# Function generator for wireless energy experiment

1. Jan 15, 2012

### baconman71

My problem is that I am trying to, in this case, send a 50 KHz signal through an LC circuit to a receiving coil (very short distance by the way) so that the secondary coil can power an ipod. It's more so a first attempt and an experiment because the 50 KHz isn't a very powerful signal but I am fairly new to the realm of electronics and while I understand these concepts I don't really know what options I have. Any suggestions? I have heard of op amps and even integrated function generators. And are sine waves better to use than square waves for this? I'm assuming so. I want to be able to make it so that I can gain some experience from it instead of it coming out of an expensive function generator. And I would also need to send across a good bit of voltage due to losses in efficiency concerning that it is my first attempt.
Thanks you guys!!

2. Jan 15, 2012

### Bobbywhy

baconman71, Welcome to Physics Forums!

Let me respond to your second question first: Sine waves are definitely the ones to use when driving an LC circuit. Square waves switch from zero volts, for example, to some positive voltage almost instantly. Inductors (the "L" part) resist a change in current flow. This means the current (energy) delivered to the coil during the first half-cycle would not have time to reach maximum before the square wave switched to its other state.

As for transfer of power to charge the I Pod: I would ask how much power is needed to supply the I Pod? Then consider how much the secondary coil output is supplying. The LC circuit should be "tuned" to your frequency of 50 KHz so it is most efficient. Same for your transmitting coil. Now, to get sufficient power transferred you must increase the amount of power you transmit. For this you would apply a stronger drive signal to the first coil by amplifying the signal from the signal generator. It may be a matter of a trial and error process, but I have outlined the basics.

3. Jan 15, 2012

### vk6kro

Variations on this question come up frequently.

Here is one from about a month ago:

It would probably answer most of your questions. Yes, you would use a sinewave.

This diagram from that post is a possible way you would connect it:

https://www.physicsforums.com/showpost.php?p=3666447&postcount=11
The component values would be different, though, because the frequency for that one was 4 MHz.

As a guide, a capacitor of 0.018 uF and an inductor of 500 uH will resonate at about 53 KHz.

4. Jan 16, 2012

### baconman71

Oh wow that's much more than I expected! Thanks a lot guys. So for the function generator part what would be a fairly easy sine wave generator? The 50 KHz is just the first of my attempts now because I am doing an actual project on it where I will be going higher (with limits of course) but I'm not sure what designs would be good to try out. Any suggestions?

5. Jan 16, 2012

### skeptic2

I'm going to make the same recommendation I made in the other thread, that you use an oscillator like a colpitts or hartley instead of a function generator. If you use a function generator you'll have to amplify its output anyway.

I know doing an oscillator like that may be somewhat of a stretch for you but that's how you learn. First read up on how those oscillators work and if there's anything that's not clear, don't be afraid to ask us.

6. Jan 16, 2012

### vk6kro

Unless you have some reason to choose 50 KHz as your frequency, you will find a frequency of about 4 MHz a lot easier to use and more efficient.

Inductors for 50 KHz will be large, expensive and tedious to wind. Coils for 4 MHz are still a challenge, but nowhere near as difficult. And they will work better.

I have modified the above diagram to show how to turn the first tuned circuit into an oscillator.

The oscillator is a Hartley oscillator and the exact components will depend on the frequency used.

7. Jan 16, 2012

### Mike_In_Plano

I worked in the medical business on wireless power transfer for implants. The preferred circuit was a colpitts oscillator with a matching network driving at about 2 MHz.

I did some work with Royer oscillators, and got better efficiency, and a more friendly circuit overall. The following post I put up to help others with the circuit. Most of the parts you can build or purchase at Digi-Key. It does require that you build a small transformer on a ferrite core, but I've done this with a number of cores and it isn't very fussy.

Enjoy!

Last edited by a moderator: May 5, 2017
8. Jan 17, 2012

### baconman71

Ok so a fixed sine wave sounds good. I've decided to abandon my 50 KHz idea for its inefficiency and the fact that it probably wouldn't work well with the function generator. So when you get higher in frequency you get more efficient right? But then at the same time you get more impedance? So is there a point when you send power over so quickly that it really doesn't matter to go higher because it would only increase the impedance and it wouldn't change? This is another topic that I'm having trouble finding out.

9. Jan 17, 2012

### baconman71

I actually just started to think about this and I remembered that the inductor and capacitor in series provides 0 impedance and then in parallel it has infinite impedance? am I correct on that part? what connection should it be for the primary and secondary coil?

10. Jan 17, 2012

### vk6kro

But then at the same time you get more impedance? So is there a point when you send power over so quickly that it really doesn't matter to go higher because it would only increase the impedance and it wouldn't change? This is another topic that I'm having trouble finding out.

This seems a bit mixed up.
The impedance of an inductor increases with frequency, but that of a capacitor decreases.

The speed of travel of radio waves is the same regardless of frequency.

What would be good it to be able to listen with a radio receiver to make sure you are not interfering with any other transmission. A small signal may be radiated and you need to make sure it isn't interfering with anyone.

I actually just started to think about this and I remembered that the inductor and capacitor in series provides 0 impedance and then in parallel it has infinite impedance? am I correct on that part? what connection should it be for the primary and secondary coil?

If the components were perfect, this would be the case. But real series tuned circuits have low impedance, not zero and parallel tuned circuits have high impedance, not infinite.

Connection was shown in the link:
https://www.physicsforums.com/showpost.php?p=3666447&postcount=11
except that I added a transistor to it to make the signal generator unnecessary.
If you wanted to use a signal generator, you would leave out the transistor and feed it in at the left of the diagram.
Because the signal generator is probably low impedance, it can drive the series tuend circuit which is also low impedance. This results in maximum current in the coil and capacitor.

11. Jan 17, 2012

### baconman71

Ok that makes much more sense. I think that I might look at a frequency of 3.75 MHz. The allocation chart for frequencies has reserved that to amateurs.

http://www.ntia.doc.gov/files/ntia/publications/2003-allochrt.pdf

I feel like that will work. And for the size of the inductor itself is it ideal to have a larger coil for both sending and receiving?

12. Jan 17, 2012

### kougou

welcome to the forum

Last edited by a moderator: May 5, 2017
13. Jan 17, 2012

### vk6kro

Ok that makes much more sense. I think that I might look at a frequency of 3.75 MHz. The allocation chart for frequencies has reserved that to amateurs.

This allocation is for licensed Radio Amateurs. If you do not have a licence, you would be taking quite a risk if you caused interference on that frequency or any part of the band of 3.5 MHz to 4.0 MHz.

If you make the device tunable, you could use a suitable receiver to get it above 4.0 MHz. Radiation from it should be very small, but any interference it causes is your responsibility.

14. Jan 17, 2012

### baconman71

Ok so not that then. But what says that I can use the signal just above 4 MHz? I'm just not sure on how to be able to tell.

15. Jan 17, 2012

### vk6kro

You would have to listen and check the allocation list to see how this frequency range is used in your area.

Any radiation is illegal if it radiates beyond the borders of your property, however it is difficult to be sure that this is not the case.
If you have a metal shed or garage, this would be a good place to try this sort of experiment.

Most Radio Amateurs would welcome the chance to check your device to see if it is generating any interference as this would demonstrate a responsible attitude on your part. So, if you can contact a nearby Amateur, you might like to consider doing this.