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Just wrap some wire around a power cord, rectify and filter whatever is induced, and measure voltage drop over a known resistance. From there you can calculate current and power. If you are not getting enough, experiment with more wraps.

This seems right to me (see sig below), but remember to just wrap one conductor of the AC line, or the two will cancel each other out. It's basically just a transformer. The supply side just happens to be powering something else as well.

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Thanks for the replies chaps, I was hoping that trial and error wouldn't have to be my first step but who am I to shun the tried and tested technique!

 

If what GeekDoc says is true, I'm not going to be able to do what I wanted. I wanted to develop a contactless, batteryless remote power indicator - a board you can strap to an AC supply cord (say for a lamp) which is powered inductively and provides feedback on whether power is currently applied or not via RF.

 

Looks like a CR3032 battery holder is going on the BOM.

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I can tell you absolutely that you can get some serious voltage by just wrapping about 10 turns around a standard PC power cord. I tend to fidget, and once I wrapped a small piece of scrap wire around a power cord. I managed to get a nice little wake-up shock in the process. It can be done, and you don't need anything fancy.

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it is *amazing* how little current the msp430 draws in sleep mode.

 

My thoughts exactly! I was astounded when I saw the tables for power consumption for the 430.

 

I've had a bit of a play today, but I don't think I'll be able to get anything meaningful done without building a circuit to charge a cap. 20 turns of .75mm wire round a 50Hz 240 a.c. power cable doesn't give enough juice to light a LED (I suspected it wouldn't but I got hopeful after hearing of rockets4kids experience). So the next step is rectification, smoothing and messing about with wire size and number of turns.

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Just a little update on this, feel free to throw in any suggestions/explain the basics to me :oops: ...

 

My intended application isn't going to be feasible I wouldn't have thought (RF TX/RX), but there may be another application (datalogging?) that this could prove useful for.

 

I knocked together a rectifier and put an electrolytic cap on the output (100uF), and attached a 10k load. Pushing the resolution limits of my multimeter, 0.002V drop across the resistor was recorded. (0.002V/10k = 200nA ?)

 

Leaving the load disconnected and allowing the cap to charge shows a charging rate of around 0.02V/min. After ~45 mins V across the cap had built to around 0.7V.

 

Firstly, principles is not my strong point, so have I made some basic errors here, or do my figures make sense?

 

Secondly, If I am correct in my figures, 200nA with a 0.2V drop indicates 0.4nW instantaneous power. As an 430F2xxx device needs 900uW at 1 MIPS, a charge time/powered ratio of 22000000:1 is needed with this setup (or 62 hours charge for 0.01 seconds activity).

 

My thoughts so far are that with an FRAM chip (I think you can write individual registers instead of blocks of flash?) and the right application (sample temp and record once every hour), an inductively powered MCU could be possible. I don't really know what power and time would be used waking the chip, ADC's, etc., but for my application, it's a no go...

 

...unless I can code these issues out...

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Pics would mean showing the state of my "workstation" to the world :oops: But there really isn't anything to see. 4 x diodes arranged for the rectifier, a cap across the output. The inductor is literally a wire wound round the power line 20 or so times.

 

Adding a few more coils won't help this setup much. I could improve the power harvested by building a better inductor (an iron core surrounding the power line, coils around the iron core), but that's too involved for my application. Actually, there'll be something I can salvage from my parts bin, I'll have a look tomorrow.

 

I'm reading a 0.5V drop across the diodes of the bridge (lower than I expected tbh), and they're big ugly looking diodes, so I could probably get a more efficient rectifier setup, but from my preliminary examinations I'll need to do more than eek out another 5% from the bridge.

 

I suppose it's not dead in the water yet, it's just going to need more specialist hardware.

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