent ballast circuit using a single chip FAN7711 from Fairchild semiconductor.

The proposed mains 220V operated, 20 watt fluorescent ballast circuit is built around an LCC resonant tank and a half-bridge network.

Implementing Zero Voltage Switching

The main features and operations may be understood with the following explanation:

In order to execute a zero-voltage switching (ZVS) through the half-bridge inverter circuit, the LCC undergoes a high frequency operation beyond the resonant frequency, fixed by the components L, Cs, Cp and RL where RL is the equivalent to the lamps impedance, and it also crucial with the transfer operation of the LCC resonant tank circuit.

The in-built oscillator stage inside the IC FAN7711 is capable of generating optimal driving pulses in order to implement effective lamp ignition and enhance lamp life.

During the procedures, the oscillation frequency goes through the following transitions:

Preheating frequency > Ignition frequency> normal running frequency.

Initially, the lamp impedance is relatively high, but once it’s ignited the impedance is brought down substantially. Due to this high initial impedance, the resonant peak could be very high too, owing to this the lamp is fired at a higher frequency than the resonant frequency.

During the operations, the current fundamentally flows via Cp which becomes responsible for linking up the two filaments of the tube and producing ground path for the passing current.

In the course, the passing current preheats the filament for a quick striking.

The amount of amps that may be required for this is adjustable and could be done by setting up the capacitance of Cp.

Calculating Preheating Frequency

The preheating frequency which forms the driving frequency may be expressed as:

fPRE = 1.6 x fOSC

As soon as the above preheating is concluded, the IC pulls down the frequency, increasing the voltage to the lamp for executing the intended ignition of the lamp.

The ignition frequency which is also the function of CPH voltage may be written as:

fIG = [0.3 x (5 – VCPH) + 1] x fOSC

Where VCPH is the voltage rating of the capacitor used

Once the above operations are implemented, the lamp witnesses a constant frequency drive via an external resistor Rt for the required sustained illumination.

Circuit Diagram


Parts list for the above 20 watt electronic ballast circuit



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I am an electronic engineer (dipIETE ), hobbyist, inventor, schematic/PCB designer, manufacturer. I am also the founder of the website: https://www.homemade-circuits.com/, where I love sharing my innovative circuit ideas and tutorials. If you have any circuit related query, you may interact through comments, I'll be most happy to help!

10 thoughts on “Mains 20 Watt Electronic Ballast Circuit”

  1. dear sir, i couldn't find this ic, and i don't have coiles , can you give me ballast circuit for 15-20w flourcent, i already have one. But it needs 6.8nf 2kv capacitor, i couldn't find it too

  2. Thanks for the post. What is the main supply used in this circuit? Is it possible to substitute the schematics to the left of the circuit with a 12VDC supply (or other DC value)? I have yet to select my tube – which components would need to be modified for a tube with different specs? Thanks is advance.

  3. hi, swagatam can i use 40w tube in this circuit? also i have similar ic called IR2153 could use in this configuration?

  4. Sir can u do for me a high frequency smps inverter for electric fishing used 12 car battery , inverter used in minimum 1.5 metre dept water respectively , shocked by out put connected in two aluminum rodes used a calling bell switch and rodes dipped switch on shocked, sir its the working , can you sent me circuit this id or published here ! arundas118@gmail.com

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