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| How to drive small inductive loads up to RF? |
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| horror-vacui:
--- Quote from: geggi1 on June 26, 2022, 08:56:55 pm ---A former colleague of did some testing with a audio amplifier. I believe it was class D or E amp. The amplifiers are pretty robuste and wold most likely be able to handle your tests as long as you dont drive it to hard. --- End quote --- That's true, but I am sure that I would need a higher frequency, than the audio range. |
| dmendesf:
You need a power RF amplifier. You can test some used for cable TV: BGY888 for main frequencies (50-900MHz) or R2005240P12 for reverse frequencies (5-45MHz). There are others. |
| horror-vacui:
--- Quote from: Bud on June 26, 2022, 08:01:39 pm ---I do not think what you want is feasible. Especially given the "tiny coil" requirement. --- End quote --- That's why I want to check it out. My calculations indicate that it could work, now I just want to test those calculations. The received voltage is proportional to the "effective area" x "frequency" product, so there is a chance for power transfer with a small coil. I am not sure how well it works, but EM micro announced an passive UHF RFID tag chip with integrated coil packaged into a 1.5 x 1.5 mm chip. That is tiny as well! They claim 10mm readout distance. |
| T3sl4co1l:
Easy. Don't. Resonate with a cap. That's what everyone does, and with very good reason! The fundamental problem is the very poor coupling between coils. Which means reactance dominates over real power transfer, by a factor (Q factor) of... 2, 5, 10 or more -- roughly, the inverse of the coupling factor. With judicious use of ferrites (pole pieces and shielding), you can get a Q factor under 1, but only when things are very closely spaced, and the alignment must be pretty close as well. Basically, you get something like a split-bobbin transformer in that case, and any air gap or misalignment counts as air gap between windings -- at expense of coupling. Works for something like a cordless phone or toothbrush in a cradle, not so much for random sized items on a charging pad. Resonance reduces the control problem to a matter of frequency, amplitude and/or phase control. Whereas direct drive simply draws whatever current the reactance does (again, because reactance is dominant), resonant can draw much more power (from the inverter), because load resistance acts to dampen resonance, and its absence therefore requires a control strategy. In exchange, you have a lot of time to solve the control problem: rather than the per-cycle control that a lot of switchmode controllers employ, you can spend several cycles working out what frequency to choose, or amplitude to modulate to, and merely a peak current fault (latched) will do to protect the inverter. This is because bandwidth is also determined by Q factor, and a high Q means low bandwidth, means current changes less, from cycle to cycle. Tim |
| horror-vacui:
--- Quote from: T3sl4co1l on June 26, 2022, 09:52:57 pm ---Resonate with a cap. That's what everyone does, and with very good reason! --- End quote --- That will be the plan for the project, but first I wish to do some experiments. The resonance can be used to increase the current in the TX coil, which in turn increases the magnetic field, for the same voltage drive (equivalent with reducing the impedance), but it generates the same magnetic field as a non-resonant coil with the same current. While it is great to have a less reactive load, it "only" makes the life of the driving source harder. Of course if there is nothing out there, I will use different caps for every frequency in my measurements. --- Quote from: T3sl4co1l on June 26, 2022, 09:52:57 pm ---The fundamental problem is the very poor coupling between coils. --- End quote --- In itself it is not a real problem for me. Efficiency is not one of my concerns. The only issue I see, is that the inductive load makes it harder to drive. --- Quote from: T3sl4co1l on June 26, 2022, 09:52:57 pm ---any air gap or misalignment counts as air gap between windings -- at expense of coupling. --- End quote --- I am aware of this, and this is also something I would like to have first hand experience with. |
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