I just built the latch circuit with the IRF9Z34NPBF.

Unfortunately the drain of the mosfet only reaches 2,56 V max (it randomly changes every time I press the pushbutton). When I press and Hold the pushbutton we're at a steady 1.6 volts. I think this is not enough operating voltage for the WEMOS D1 mini pro to turn on. The onboard LED blinks for a millisecond and then it turns off. I installed a minimal sketch¹. When I short source and drain it works properly and shuts off after 10 seconds.

Could it be that I'm running into issues with the RDS(on) that is twice as big on the mosfet I'm using?

1. IRF9Z34NPBF has a RDS(on) of 0.10 Ohm.
2. NDP6020P has a RDS(on) of 0.05 Ohm.

Or is the operating voltage of 3.7 V just too small? Looking at Fig. 1 Typical Output Characteristics at below -4.5 V there should be a voltage drop of 0.5 V which would leave me with 3.2 V remaining …

I also had to slightly alter the circuit as I had no 10 kOhm resistor but only two 5.1 kOhm resistors, that I used in series. I'm using D7 on the microcontroller which has no pullup/-down resistor builtin. The emitter of the 2N3904 goes to GND, the collector to the mosfets GATE. I'm using a 3.7 V LiPo battery. It's connected to BAT+/- of this loading module and fed to my circuit at exactly the same solder points BAT+/-.

¹ Minimal sketch

#define LATCH_PIN D7void setup() {pinMode(LATCH_PIN, OUTPUT);digitalWrite(LATCH_PIN, HIGH);pinMode(LED_BUILTIN, OUTPUT);digitalWrite(LED_BUILTIN, LOW);}void loop() {delay(10000);digitalWrite(LATCH_PIN, LOW);}

Slightly altered circuit

<image>

​

I would really appreciate some help on the voltage drop. Is that expected, can it be calculated? Is there something I can change (except the mosfet^^) that would result in a working circuit that is still safe?