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Electromagnetic Ring Launcher -- Building a Science Museum Classic | Design News | designnews.com

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| November 21, 2016

In science museums and physics classrooms, there is a very common demonstration called "jumping loop" or "electromagnetic loop transmitter". The experiment involved inserting a cylindrical iron core a few centimeters long into a large solenoid while a copper ring passed through the extended iron core. . When the solenoid is powered by AC power, the magnetic ring jumps out of the magnetic core.

There are many reasons why this experiment is so popular and important in science and engineering. First, it is interesting to observe the metal ring jumping out or hovering. Secondly, it uses Faraday's law of induction, Lenz's law, mutual inductance and electromagnetic induction force to make the ring hover or jump. The main problem with this conventional ring transmitter is its bulkiness and weight, because it requires a large number of thick copper wires for the solenoid and the heavier core inside. In addition, since it works with mains voltage (115 V or 230 V, alternating current), it is not safe to operate. Calculations show that the efficiency of ringing transmission is several times higher than that of AC mains (50/60Hz).

In this project, I used a square wave generator with a 555 timer IC whose frequency is adjustable from 700 Hz to 18 kHz. Its output drives the power MOSFET. The MOSFET drives a small coil of about 50 to 60 turns, which is wound on a 10 cm long ferrite cylinder instead of an iron core. The copper ring passes through the extension of the ferrite cylinder. Place a 16 micro Farad film capacitor in parallel with the coil to achieve parallel resonance. At resonance, the current flowing through the coil can reach several times the current provided by the power supply. When thick copper wire (AWG#14) is used to make the coil, the resistance of the coil will be reduced, resulting in a high quality factor (Q) of the coil. The current maintained by the high Q of the coil is nearly 8 times higher than the current that the power supply can provide. High primary currents are essential for inducing high currents in the copper ring, and the interacting magnetic field will float the ring. The circuit only needs 24V DC for floating, hovering and launching loops. A 10 ohm resistor is used in series with a 24V power supply. As the oscillator frequency increases slowly, the power supply current gradually decreases. At the resonant frequency, the power supply current reaches a minimum (~1.2 A), and at this point, the copper ring is suspended and hovering in the middle of the extended ferrite rod. The other switch is used to short-circuit the 10 ohm resistor. When short-circuited, the ring will jump out of the rod a few centimeters. Now, keep the 10 ohm resistor shorted, if you turn on the power, the loop jumps to a few tens of centimeters above the rod. This one

Show these effects.

This circuit consists of a square wave oscillator implemented by a 555 timer IC, a power MOSFET and a MOSFET driver circuit (Figure 1). This circuit requires two power supplies, a 15V, 0.8A power supply for the oscillator and MOSFET driver, and a 24V, 4A power supply for the coil.

In order to achieve a duty cycle close to 50%, the resistor R1 is chosen to be 180 Ohm, which is much smaller than R2 + R7 (minimum ~4.7k). By changing R2 from 100K to 0 Ohm, a square wave output from 700 Hz to 18 kHz can be obtained. The square wave output on pin 3 of the 555 timer IC should not be used directly to drive the MOSFET (Q3), for the gate capacitance. The MOSFET driver is implemented using two transistors Q1 and Q2. To limit the initial high gate current, R5 is used. High power and high current MOSFET (Q3) is used to drive the coil capacitor combination. The fast recovery diode D1 is used to enable the LC circuit to run freely during the off time of the MOSFET. When the MOSFET is on, an inductor of 5 microhenries (L1) is used to limit the initial large current. L1 can be easily made by winding about 40 turns on a plastic tube with a diameter of 1 cm. When the MOSFET is turned on, the energy is transferred to the LC circuit. When the MOSFET is turned off, the energy stored in the capacitor C and the coil L begins to flow between L and C.

When the switching frequency of the MOSFET matches the resonance frequency of the LC circuit, the LC circuit will use the least energy to maintain oscillation. In this case, although the current drawn from the power supply is small, the current flowing in the LC circuit is much larger. This high current generates a strong magnetic field in the ferrite core. The copper ring passing through the core serves as a low-resistance single-turn coil. The alternating magnetic field in the ferrite core induces a voltage in the copper ring, so high current also flows through the ring. These two interacting fields force the ring to jump out of the core. After working for a period of time, especially during tuning, MOSFET Q3 and diode D1 become hot. These two devices require two small heat sinks. The PCB layout of the circuit is shown in Figure 2. The 10 ohm power resistor (R8) is not shown on the PCB because it is a panel mount type. R8 should be screwed onto the aluminum housing as shown in Figure 3.

figure 2. PCB layout

image 3. Circuit board in the housing

Figure 4. Front panel

In order to make a suitable bobbin and ferrite cylinder base, an unclad FR4 board was used. As shown in Figure 5, cut and tighten several pieces. At the top, a hole was punched for the ferrite cylinder. Nowadays, since long ferrite rods are no longer common, two ferrite cylinders with holes are used. Each cylinder is 5 cm long, and these cylinders are connected by long nylon brackets and nylon screws. After making the coil, use an LCR meter to measure its inductance and resistance. Connect two ~8 micro Farad capacitors in parallel to make ~16 micro Farad, and connect this combination in parallel with the coil. Now you can use the well-known formula to calculate the resonance frequency:

The resonant frequency can also be measured using an oscilloscope and function generator. This process can be found in many places on the Internet.

The detailed specifications are as follows.

Diameter (coil former): ~27 mm, length: 16 mm, number of turns, N: 50~60, wire size: #14 AWG. Insert a ferrite cylinder with a diameter of about 16 mm into the coil (use nylon screws and supports to connect two cylindrical ferrite rods). The measured inductance is approximately 110 mH (approximately 235 mH with a ferrite core inside). The measured resistance is about 0.1W, C = 16 mF, the measured resonant frequency,

About 2.6 kHz

 Figure 5. A coil of 16 microfarad capacitors is connected in parallel. Shown is a copper ring passing through an extending ferrite cylinder.

The entire system consists of boxed circuit boards, power supplies and coils; as shown in Figure 6.

Figure 6. Photo of the complete system

In order to set the best operating conditions, the coil capacitor circuit should be set to resonance. Without using any expensive equipment, we can easily determine this situation according to the schematic block diagram shown in Figure 7.

Figure 7. Set resonance

Before connecting the 24V power supply, we must ensure that the 10 Ohm resistor parallel switch (S2) is turned on. Now, connect the 24V power supply to the ammeter in series with the circuit, and the potentiometer R2 slowly changes from high to low, which causes the frequency to change from low to high. As the frequency increases and the current decreases, we can see that the ring begins to float. At resonance, the current reaches its minimum value at ~1.2 A. At resonance, the copper ring is suspended about 2 cm above the coil. Now, if the 10 Ohm resistor is short-circuited by closing S2, the loop will jump out of the ferrite rod. Keep the switch S2 closed, if you turn the power switch S1 from OFF to ON, the ring will jump to several tens of centimeters above the rod. All these tests are shown in the video. The circuit can even operate at a voltage higher than 24V. If driven by 48V, higher jumps can be seen.

When the coil is in resonance, the oscilloscope waveform is shown in Figure 8.

Figure 8. When the coil is driven in resonance, the waveforms of the MOSFET gate, drain, coil high side and both ends of the coil.



ceramic capacitor



0.1 microfarad








Polarized capacitor






Fast recovery diode




18 volts

Zener diode




Timer IC



5 hours


~40 open 1cm diameter. plastic pipe



NPN transistor




PNP transistor





70017262 (equivalent  



180 ohm












4.7 ohm



10 ohm






4.7 thousand



10 ohm, 25W

Power resistance


2-pin terminal, 4





15V, 800 mA

Power supply 1



24V, 4.5A

Power supply 2








About 238 uH

Ferrite core coil


8.2 uF + 8.2 uF

2 parallel capacitors


2 terminals (red)


2 terminals (black)




Ferrite cylinder-2

Etiquette #2643625202

Other items needed for this project include screws, nylon screws, and connecting wires.

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