How to eliminate contamination from a frequency generator?

[void109]

That's it.

I just took a good look at your schematic. Once the induction coil is charged up from the battery, it has nowhere to go but back into the battery. Does your MOSFET get hot or is it on a heat sink?

Unfortunately it got very hot yesterday, I left it running with everything hooked up and melted the ground from my scope probe into the mosfet case.  Oops.

[MrMag]

Yeah, it's like a direct short across the battery. The only thing different is the coil. You might have to hook the secondary up to something to help take away some of the energy.

[void109]

As the voltage was rising the frequency for "optimal charging" kept shifting, and if it wasnt just right, it would swap from -50mA to 1A.  It was during one of these switches to 1A (my back was to it while I was working so I didnt notice) that it melted the probe ground wire.

When its just right, it doesn't get hot.

I put that cap in and have it running - showing -40mA on the ammeter.  I put a yellow led with a 1K resistor across the battery leads and the voltage was slowly dropping, removed the led, its charging again.  Have to get some work done, I'll look at it more later.

[MrMag]

As the voltage was rising the frequency for "optimal charging" kept shifting, and if it wasnt just right, it would swap from -50mA to 1A.  It was during one of these switches to 1A (my back was to it while I was working so I didnt notice) that it melted the probe ground wire.

When its just right, it doesn't get hot.

I put that cap in and have it running - showing -40mA on the ammeter.  I put a yellow led with a 1K resistor across the battery leads and the voltage was slowly dropping, removed the led, its charging again.  Have to get some work done, I'll look at it more later.

I wonder if you hit the resonant frequency or one of it's harmonics? I would think that if it was timed just right, the load may not reach the MOSFET. This would keep it running cool and you would definitely see -mA going to the battery. Do you have a scope?

[gyulasun]

Hi void109,

Thanks for the schematics and further info.

Using opto coupler is expensive at 30MHz, much simpler to use transformer or capacitive coupling.
Re on capacitive coupling: as you do not use a resistor between the gate and source in your modified capacitive coupled version, the gate electrode is floating and has no DC return, (the gate capacitance has no real resistive path to discharge) you may wish to place a 51 Ohm 1/2W or 1W resistor directly between the gate and source. (Earlier your generator output resistance (50 Ohm) insured the DC return of the gate.)

However, to completely isolate your circuit from the generator, I suggest either using a series 100nF or 1uF coupling capacitor in the ground line too (just connect the ground point of the gen output (this is usually the coax cable outer shield) to the source electrode of the FET via a 2nd coupling capacitor, this way you get rid of the generator completely. Of course the gate coupling 1st capacitor and the 51 Ohm resistor between the gate-source also remains there.

Or alternatively you could use a 1:1 wideband transformer, just make a 10 turn bifilar coil on any ferrite torod core at hand (not critical) and connect it as I shown in the attached modified schematics. Some theory on wideband transformers is here:
http://www.minicircuits.com/pages/pdfs/tran14-2.pdf
Use a 0.5-0.6mm OD enameled copper wire and twist two wires (7-8 twists /inch) together for a length that is enough to make 8-10 turns on the toroid.  It will surely work between 25-35MHz.

What I think now on you circuit is that it somehow oscillates at either the input or some other frequency and in this oscillator the battery is included as a nonlinear component (mainly as a capacitor but it may also behave as a low Q  LC circuit too) and takes up "pseudo charge", though you may run the setup for a much longer time to see if the charge taken is really stored or not.  IT is possible when you use the 51 Ohm resistor between the gate-source to terminate it from DC point of view too, and you use either the double capacitor or the transformer input isolation,  the tuning for the "sweet point" will maybe be more difficult, though it shouldn't because in your circuit shown in your first schematic the gate-source was terminated by the generator for DC return too.
IT would better to see some scope shots both on the gate and the drain with respect to the source electrode, and also separately across the battery,  use DC coupling and indicate the zero line.
Maybe you wish to use a second MOSFET to check the "toasted" one if it is all right... 

rgds,  Gyula