Introduction: Electronic Ballast
This Compact, lightweight, high-frequency electronic ballast is designed to operate off the 230V AC mains supply at 50-60 Hz and is suitable for 36 – 40 W fluorescent tubes.
The advantages of such a ballast are:
- Saving in Energy
- Longevity of tube
- Low noise
- Operates at low voltages
- No Flickering
- No heat
- Instant-Start operation
- Minimal Weight
- No Starter Required
This unit cost me less than $2 to build.
Step 1: Caution
This unit operates off the AC mains with a voltage of 230 Volts and voltages generated within the unit can reach 600 to 800 Volts. Extreme caution must be exercised while building, testing and using this unit.
At a minimum a VARIAC (Variable AC transformer), Isolation transformer and meters for monitoring the input voltage and current are required during initial testing of the prototype units.
It is strongly suggested that you go through the excellent Instructable "Work safely with high voltage" by kovo:
Warning & Disclaimer:
All content provided here is for informational purposes only. I make no representations as to the accuracy or completeness of any information. I will not be liable for any errors or omissions in this information. I will not be liable for any losses, injuries, or damages from the use of this information.
Step 2: Circuit and Description
The schematic of the unit conforms to that of a “Half-Bridge Resonant Converter”, and operates as follows:
The AC mains input is fed at AC1-AC2 and full-wave rectified by diodes D3, D4, D10 and D11. The rectified output is filtered by the high-voltage capacitor C5 to form the DC supply voltage to the half-bridge converter.
A mid-point DC voltage is formed by the diode-capacitor network D2, D5, D8, D12 and C2, C7.
The half-bridge transistors Q1, Q2 effectively switch the load between the collector junction of Q2 and the junction of diodes D5 and D8. High frequency filtering of mid-point DC voltage is provided by capacitors C1 and C6.
The effective load is a series combination of the base-drive toroid-transformer, resonant-inductor and florescent tube.
The series-resonant frequency is determined by the 1.3mH inductor and the series combination of capacitors C3 and C4. The circuit resonates at a nominal 60 kHz.
The toroid transformer provides the base-drive to transistors Q1 and Q2 in the correct phase so that the transistors switch on-off alternately.
Resistors R4 and R9 provide a dc-bias path for Q1 and Q2. R3 and R8 limit the base drive and resistors R5 and R10 limit the collector currents of Q1 and Q2.
To ensure start-up the voltage across C8 builds-up through R1 and R7 till it crosses the breakdown voltage of DIAC D6. A sharp pulse is fed to the base of Q2 when this occurs. D9 inhibits this triggering once the resonant-converter starts-up.
On initial start-up a very high AC voltage is impressed across the fluorescent tube terminals TL1-TL2 the tube ionizes and breaks down setting up the discharge between the terminals. Once this occurs a voltage of 80-100 volts across the tube is sufficient for generating the light output. This voltage and the current through the tube are regulated by the inductor L1.
Step 3: Waveforms
The half-bridge output drive drive voltage to the load at Pin 4 of the toroid transformer is a 60 kHz nominal square-wave modulated by the AC ripple voltage at 50 Hz.
Mid-point DC voltage:
The mid-point voltage of approximately 200 Volts DC with a ripple of 200 Volts at 50 Hz is seen.
The input-voltage seen by the toroid-transformer is 18 V p-p at a nominal 60 khz.
The resonant-voltage at the tube terminal TL3 is 600 V p-p at 60 kHz.
Step 4: Bill of Materials
The table provides the bill of materials required for this unit.
Step 5: Wound Components 1 - Toroid Transformer:
This toroid-transformer provides the drive to transistors Q1 and Q2 in the correct phase and amplitude to create a positive-feedback system which oscillates at the resonant frequency. The transformer is wound using single-strand insulated wire with primary of 4 Turns and two secondary’s of 5 Turns each. The toroid is a ferrite core from Mahindra-Hinoday with part number T 10 X 3 and material MSB5S. The direction of winding should be exactly as shown in the images.
Step 6: Wound Components 2 - Resonant Inductor:
Is a wound inductor with air-gap using an EE core from Mahindra-Hinoday with part number EE 19X8X5 and material MSB5S. A 0.33 mm air-gap is created by pasting three layers of standard insulation tape on the three legs of the EE core. The winding is 130 Turns of 2 X #38 gauge insulated copper wire. The inductance value is 1.33 mH and resistance 1.9 Ohms.
Step 7: Printed Circuit Boards:
The images show the component placement details both by value and name.
The physical placement of components on the wired PCB is also shown.
The Choke_BOT.pdf file gives the 1:1 size of the PCB bottom on A4 size paper. This can be used to fabricate the PCB.