Electronics > Beginners
NFC field generator
T3sl4co1l:
Yes, 13.56 is the free ISM band in that range. It's rather narrow though, which severely limits applications: induction and dielectric heating needs an active tuner (mmm, servo driven vacuum variable caps!) or an amp with a huge tolerance for SWR; NFC and other transmissions must be low bitrate; etc.
NFC at least being near field might permit more data rate up close (it could also be driven with QAM rather than ASK, giving more bits per symbol), while still meeting the limits in the adjacent shortwave bands; although I don't think anything can do that. If nothing else, a subtle modulation scheme wouldn't be easy to manage with self-powered hardware.
Tim
judge:
I'm not getting anywhere making an amplifier using a single IR530, either in LTSpice or in reality. Reality matches what I get in LTSpice though (which is at most 1:1), which is always nice. On the other hand, I did manage to light up the tags by adding capacitance in series with the coil to form a resonant circuit:
David Hess:
--- Quote from: Berni on November 28, 2018, 03:50:11 pm ---You also might want to have a capacitor wired across your coil and tune it to a value that makes the capacitor and coil oscillate as a LC circuit at the desired frequency. Take it step by step and play with it until you get it working.
--- End quote ---
Making the coil a tuned circuit allows for much higher circulating current between the coil and capacitor while lowering the current through the amplifier so a less powerful amplifier can be used.
Berni:
--- Quote from: judge on December 26, 2019, 11:47:38 pm ---I'm not getting anywhere making an amplifier using a single IR530, either in LTSpice or in reality. Reality matches what I get in LTSpice though (which is at most 1:1), which is always nice. On the other hand, I did manage to light up the tags by adding capacitance in series with the coil to form a resonant circuit:
(Attachment Link)
--- End quote ---
Yes resonant LC circuits are the way to go for wireless power transfer.
The resonant nature of it causes the amplitude of the signal to rise significantly despite a small input power. This is because such circuits can 'recycle' energy really well. The inductor and capacitor exchange energy back and forth without you providing any input power. The amplitude of the oscillation only dropping slowly according to resistive looses and as much energy is transferred out to something else. So you only need enough input power to cover the system looses + transferred power.
Same trick tends to be used on the receiving side too. Being a resonant LC circuit it can develop a large amplitude from a tiny input power, so very little magnetic coupling is needed to the transmitting coil in order to develop enough voltage for powering a LED. However this only works well if both the transmitting and receiving circuit are tuned to the same frequency.
The lower losses you can get in your transmitting tank circuit (Low resistance coil, high Q caps etc) and the closer you can match the transmitter and receiver frequency the less input power you will need to get the same range/performance.
judge:
Further messing around with LTSpice shows me what kind of circuit I need to make the amplifier work, but with the frequency, peak-to-peak voltage and possible bias voltage I have available, the IRF530 isn't much use. The peak-to-peak input signal is around 2V, which just doesn't seem to be enough to completely turn the mosfet on/off.
A mosfet I've used for other projects - the AO4294 - is much faster and has a lower gate threshold voltage. I might see if I can get that to work in LTSpice, but does anyone have ideas for a plan B? I want the circuitry to be small and unobtrusive - this is for a wearable project.
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