# How to maximize magnetic field strength?

#### praondevou

Joined Jul 9, 2011
2,942
I'm trying to build a circuit that uses a MOSFET fullbridge driving a LC parallel tank circuit (with a smaller inductor in series to smooth current).

The main inductor is an air coil whose magnetic field is supposed to be picked up by another resonant LC.

I'm in doubt in how to maximize the magnetic field strength for a given DC input voltage (12V) and oscillator resonant frequency. Right now I'm using 375uH and 4.7nF, i.e. around 120kHz resoncance frequency.

I'm aware that increasing the number of turns should increase the magnetic field too, but it will also lower the quality factor...

How can I find the best combination of inductor and capacitor value?

Joined Dec 26, 2010
2,148
Since the bridge driver output impedance may be quite low, it may be easier to obtain an undamped result with a series resonator. If the Q can be made as large as possible, this will increase the voltage magnification of the system, so as to obtain the largest possible coil current for a given inductance and supply voltage.

#### strantor

Joined Oct 3, 2010
6,042

#### jimkeith

Joined Oct 26, 2011
540
I believe that the number of turns will make no difference as the ampere turns at the same resonant freq will be constant.

My best guess is that if you shape the field by stretching the coil, the effective range will increase--the same goes for the receiver. Of course, stretching the coil will lower the inductance so re-tuning will be necessary by reducing the capacitance.

I remember fooling around with a couple of coils in lab about 50 years ago--we were able to receive a measurable signal at about 5 ft. without much effort.

A good active filter /amplifier at the receiver should multiply this--also crank up the voltage...

Make the limiting inductor about 10% of the inductance of the tank inductor--it could be mutually coupled, but loosley so the square wave harmonics are still limited--mutual coupling it by adding it off of the end of the tank will further focus the field lengthwise.

'6 step' waveform will reduce losses without significant effect on the fundamental.

Good idea adjuster--didn't think of the series resonant config--get some pretty high currents and higher voltages and higher ampere turns...

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#### thatoneguy

Joined Feb 19, 2009
6,359
point the coils at each other, and as close together end to end as possible, such as interleaved.

#### praondevou

Joined Jul 9, 2011
2,942
Since the bridge driver output impedance may be quite low, it may be easier to obtain an undamped result with a series resonator. If the Q can be made as large as possible, this will increase the voltage magnification of the system, so as to obtain the largest possible coil current for a given inductance and supply voltage.
I do not follow. I'm aware of the difference of series and parallel tank circuits. But how will I find the best combination of C and L, i.e. if increasing the inductor value will increase the magnetic field (for a given DC input voltage), how can I determine the best inductor value? There is surely a point where it's resistance becomes so high that it doesn't radiate at all. Or am I missing something here?

There are surely some formulas to get there, but I can't figure it out...

#### praondevou

Joined Jul 9, 2011
2,942
Simply speaking , why would I choose 4.7nF and 375uH instead of 1nF and 1.76mH? for example, there has to be a reason why engineers choose a certain amount of turns for induction heaters and induction ovens etc...

I believe that the number of turns will make no difference as the ampere turns at the same resonant freq will be constant.
What about wire resistance?

point the coils at each other, and as close together end to end as possible, such as interleaved.
Yes the directivity of the coils is important. The question is how to find the point of max emitted field strength for a given frequency and input voltage.

I remember reading something about that in this tutorial.
Yes that's the theory behind it.

However he wrote "The AC power drives a transformer which is coupled to a series LC tank. The inverter frequency is set to the tank's resonant frequency, allowing the generation of very high currents within the tank's coil."

I thought in a series resonant LC the voltage on each component can be MUCH higher, in a parallel it will be current that will increase inside the tank circuit...

I thought the advantage of a parallel tank circuit would be that the driver bridge doesn't has to support the current that goes through the coil. It doesn't matter for my circuit though, since I don't need much current.

Joined Dec 26, 2010
2,148
I do not follow. I'm aware of the difference of series and parallel tank circuits. But how will I find the best combination of C and L, i.e. if increasing the inductor value will increase the magnetic field (for a given DC input voltage), how can I determine the best inductor value? There is surely a point where it's resistance becomes so high that it doesn't radiate at all. Or am I missing something here?

There are surely some formulas to get there, but I can't figure it out...
It really depends what you are trying to do. If you are trying to make an induction heater, you would be better to stick to the sort of circuits normally used for that purpose, as they are quite specialised.

I had imagined you were trying to induce a large voltage in a secondary coil, in which case there might be an interest in getting some magnification out of the primary circuit by making it series resonant. (In which case, it would be important to use components whose reactances at the working frequency are many times the driving point impedance.) The voltage increase obtained this way would however also be dependant on the component Q values, and generally this may be a less predictable and reliable arrangement.

#### praondevou

Joined Jul 9, 2011
2,942
Since the bridge driver output impedance may be quite low, it may be easier to obtain an undamped result with a series resonator. If the Q can be made as large as possible, this will increase the voltage magnification of the system, so as to obtain the largest possible coil current for a given inductance and supply voltage.
It's not an induction heater, it's a wireless charger.

Following your suggestion I just tried it with a series tank LC and yes the results are promising. I get a much higher output voltage at the receiving coil. I guess I had too much of a loss on the series L.

Only problem, I have to find high voltage caps now...

At 3V DC bus voltage on the bridge I have 112V on my caps. I think their voltage rating was 50V...

I know that I'll go the easy way by not trying to find the best LC combination, but just use something that works the way I need.

However, GREAT!

Thanks

#### thatoneguy

Joined Feb 19, 2009
6,359
Yes the directivity of the coils is important. The question is how to find the point of max emitted field strength for a given frequency and input voltage.
Inductors act a bit like a circular polarized antenna (like the helical antenna used for satcom in the movies). Pointing two circular polarized antennas at each other, particularly if they are wound in the same direction (mount inductors so the coils look like a continual screw), will add to the coupling coefficient.