Re: Question about auto coil -

December 21, 2003

Hi ,

The original machine I made runs easily up to 1.6 Mhz although there

is some drop off above 1.3 Mhz. It will only just light a plasma at

1.8 Mhz. But it also runs stone cold in operation, the MOSFETS

usually don't even get warm. I have them on a large heatsink, but

it's unnecessary, the bodies are not even perceptibly warm at any time.

The caps really need to be above the built in zener rating of the

MOSFET's, which is 55V for the originals I specified, because the

supply line will see spikes of at least this voltage. It sounds to me

like you have problem in the driver stage (not enough voltage to turn

on the MOSFET's) or perhaps your toroid is a different specification.

I haven't bothered to develop this design, it was simply something I

came up with in an hour at the time when I wanted a quick plasma

ignition circuit! I would suggest that it could be improved by using

a lower permeability toroid, a greater number of primary turns per

phase, maybe 2 or 3, and higher voltage MOSFET's. It will also work

with a ratio of 30:1 with a 55v MOSFET cutoff. Remember that the

voltage across the MOSFET's is NOT simply the value of the DC supply,

but rather what the voltage bounces up to when current is drawn

through the primary winding. In the original it is limited to 55V by

the MOSFET's. If you used higher voltage MOSFET's say 100v ones, then

the primary voltage would bounce up to 100v and you could get away

with a 15:1 turns ratio on the toroid. If you use sufficiently high

voltage MOSFET's you can find a point where the internal zener of the

MOSFET never triggers which is more efficient. In practice, it's the

internal Zener that usually burns out when using the lower voltage

MOSFETS. I only chose those ones because of their high current rating

and low gate capacitance.

It needs a bit of experimentation to get the best configuration. I

only offered the design as a starting point for anyone interested in

experimenting further. The original worked well for the experiments I

performed at the time and I never bothered to improve it as is.

As Bil mentioned, I often used to use it with a fixed modulation

frequency in the audio range and used the carrier to set the actual

MOR in the No 4 range. The audio modulation is really only required

to keep the power level down. You could easily use say 2128 and 1.604

Mhz together for cancer for example. Although, because the output is

very square, the 3rd harmonic is strong, so to get 1.604 Mhz, you can

just as easily use 1/3 of that (i.e. 534,666) at higher overall power

and still get a strong harmonic at 1.604 MHz. I only use this as an

example because it is probably the highest frequency that is needed in

practice.

Hope this helps.

Best wishes

Aubrey

> Hi Bil, Thanks for the link to Don Maloneey's device. I note that he

is using different Mosfets ( I cant read the number) and a very

susbstantial cap bank for bypass- these must be rated at 100v or over,

judging by their size ; with 6v I am using the (much smaller) 4x

5000UF 35VW mini caps which take up a fraction of the space and still

work fine with the original mosfets. My only need is a much more

effective heatsink than I am using at present.

>

> Could I reiterate my query about the driver you are using?

>

> Andy

December 22, 2003

Hi Alvin,

Thanks for the MOSFET info, I haven't looked them up yet, but my first

thought was, like Bil, whether the gate capacitances were low enough.

When I first made my machine, I used much heavier high voltage

MOSFET's but I couldn't drive them with any reasonable rise time

because of the high gate capacitance. The drivers needed to source

huge currents and got very hot as well as the MOSFET's themselves

which got hot because of the slow transition time (i.e. more time

spent in linear regions rather than saturation). At very high

frequencies I found that they didn't fully switch either on or off and

this was a significant limiting factor.

My impression is that putting a resistor on the gate (did you mean in

series or to ground in parallel?) wouldn't make such a big difference.

If in series it would simply extend the switch on/off time making

the problem worse. And in parallel, I already use higher values on

higher capacitances, although often the impedance of the gate at the

frequencies in question is VERY much lower than the impedance of the

resistor so the difference is negligible. Or did I misunderstand

something? An inductor in some cases would be better than a resistor

(i.e. a tuned series resonant circuit with the gate capacitance) but

would be limited to only one carrier frequency.

I don't use that circuit any more, I only made it for some specific

work which is why I never developed it, and I don't usually have any

problem when I do use it. I blew a few MOSFET's at the beginning, but

once I got wise to its idiosyncracies I was able to use it easily

without blowing any MOSFET's.

But I would be interested in alternatives and appreciate the info.

I'll look up the MOSFET's you suggested. Can you give any more

details? Such as what frequency ranges you can achieve with what

MOSFET's? How hot do they (and the drivers) get? What do the signals

look like? What is the failure rate like? etc etc. Any additional

info would be appreciated. Thanks.

BTW I got my toroid from RS Components in the UK. Dave Jeffers who

runs the Rifers list found a source in the US, you can ask him. But

there are other toroids that would work just as well if not better.

I've used an FT240-43 toroid without any major problems. And a type

125 material is even better for frequency response as well. But I

have to order these from the US, I haven't found a source for them in

England.

Best wishes

Aubrey

> Hi Bil

> all that is required in the case of the HUF transistors

> is to change the resistor value on the gate to a higher

> value and they will work fine..The IRFP048 is being

> used in the circuit at the CICBS web site which has the

> well lit phanotron and it sure works good too.

> I think that the difference in the capacitance has little to

> do with the difference in performance..Give it a try and you will

> see.

> Alvin

December 22, 2003

Aubrey,

Have tryed to contact you via your e-mail address on your web site?

Really enjoy your posts. I have built a 2KV switching supply that will

operate to 3 mhz easily and modulation of 1 mhz nicely. It does not use

a transformer and direcly switches +/- 1kv supplyies using HV fets. The

output is very square unlike going through a transformer. Will be glad

to send you the design. I am using special gate driver IC's to get very

fast rise times. Using a couple of Microwave oven transformers for the

supplies and the Iinternational Rectifier HV FET's sell for $7 each " using

4 in the circuit " . The gate drivers are only $2 each, so have less than

$100 in the package.

Look forward to hearing fro you,

Karl Harrar

>

> Hi Alvin,

>

> Thanks for the MOSFET info, I haven't looked them up yet, but my first

> thought was, like Bil, whether the gate capacitances were low enough.

>

> When I first made my machine, I used much heavier high voltage

> MOSFET's but I couldn't drive them with any reasonable rise time

> because of the high gate capacitance. The drivers needed to source

> huge currents and got very hot as well as the MOSFET's themselves

> which got hot because of the slow transition time (i.e. more time

> spent in linear regions rather than saturation). At very high

> frequencies I found that they didn't fully switch either on or off and

> this was a significant limiting factor.

>

> My impression is that putting a resistor on the gate (did you mean in

> series or to ground in parallel?) wouldn't make such a big difference.

> If in series it would simply extend the switch on/off time making

> the problem worse. And in parallel, I already use higher values on

> higher capacitances, although often the impedance of the gate at the

> frequencies in question is VERY much lower than the impedance of the

> resistor so the difference is negligible. Or did I misunderstand

> something? An inductor in some cases would be better than a resistor

> (i.e. a tuned series resonant circuit with the gate capacitance) but

> would be limited to only one carrier frequency.

>

> I don't use that circuit any more, I only made it for some specific

> work which is why I never developed it, and I don't usually have any

> problem when I do use it. I blew a few MOSFET's at the beginning, but

> once I got wise to its idiosyncracies I was able to use it easily

> without blowing any MOSFET's.

>

> But I would be interested in alternatives and appreciate the info.

> I'll look up the MOSFET's you suggested. Can you give any more

> details? Such as what frequency ranges you can achieve with what

> MOSFET's? How hot do they (and the drivers) get? What do the signals

> look like? What is the failure rate like? etc etc. Any additional

> info would be appreciated. Thanks.

>

> BTW I got my toroid from RS Components in the UK. Dave Jeffers who

> runs the Rifers list found a source in the US, you can ask him. But

> there are other toroids that would work just as well if not better.

> I've used an FT240-43 toroid without any major problems. And a type

> 125 material is even better for frequency response as well. But I

> have to order these from the US, I haven't found a source for them in

> England.

>

> Best wishes

>

> Aubrey

>

> --- In Rife , Alvin Rose >;ajroseca@n...!!

>

December 23, 2003

Hi Karl,

Thanks for this. I , for one, would be most interested to see your

design/schematic!

Andy

Re: Re: Question about auto coil -