3v OU Flashlight

[verpies]

I really don't understand how the mosfet in this circuit is supposed to be switched. The inverter chip is only supplied with the 3 volts input power and the sine wave input is only a couple of volts p-p. How can the output of the inverter stage reach the 4.5 volt (approx) gate threshold voltage of a mosfet?

Some MOSFETs have much lower V_GS(TH) than 4.5V. 
For example the M8208 has a typical gate threshold voltage of 1.2V and as low as 0.5V.

[gyulasun]

Yes,  the MOSFET family with low gate-source threshold voltage is called logic level gate type if you wish to search for them. Several manufacturers produce them.

Gyula

[TinselKoala]

Well, maybe I'll be able to find a logic-level mosfet locally. Does anyone know the part number specified for the schematic?

Meanwhile I'm 95 percent finished with a build. I'm experimenting with a different construction technique than I usually use. I ordered 3 MC34063A chips for 3.99 US from an Ebay seller, they should be here in a couple of days. Saturday or Monday, probably. Still need a couple of resistors and a capacitor, which I'll get from my local supplier tomorrow. I've got an IRF3205 in there now even though I don't expect it to work. The wires are to the transformator which I haven't wound yet.

[magpwr]

I really don't understand how the mosfet in this circuit is supposed to be switched. The inverter chip is only supplied with the 3 volts input power and the sine wave input is only a couple of volts p-p. How can the output of the inverter stage reach the 4.5 volt (approx) gate threshold voltage of a mosfet?

hi TinselKoala,

The fav 600volt mosfet 47N60C3 which i normally use in my experiment do have a gate turn on voltage threshold from 2volts...4volts for low current.For max current from mosfet gate plateau voltage is 5.5v.
Do check datasheet for your selected mosfet.

[TinselKoala]

OK.... here's something of a teaser. I did get the mosfet to switch. See the scopeshot below.

I don't have the MC34063 or some other components on the "input" side, nor any coil yet. I'm just testing the inverter-mosfet "output" side to see if the inverter functions and drives the mosfet.

So I'm giving the board about a 3V supply to power the chip. And I'm using the Interstate Function Generator to inject a sine wave signal into the Pin 13, monitoring on the bottom trace of the scope. And I'm monitoring the mosfet Drain response on the top trace. Both traces are set to 2 v/div. The scopeshot shows one possible response waveform. But there are others! My setup is very sensitive to the input voltage to the board; a hair more than 3 volts and it works better, the mosfet will now turn fully on.... but with just a bit more, the mosfet then _stays_ fully on  100 percent .... impossible to explain in text really, so I shot a video, but it will be an hour or so before it's done processing and uploading. Meanwhile... the sinus signal to the inverter does result in a response at the mosfet, at least I know that much.

The nice square flat tops on the Drain trace are at the supply voltage and indicate the mosfet fully OFF.