Dear community,

I have a 1mH coil and want to drive it in resonance digitally controllable between 20kHz and 30kHz with up to 1Amp P2P. Any Idea's how to achieve it? My idea's so far:
- half or full bridge mosfet drive with PWM from DSP/uC and fixed C (i.e. 60nF for 20kHz) works well
- varactors / variable would be nice, but I don't have experience with them
--> they seam not to be available for this capacitance range (rather at pF)
--> I would need something like >10V and in the range of >100mA which seams also too much

Any ideas / hints how I could achieve this ?
BR, BastelBaus

 

Would small frequency steps be OK? I do not have an easy answer as to varying capacitance by nF otherwise. I believe you are correct that varactor diodes work in the pF range, and probably only with low signal levels.

I am proposing binary steps switching added capacitors in parallel.

 

Thanks!, this was also my thought, 16 steps with 4 fets/caps might work but is a lot of components …

 

Or maybe two opamp and variable resistor as capacitance multiplier?
https://www.ednasia.com/synthesize-variable-in-circuit-rs-ls-and-cs/

 

Boundary_A_La_Mode you might be right, some more details;

I want to rebuild this 3D coil sender: https://www.amfitrack.com/ (or link)
- 3 coils not 1
- currents ~0.5A, frequency 20..30Khz video@1m11s

- in 32 steps link


video@51s:

Or probably the same IC: video@2m32s


There are three lines with the same IC (2*7 pins + 2*2 pins) in the first video followed by several different caps and the 3D coil afterwards (and one small 6pin IC device, might be a half bridge?) . What kind of IC is this 18pin device? Does anyone know it ?

BR,
BastelBaus

 

You want to drive a coil to resonance?? Digitally?? All we are given is an inductance value of one millihenry.

What resonant frequency have you chosen??
A plain inductance seldom resonates. A capacitance is required to achieve resonance.

 

You want to drive a coil to resonance?? Digitally?? All we are given is an inductance value of one millihenry.

What resonant frequency have you chosen??
A plain inductance seldom resonates. A capacitance is required to achieve resonance.

Dear MrBill2,
please see my first post, 20 to 30kHz variable, with this 1mH between 60 to 160nF series capacitance to achieve the resonance.
So the my question how to vary the capacity in this range to let about 500mA flow trough he coil and variable capacitance leading to a voltage at the cap of about +/-50V.
BR, BastelBaus

 

OK, there is an exact formula for calculation of the resonant frequency given the inductance and capacitance values. So solve the equation for the capacitance. then use the stated inductance and frequency. That will give you a capacitance value that will be close. Then you can use a signal generator to determine the actual resonant frequency, and if more capacitance is required use an adjustable mica compression capacitor to set the exact frequency.

 

OK, there is an exact formula for calculation of the resonant frequency given the inductance and capacitance values. So solve the equation for the capacitance. then use the stated inductance and frequency. That will give you a capacitance value that will be close. Then you can use a signal generator to determine the actual resonant frequency, and if more capacitance is required use an adjustable mica compression capacitor to set the exact frequency.

Thanks William, then using a stepper motor with a driver would make it electrical controllable scnr …. Varactors are the parts normally used which have a variable capacitance depending on the dc voltage applied. But they do not exist in the 50nF range I need. So the other option might be using analog muxes to switch a capacitance bank as Boundary proposed …
BR
BastelBaus

 

Normally when a resonant circuit is used the inductance is selected to allow a convenient capacitance value to achieve resonance. Starting with a fixed inductance may lead to requiring some rather inconvenient capacitance values.

 

Normally when a resonant circuit is used the inductance is selected to allow a convenient capacitance value to achieve resonance.

I could see where it might be easier to tune an inductor than a large value capacitor. Older forum participants have probably seen variable capacitors (in AM broadcast radios for example) but I think those are mostly in the picofarads. I have not investigated but in theory I would think that a ferrite or other magnetic core could be moved to vary an inductor in the uH range.

 

See

 

Looks amazing Bordodynov. How is this called and where can I look how this works?

 

I used the Qspice simulator. There are special materials for rebuilding resonant circuits at high frequencies, but regular ferrites will do, but with a smaller effect.