Hello I=92m trying to build an automatic recording unit, for a 3V recorder. I=92ve done it before, about 20 years ago, but I can't find it or the schematics anymore... I've found this scheme several days ago, built it - but it doesn't work. Then I've made a simple experiment: I've connected the VN10K to the recorder, a variable P.S between the Gate and the S ( + to G, - to S). This was the only way the small motor of the recorder began to work. At about 2-3 Volts (G-S), the current suddenly rose to about 100mA. Less than that it hasn't work. Something is very unclear to me about it - and I'm asking for help. As far as I remember, I've used in the past very high resistors (10-20M) connected to the gate, but now I'm not able to remember how I did it... Here is the schematic:
So, here is the full "experiment": I've done the experiment outside the board. Ids is about 70mA. The FET has an interior diode, so if I connect it wrong - recorder's motor will work with nothing connected to the gate.
Here is how I've tried to connect the FET - without the PCB, "on the table"...
- It gets from the recorder's battery & motor 3V. The +3v is connected to D and the ground to S. The gate is NOT connected - the motor is not running.
- Connecting external PSU, (+ to G and - to S). Rising the voltage from 0 to 2V. The motor is NOT running, gut the Isg is rising up to
- Connecting the external PSU (- to G + to S). Rising the voltage from 0 to 2V. The motor IS running, gut the Isg is rising up to 100mA.
So, this is the main part which confuses me. It is connected to the PSU INVERTED, meaning the G gets - and not +. As I have many FETs, I thought that the one I'm using is faulty - however I've tried 3 more similar FETs - ALL act the same. This is not the same I've built so many years ago - but I cannot understand it at all.
Thanks in advance for any help, Petru