As long as your having fun Arthur ...

Dr Winkler of the US Naval Observatory is credited with saying, and I'm paraphrasing ... Time is man-made and is irrevealent.

 

I am having fun with this "project"
I would be sooo happy when I get it to work.

Oh, and dont worry I already know what i want to do next =)

 

about the diode bridge the diodes do not all point in the same direction. the bottem is like a 'V' and the top is like a '^'

If the catodes join they do not point in the same direction
So I was wrong.

Okay, you got me on that one.

But what I meant is, if you consider the diodes to be arrows you'll see that all four point towards an apex -- that apex is the (+) output. So, the easiest way to remember how to draw a bridge is to make all those arrows (cathodes) point upward. Then you'll know the top is (+) and the bottom is (-).

 

Right, what you said.

 

Ok here are the changes. Can I build it yet? (only if it will work with the circuit)

 

Unless R6 serves no purpose. Get rid of it. It would just burn up anyway, unless rated for 50 watts or more.

 

Yes the 555 will (should output around 680kHz) work. It worked fine with a smaller setup.

What is Ciss and how is it 6.6nF?

I have an LCR meter I can mesuer L but whats Ls?

I have 3 coils i might use.
1. Short wrapped evenly around 7 Screws. .002mH
2. the green coil from the 3 pack at radio shack 1.2mH
3. a long coil wrapped around a big ceramic(i think) cup .25mH

I think the best coil is the one with 1.2mH but the short one also worked. so did the cup.

Do any of these coils have a natural resonant frequency of around 680kHz? You will have to drive the coil at resonance if you are replicating MIT's experiment.

Also, your secondary (receiving) coil should be identical to your primary (transmitting) coil, if possible, so it has the same resonant frequency. For example, you could get 2 of the green wire Radio Shack coils.

I've actually been planning to work on something similar to this, but I'll probably just use a function generator instead of the 555.

 

yes the coils are both 680Khz resonant frequency

 

Yes, do delete R6. It is only an electron eater, and good for frying bacon.

 

Ok, so the parts i ordered from futurlec finally came.

I got the IRF540 fet some heatsinks a bigger protoboard and other stuff...

So I decided to build this and i have a few problems.

The secondary coil led wont stay on. It only stays on when you rapidly pull the primary coil it out of the socket and put it back in. It Works well when you connect the primary coil right to the 555 output but then the 555 gets hot and smells bad. it wont work from the fet

What is the smoke that comes out of fried components and why does it smell sooo bad?

 

We call that smell "the agony of defeat."

Fortunately, the ashes of disaster are marvelous fertilizer for the eventual blossoming of the roses of success.

Do you have any access to an O-scope? Can you verify pulses across your coil?

Failing that, can you get some voltage readings on the FET drain?

 

OK I got it to work.. but then i tried to change something and it wont work anymore(imo i think the fet is not switching on and off. power is going trough the coil because it gets warm(also i killed like over 10 555's). but I got videos of it working.

The resistor that connects the gate of the fet to the output of the 555 gets hot. Why does it get hot? how can i fix this?

 

If the gate resistor is getting hot, it is carrying too much current. This is probably why your 555s are burning up. Your FET, at the speeds you are driving it, needs more gate current than the 555 can source. I'll bet your FET is not turning all the way on or off. Such would severely limit power to your coil.

You'll need to put a FET driver between the 555 and your FET. The best tutorial I've seen on FET drivers is here: http://focus.ti.com/lit/ml/slup169/slup169.pdf

 

how much current does the fet need? I looked at the datasheet and could not find a Ig(gate current). can i use any fet driver? i have 1 but i will have to look for it.

Found it! its a TPS2811P

I have a question about the data sheet. there is a green picture labled BONDING PAD ASSIGNMENTS what is that a picture of?

 

If the gate resistor is getting hot, it is carrying too much current. This is probably why your 555s are burning up. Your FET, at the speeds you are driving it, needs more gate current than the 555 can source. I'll bet your FET is not turning all the way on or off. Such would severely limit power to your coil.

You'll need to put a FET driver between the 555 and your FET. The best tutorial I've seen on FET drivers is here: http://focus.ti.com/lit/ml/slup169/slup169.pdf

I ran a sim on a bipolar 555 driving the IRF540, with the 10 ohm gate resistor. The 555 dissipated around 460mW, and the 10 ohm resistor dissipated about 170mW. This is with a 12V supply. It looks to me like a 1/8 watt resistor would burn your fingers, or a 1/4 watter would get pretty warm. The 555, according to the datasheet (thermal resistance is 106deg C/watt), will reach around 75C, which is above the 70C max operating temperature.
As I see it, you have several choices, based on the fact that the power to charge and discharge a capacitor is P=F*C*V^2, where F is your frequency, C is the gate capacitance in this case, and V is the peak-to-peak voltage swing.

1. You can reduce F, reducing power proportionally.
2. You can use a smaller FET, which will reduce C and therefore power.
3. You can reduce V and get a lot of bang for your buck, since power is proportional to V squared. For example, reduce V from 12 to 10 and power drops by a factor of 1.44. Keep in mind that the output swing of a bipolar 555 is not rail-to-rail, so the calculation needs to account for that.
4. You can use a 1/2 watt resistor, and a heat sink on the 555.

Personally, I would start with option 2. You don't need 44 milliohms Rds(on). Option 3 is attractive, but it will presumably reduce your ultimate output power, assuming you ever get any.

 

That's only half the problem, Ron. He still needs to turn the FET fully on and fully off. He needs a driver!

(Good call on dropping Vcc for the 555, though!)

 

That's only half the problem, Ron. He still needs to turn the FET fully on and fully off. He needs a driver!

(Good call on dropping Vcc for the 555, though!)

Well, a driver would certainly help if he sticks with the IRF540. The sim shows that the gate voltage is reaching the rails with time to spare, but it also shows the bipolar 555 going all the way to the rails, which it won't do, and it shows transient gate current to be +/- 400mA, when the datasheet says it can source or sink 200mA. As I said though, I think a smaller FET would solve all the problems.
EDIT: Maybe I should clarify what I mean by "smaller FET". I don't mean the package size. I'm speaking of the geometry of the die (chip) inside. A smaller die will generally have lower gate capacitance, higher on resistance, and less power handling capability.

 

If he's trying to replicate the MIT experiment, wouldn't less power be counterproductive?

 

If he's trying to replicate the MIT experiment, wouldn't less power be counterproductive?

If he leaves the 100 ohm resistor in series with the drain, the Rds(on) is irrelevant. He could reduce it to 10 ohms and still get away with a much smaller FET.
If he has a driver at hand, then go for it.
I was not aware of an MIT experiment. Was that mentioned somewhere in the thread?

 

Arthur,

I tried to warn you early-on that the 555 was not going to fly at those speeds, and drive such a circuit. I even did some math for you to explain it.

But welcome to tool time, and try watching a few episodes of The Red Green Show. A little duct tape on that 555 should help keep the smell in --- ohh, just kidding.