driver circuit (Fig.1). The circuit needs two power supplies, a 15V, 0.8A supply to deliver power to the oscillator and MOSFET driver and a 24V, 4A supply is powering up the coil.
To achieve near-50% duty cycle resistor R1 is chosen 180 Ohm, which is a much smaller value compared to R2+R7 (minimum~4.7k). By varying R2 from 100K to 0 Ohm, square wave output is obtained from 700Hz to 18 kHz. This square wave output at pin-3 of 555 timer IC should not be used directly to drive the MOSFET (Q3), for the gate capacitance. A MOSFET driver is implemented using two transistors, Q1 and Q2. To limit the initial high gate current, R5 is used. A high power and high current MOSFET (Q3) is used to drive the coil-capacitor combination. A fast recovery diode, D1 is used to leave the LC circuit to run free during the OFF time of the MOSFET. An inductor of 5 micro Henry (L1) is used to limit the initial high current, when the MOSFET is switched ON. This L1 can be made easily by winding ~40 turns on a piece of plastic pipe of 1cm diameter. When the MOSFET is ON energy is delivered to the LC circuit, when the MOSFET is OFF, the stored energy in the capacitor C and coil L begins to flow in-between L and C.
When the switching frequency of the MOSFET matches the resonance frequency of the LC circuit, minimum energy is used by the LC circuit to sustain the oscillation. In this situation, though low current is drawn from the supply, much higher current flows in the LC circuit. This high current creates intense magnetic field in the ferrite core. The copper ring which runs through the core, acts as a single turn coil of low resistance. The alternating magnetic field in the ferrite core induces a voltage in the copper ring, thereby high current flows through the ring as well. These two interacting field forces the ring jump out of the core. The MOSFET Q3 and diode D1 gets heated after sometime of operation, especially during tuning. Two small heat sinks are needed for these two devices. The PCB layout of the circuit is shown in Fig.2. The 10 Ohm power resister (R8) is not shown in the PCB, as it is a panel mounted type. R8 should be screwed to aluminum enclosure as shown in Fig.3.
Fig.2. PCB Layout
Fig.3. Circuit-PCB inside enclosure
Fig.4. Front panel
The Coil with Ferrite Core:
To make a suitable coil holder and a base for the ferrite cylinder, uncladed FR4 sheet was used. Several pieces were cut and screwed as shown in Fig.5. On the top piece, a hole was made for the ferrite cylinder to go in. As long ferrite rods are becoming uncommon now a days, instead two ferrite cylinders with hole inside were used. Each cylinder is 5 cm long, these are joined by a long nylon stand-off and nylon screws. After the coil is made