I have this circuit built, and the first batch of 6 pieces all worked.
Now the second time, I made 4, and only 1 works, the others all do the following.
When the voltage goes below 16.2V, the output drops to 7.8V and something buzzes softly. The voltage then after a bit, does go down to zero, but it cannot be that low for my application... Is it a problem with the Zener Diode, the latch mechanism, the MOSFET, or what?
Perhaps you should tell us what the intended function is, what the power source is, and the characteristics of the load.
Check very carefully the component placements between the good and bad boards. a consistent problem across a batch of boards usually comes down to assembly issues.
There are no buzz-able components in the schematic. Try to determine the location of the noise.
This schematic is added in front of a small capacity buck converter circuit, using the MP2357 buck regulator.
The Converter output voltage is adjustable between 5V and 10V to run a small fan.
The power supply is a lithium cordless tool battery, which needs a low voltage protection at about 16V.
My zener diode is in a SOT-23-3 package, so it cannot really be backwards, and I tested the forward and reverse-biased voltage, and it is the same, even on the ones that work and do not work.
I checked almost all of the components to make sure they are in the correct places...
I ordered some parts that should arrive soon to try swapping out the Zener diode among other things...
Would it have to do with static in assembly, because the first ones I made sure to always be at the static-free station, and the second batch, I did not use as much caution as I should have...
Would it have to do with static in assembly, because the first ones I made sure to always be at the static-free station, and the second batch, I did not use as much caution as I should have...
C2 looks like a mistake to me. You could try putting it in parallel with R1 instead.
The possible issue is that in real life, the battery will have resistance, so when the MOSFET turns off, the battery voltage will increase. Having a capacitor in parallel with R8 will tend to make the MOSFET turn on again as the battery voltage increases.
The MOSFET has a gate threshold voltage between 1 and 3 volts. https://www.onsemi.com/pub/Collateral/FDS4685-D.PDF and it could be that the second batch has a lower gate threshold voltage, making turning back on more likely.
You could try simulating with some internal resistance in the battery.