High frequency alternating magnetic field generator

[rashmie]

i have been working on generating high frequency alternating magnetic field (amf) generator (100-500 khz range) to magnetize magnetic nanoparticles contained in a plastic test-tube wound with copper wires around which amf has to be generated. what is the best option to choose for such an application (transistor/opamp/transfomer/mosfet etc??)?.. looking forward for your suggestions

 

[Bob S]

You should first estimate the required magnetic field (Tesla) and volume. This, along with the number of turns on the coil will help in designing the H-bridge driver. In general, because V = L dI/dt, getting the required peak coil current I with the applied voltage rating of the driver depends on the coil inductance, which depends on number of coil turns, volume, etc. The maximum field is proportional to the amp turns (NI) in the coil. Inductance, which is proportional to N², limits the maximum current. In general, lower frequency is better, because dI/dt is lower. Why do you need >100 kHz for this?

The driver could be either integrated H-bridge circuits (e.g., LMD182XX), or IGBTs, depending on the voltage and current requirements. Really high coil currents will need water cooled coils. Sometimes, ferrite can be used to increase the magnetic field, and Litz wire can be used to reduce coil eddy current losses.

Please describe maximum field, coil geometry, and volume.

Bob S

 

[rashmie]

thank you for the reply..we require around 20 to 50 mT magnetic field, the inductance of coil is 10.6mH and capacitance 0.6 microfarad when measured across a LC multimeter. the coil has 1500 turns made of copper wire (we used normal Cu wires due to unavailability of litz wires).to produce our required field we need around 10-150mA current in the coil.we need high frequency since we want to heat the magnetic nanoparticles(neel and brown relaxation) which is possible at higher frequencies.is there a any possibility to reduce the inductance of the coil and make it resonate at high frequency?

 

[Bob S]

Using 1500 turns, 150 mA, and a coil length (guess) of 5 cm, for an air-core solenoid, I calculate a peak field of only about 5.6 mT (using the simple solenoid formula). See

http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html

The calculated inductance is 17 mH. See

http://hyperphysics.phy-astr.gsu.edu/hbase/electric/indsol.html).

Tell me more about your geometry and dimensions. With 1500 turns, you will have resonances in the audio range due to self (inter-turn) capacitances, so the coil will never go to 100 kHz.

Bob S

 

[rashmie]

so how can the coil be modified to get an alternating magnetic field in the high frequency range(kHz)?

 

[Bob S]

How did you get 50 mT with 150 mA and 1500 turns?

What are the dimensions of the plastic tube with sample (length, diameter)? What is the duty cycle, on time, off time? What is the coil geometry (solenoid?)? Are you using any iron or ferrite, or is it air core? Does your magnetic sample have a permeability >1?

What electronics do you have available; signal generator, ac voltmeter, dc power supplies, etc.? Do you have access to electronic components, like LMD18200, HIP4080A, etc.?

Bob S

 

[Bob S]

Here is a circuit to think about. A capacitor C is charged to a voltage V, and discharged into an inductor L. The peak current in the inductor is I = V sqrt(C/L). The resonant frequency is f = (1/2pi)sqrt(1/LC). Very approximately the peak field(Tesla) in the coil is B =μ₀NI/z, where N = number of turns and z is coil length. See simulation in thumbnail. For L = 18 uH, C = 0.14 uF, V = 1000 volts, the peak current is about 85 amps, and the frequency is 100 kHz. For a 50-turn two-layer coil 4 cm long and 1.8 cm diameter, the peak field is about 130 mT.

This circuit is very similar to the circuit used in CD (capacitor discharge) automobile ignitions.

Bob S