I'm just starting out with electronics and have built several Arduino project connecting modules with software (i'm a software engineer), but now I want to get more involved with the electronics side of the build. I recently watched Dave's video on the World's simplest soft latching power supply and wanted to follow along with each stage of his design. This meant I tried to recreate the latch circuit starting with the initial 2 transistor solution.
Based on the whiteboard diagram @3:26 I tried to mimic the circuit but wasn't able to make it work without some changes - only I don't understand why I needed the changes. I'll do my best to explain the circuit, but please see the attached images...
For the top transistor I used a 2N3906 and for the bottom transistor I used a 2N3904. I played with the resistors a bit and had something working using 100K for each, but really it didn't seem to matter what resistor values I used. I also used a 9v battery for the power supply and a green LED with a 1K resistor in series for the load.
So here's the problem - Whenever I connected the based of the lower transistor to Vout, using a resistor ranging from 330 ohm to 1M ohm, the circuit immediately turned on? So first off I don't understand why there's a current flowing to Vout when the base of the top resistor is zero? Could this be a bad/cheap Chinese transistors problem? If I used a 1M ohm resistor the current should be in the micro-amp range so way below the required current to turn on the gate, right?
To make the circuit work I could move the resistor connected to the base of the lower transistor to the far side of the load LED. I guessing this is because the LED requires 2+ volts before it conducts and so unless it's on then there's truly no current flowing.
Any help to understand why the 'Always on' circuit wasn't able to toggle on and off as Dave suggested would be very much appreciated.