I have a slightly movable (torsional (freely for infinite degrees around the z-axis) and translational (+- 40mm in the xy plane)) microcontroller in communication with a fixed one. I'd like to use it without having a dedicated battery. Cables would transmit too much mechanical force (own weight, spring-action), coils would pick up too much noise and require ac/dc conversion in the mobile component, so I was thinking about using a solar cell on the mobile component for power, and either a dedicated TX/RX part, or to solve the RX with the solar cell as well (there won't be much communication, so the (off-)duty cycle won't be much of an issue)

There are multiple papers about the principle, but I am stuck on a practical issue: the mobile component will need about 20mA @ 3.4V (~0.07W) continuously, and i cannot find the values i need to compute whether this is feasible in a small package.

A silicon solar cell has about 10% efficiency, but that is for full spectrum sunlight, and I would be using an LED at 950 nm (because silicon solar cells seem to be more efficient there - only some sources say the reverse, which might be due to NIR not having the most energy in sunlight -), and I’d ballpark an LED at 50% plus the solar cell would only get half of the light at any given moment, to account for movement, so I’d have to power the LED (array?) with 40*20 mA=800 mA, which seems a lot.

Also, I cannot seem to find upper limits for radiation densities for small-spectrum light on silicon cells (amorphous? Crystalline?) - is it the same as for full spectrum, ie 10000W/m2 ?

Alternative power conduits are welcome, but my main focus is on scaling the LED(laser?) and solar cell.

EDIT: there will be plasma cutting and arc welding going on in the near vicinity, which is why i thought coils were out. The welding and cutting equipment is not related to the project i am talking about here, so neither can i use the resultant light, nor can i use the infrastructure (cables). The MCU itself is shielded, and has functioned properly. Currently it is powered by a small lipo unit i got from a minicopter. I experimented with the coils from an electric toothbrush, but got bad results for the translational movement (did not try with welder on ). Movement is +-40mm in xy plane, and torsional movement is constrained to the x-axis. air gap of >50mm would be nice. There is no MCU is 10g, shielding is another 12g (though i'll try to shave that down as soon as i get my hands on thinner metal sheets), lipo is 7g

  • Solar is weak. How mobile? Human or robotics? (microwave harvesting or IR or en.wikipedia.org/wiki/Energy_harvesting) – Tony EE rocketscientist Jul 3 at 15:31
  • So, are you asking about power transmission or data communication? If about power then your argument against coils is weak. "pick up too much noise" has no relevance for power transmission. With properly designed coils the "ac/dc conversion" can be as simple as diode, cap and zener. I suspect you won't find lighter/smaller solution that gives you required power. – Maple Jul 3 at 17:26
  • @Maple I am asking about power transmission, the info about the transmission was for context. There is an arc welder and a plasma cutter operating at about 10cm distance, which is why i have doubts about the coil. Also, I tinkered with the setup from an electric tooth brush, which did not handle translation in the plane of the coils well. I'll amend my question in a bit. – loonquawl Jul 4 at 4:42
  • Interesting how @TonyEErocketscientist comment was spot-on. Sounds like robotic movement is involved. Anyway, tooth brush is bad choice for experiments. Wireless cellphone charging station is much better example, allowing for quite high current and relatively flexible positioning – Maple Jul 4 at 10:55
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    BTW Solar input is 1000W/m2. Not 10kW yet IR PD’s have a wavelength and diversity sensitivity – Tony EE rocketscientist Jul 4 at 13:32

Both arc welder and a plasma cutter require fairly thick power cables and gas hoses. Compared to them the stiffness of the power + communication cable for MCU is practically non existing.

The second thought is that arc welder produces very strong EMI, which will affect MCU and communications without proper shielding (adding more weight to the movable part)

So, my first suggestion is to consider using single STP cable for both power and communication. Note, that the cable should not go straight between those two points 10 cm apart, it should loop sufficiently to support possible need for equipment separation, maintenance or introducing additional nodes into communication. Which coincidentally makes stiffness of the cable less important.

A second suggestion can be used if arc welder is a movable part and if it uses AC current. In this case you can put a core with a coil (current transformer) around the welder cable and leech power right at the end, sort of point-of-use power supply.

The third suggestion does not require AC welding current but it does depend on your definition of "slightly movable". If you are talking about several inches then you can use toroidal transformer with air gap between the core and one of the coils, big enough to accommodate the movement, as in the picture below.

enter image description here

Also, if you still want to explore solar cell solution, note that you don't need light emitting part at all. The brightness of an ark and plasma is way more than you can achieve with any kind of LED.

  • Thank you for your answer. the MCU is already in operation, so shielding is not an issue. I'd just like to get rid of the battery that is currently in use. The welding would indeed produce ample light, but firstly it has a cowl to reduce on sparks and UV, and secondly it is neither attached to the MCU, nor always in operation, so the welding is simply a nuisance, not a solution. – loonquawl Jul 5 at 7:18
  • I've read your update and understand now that options 1, 2 and solar cell are pretty much out of picture. At the same time option #3 sound much more convincing now. The 100 mm diameter coil of very thin wire soaked in CA glue or conformal coating will weight barely 3-4 g. Loop that through 100 mm toroid with primary coil and you'll get your freedom of movement. Using high enough frequency you can deliver a lot more power than you actually need. – Maple Jul 5 at 8:12
  • In addition to the above, let me ask you this. No matter how light your device is, it is probably not floating in the thin air. It must be attached by something. So, this "something" already can be used as one conduit, e.g. ground. All you need then one very thin high strand count wire (don't need much for 20mA) and you are done. Or am I missing something? – Maple Jul 5 at 8:24
  • It is indeed floating, but not on air, but on water. It needs to be able to freely rotate around its z-axis. – loonquawl Jul 5 at 8:35
  • Ha! That was unexpected... In that case just go with two co-axial coils. If you search for "wireless charger coils" you'll find hundreds of ready-made wire coils as well as very light FPC ones. You will also find that there are several of them inside good quality charging stations, to allow for random phone placement, or in your case, for moving power receiver. – Maple Jul 5 at 9:50

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