Greetings AAC people

There is a part of a hobbyproject (which for me is a project on its own) that I would like some input on if someone is interested.

Backround
So the bigger project is one of those rotating POV LED displays. Since I don't want a battery on the spinning thing and cables will tangle I had the idea to try something cordless, and before I buy a premade solution I wanted to give it a try myself.

Aaanyhow...

The actual matter
I had an idea for generating AC through a coil and have tried to simulate it. Turned out to be called an H-Bridge and I use it to drive a "tank" circuit to get a sine wave. I tweaked some values and simulation-wise it seems to be doing what I had in mind (What I had in mind is to this point an AC sine wave of about 800-900mA at 150khz through a coil). Btw I also plan to add diodes to protect the transistors.

The schematic is attached as is the LTSpice file. V1 and V3 output a 5V square wave at 150khz but they are inverse to eachother. V2 is at a constant 12VDC.

Reasoning so far

• Since it is to me highly unlikely that I manage to construct a coil that matches the value it should have, I want to supply the frequency from a uC so that I can tune it sufficiently.
• If I by some magical luck should hit the sweet spot of resonance I wanted to limit the current in the tank circuit part, therefore there are a few resistors there. I have experimented some and ideally I wanted something in series with the inductor or capacitor to dampen the oscillation but this seems to upset the nice waveform. I also didn't want too big resistance at the driving (R7 and R8) of it, just enough to limit supply drain.
• There is still some spiking going on between the transistor pairs with the same input which is no wonder if they both doesn't change immediately.
• Since I atm imagine around 800-900mA it maybe doesn't make sense to have driving stage and powerful MOSFETS, but I thought I'd rather make it a little more reliable and variable in case I need more.

And here are some questions that float around in my head atm...
Questions

• Is this a reasonable design?
• What's a good way to limit the oscillations in the tank circuit?

I welcome thoughts and comments to any of the questions, my reasoning, the schematic and my poor abilities of electric design (I however prefer nice suggestions)

Also please ask if some piece of info is missing, thanks in advance

- Christoffer

 

Take a look at this one:
http://www.discovercircuits.com/dc-mag/Issue-1/issue-1-exciter-circuit.htm
You can buy receiver coils pre wound from mouser.

 

Interesting, before I made the latest design I tried something similar but didn't manage it as well. Also tried with a tank circuit like that but then the current didn't oscillate around 0. I'll have a closer look at that again also.

A premade coil would lessen the tuning problem for sure Found some at digikey as well now, they seem to be popular in the uH range.

Thanks

 

I took a closer look and tried to simulate a similar circuit again. Without anything I get a fairly nice wave, but when I try to limit the current with resistor(s) somehow they are starting to look more square or weird again. I feel as if I'm missing something. On the other schematic from the "driver construction" page there are some more details, but I fail too see how that works, and what "TP1" is. To me it looks like a resistive divider with an antenna

 

If you are just aiming for inductive coupling of power, do you really need a nice sine wave? Why not drive a primary coil with a square wave and use the output from a secondary coil?

 

Fair enough, it is indeed what I want. Maybe I got too carried away by nice waves...

Anyhow, as long as that doesn't affect the power too much the only thing I want is that it roughly stays within some manageable boundaries.

 

I know this isn't what your asking about, but, if the display is rotating on a shaft, why not just use slip rings and brushes? Doing it inductively you need to have an diode bridge to convert the AC to DC on the display. With slip rings the DC would made in the 'base' and transferred to the display directly to the led's.

 

I know this isn't what your asking about, but, if the display is rotating on a shaft, why not just use slip rings and brushes?

This is what I did many moons ago. The power went in by slip rings and the data went up the hollow shaft optically. The data was sent serially with self clocking.

On the rotating part, the clock was extracted and used to shift the data into a shift register. All of the intelligence was in the base to keep the rotating mass as small as possible.

I also used an led on the rotating part to create an index pulse in a photo-transistor in the base.

 

Right, yeah I am aware that it is hugely ineffective and will require some extra components on the spinning thing.

I have considered a slip ring and remember searching for one but failed to find something cheap, small and available. Perhaps you have some suggestion where to search or other keywords? (I am currently located somewhere in Germany ) I would then prefer something with three connections since I want one for data as well.

Now part of the reason why I want to do the inductive coupling is cause it "tickles my facy" ..and since I have no deadline I have time to fiddle around

However I'd like to have alternatives for when.. no IF, if I fail

 

Go to the hardware or plumbing store, and get some rubber tubing that fits your shaft, Then some copper tubing that will fit over the rubber tubing. Cut three (or what ever number of connections you need) rings from the copper tube. Slide the rings on the rubber tube, with a distance between each copper ring. Solder a wire to each ring up to your "circuit".

Go to the same hardware store or a hobby shop and get the same number of carbon 'brushes' as you have rings. The brushes are used in small hobby motors, like for RC cars/boats. Make brush holders from wood or better plastic, and mount so the brushes rub against the copper rings.

Or here - http://www.adafruit.com/products/775

http://www.thebackshed.com/windmill/articles/Sliprings2.asp

Or you could in a pinch, use a stereo jack and plug. They have three rings in them. Mount the plug to the end of the shaft and the jack to the base.

 

the only thing I want is that it roughly stays within some manageable boundaries.

A square-wave oscillator with ~ rail-to-rail output would do that and be simple to make.

 

@shortbus: Cheers for the suggestions. Self construction of that thing, I don't know... The audio connection idea is very interesting. A slightly loose one with little friction maybe.

@Alec_t: Yeah I guess I'll have to make some tests with different configurations to see what seems to work best. In any case it seems that the general opinion is that the original design is a little bit "too much" then, that the reverse polarity configuration is a tad overkill.

 

I have considered a slip ring and remember searching for one but failed to find something cheap, small and available. Perhaps you have some suggestion where to search or other keywords? (I am currently located somewhere in Germany ) I would then prefer something with three connections since I want one for data as well.

I am not sure that you can get reliable data through a slip ring. Multiple slip rings for the data might help but that adds mechanical complexity. That is why I used the optical scheme for my design.

 

I am not sure that you can get reliable data through a slip ring. Multiple slip rings for the data might help but that adds mechanical complexity.

Hehe yeah adds complexity for sure. But in which way would it be unreliable, lost connectivity, weird noise?

Otherwise when no connection at all is present I have considered IR for the data. And so far I only imagined it with "intelligence" in the rotating part.

 

Hehe yeah adds complexity for sure. But in which way would it be unreliable, lost connectivity, weird noise?

I should have been more complete in my reply. You seem to have gotten the idea anyway.

I worry about there being "weird noise" on the data signal caused by the brushes. The cause would be a variation in resistance and sometimes complete open circuit as the brush rubs against the rotating shaft. I believe that the common cure for this is is to use multiple brushes wired in parallel. This is fine if you have the skills and tools to do precision machining; harder to fabricate with hand tools.

And so far I only imagined it with "intelligence" in the rotating part.

The minimum needed in the moving part is the de-serializing and storing of the data, the power drivers, the LED's and a clean power supply.

At the next level of smarts, a microcontroller can do the de-serializing and hold the image data. The microcontroller would not do any formatting of the data for display.

The highest level of intelligence would have a more powerful controller accepting data in a formatted form and decoding it into a graphic image. At this level of intelligence, the data might be received in a standardized communication standard such as Bluetooth.

In between these two levels would be one where the data is entered into the rotating platform while it is stopped and then starting the spinning. This would be awkward if the image is changed often or on the fly.


You also need an index pulse if you want a stationary display, that is, a display that does not rotate because the image is not drawn at the same rate as the LED's spin. The index signal is used by the intelligence that "paints" the image.

One more note. Be warned that debugging can be tricky. It is hard to connect a meter or scope probe to a rotating assembly. :-(

 

You can get a capstan motor out of any old VCR;

If you make the magnet side stationary, and spin the coils side (with another motor) the coils will produce 3 phase AC that you can rectify into DC with 6 diodes. That will power your spinning device, with no brushes and nothing to wear out but the bearings.

The 3 phase AC can also be used as a sync reference to lock your rotating display.

These little pancake motors can deliver good power even at slowish RPMs.

I used one here in a little handheld generator lantern, it makes good light even at low RPM cranking;

 

@RichardO

I believe that the common cure for this is is to use multiple brushes wired in parallel. This is fine if you have the skills and tools to do precision machining; harder to fabricate with hand tools.

Yeah I doubt my mechanical construction skills on this one..

About further design I have imagined one uC in the rotating part that holds the image, "draws" what it has, and receives commands from a second uC at the base which would handle things on the ground, such as communication with the outside.

As index pulse I had the idea for a magnet(at base)+reed switch(rotating), as long as there isn't enough magnetic disturbance already floating around that is!

My internal ideas might change as I proceed

One more note. Be warned that debugging can be tricky. It is hard to connect a meter or scope probe to a rotating assembly. :-(

For the sake of debugging a (stable) slipring with extra cables would sure be handy

@THE_RB This idea I like strongly However I have recently moved across borders and don't have many old things lying around, except maybe a few computer parts. Harddrive, chassis-fan.. anything else that could be equally useful?

And cool thing you built there

 

Any motor with permanent magnets and AC driven coils will do. If your harddrive has that type of motor inside (most do) you can use that. Or a small PC fan usally has permmag motor with AC driven coils (elec commutated) so you could use that, although they can be hard to pull apart without destroying.

Basically you are using some type of AC motor that you can drive as a spinning generator, so it will power the spinning circuit.

You could also do it with a DC motor, but that has brushes to wear out and will not be usable as the position lock reference. An AC permmag motor is much cooler.

 

Hah I'll have to rummage around abit then.. and yeah the chassis fan is for sure DC. Hmm is it reasonably easy to approximate how much current you'd get out of something?

 

Small PC fans run from DC, but inside they use two AC coils, and electric commutation using hall effect sensors and transistors. That makes AC internally to drive the two AC fan coils.

Regarding the current (or power) it will generate, that will be at least the fan's normal operating current. So a 12v 100mA fan can generate 100mA for sure.

In reality things will change based on your RPM and load voltage and the amount of shaft power you have available to drive it, but generally you can get more than the 100mA if you are generating a lower voltage and have good shaft power. (generator current increases and voltage decreases as you increase the load).

Really you need to just build and test.