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design mosfet driver

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New Member
I have the following circuit: it’s a relaxation oscillator: R charges C (when mosfet is in saturation state) and when the C arrives to its maximum threshold voltage (83 v approximately), I must open the mosfet to start with the C discharge, but isolating the R, because I need to cut off this part of the circuit.
I try to open the mosfet giving a “1” to the base of 2N2222 transistor, so the opto-isolator led switch on its transistor (in fact I can’t realize this, I obtain a VSAT to high, nearly the value of the supply), closing the C-E junction of the BC639 and open the Mosfet.
All this would be the ideal that I pretend to happen, but I don’t realize my purpose, because the mosfet always conducts.
I attached the circuit sketch.

True Table: Base 2N2222 vs. Mosfet state:
base = 0 then Mosfet close (Saturation)
base = 1 then Mosfet is open

If my circuit doesn’t have a solution, why can I drive the mosfet?
Thanks and excuse me my grammar, I’m not American.
Luis Blaugen.



Well-Known Member
Most Helpful Member
Nothing in your circuit discharges the capacitor. Connect a resistor in parallel with it.
The Mosfet turns off when its source voltage is a few volts lower than its gate voltage so with a 83V supply, the capacitor charges to about 80V.

There is no positive feedback so it is not an oscillator.


New Member
in fact, there is a dispositive that discharges the capacitor, but in the drawing I didn't draw it in order to make easy the circuit.
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