Wawa:
Diagram #4 on this page delivers a constant Vgs.Maximum voltage across R2 is 1000/330 times the voltage across R1,
meaning 1000/330 * 4.3 = 13volt on a 5volt Arduino.
Zener D2 is only added for extra security.Are you sure your (unspecified) P-channel fet is happy with 5volt gate drive.
10-12volt is more common for full saturation.A common voltage divider, with high resistor values, makes the fet switch slow.
Could be very dangerous for the fet if high currents are involved.
Leo..PS
Can't of course use a 2N3904 for 70volt.
Can't use a mosfet there either.
This seem to be a unique solution using the zener as a constant voltage drop. yes I am using a logic level specific trench PMOS FDS8935, It even pretty happy with 3.3v.
I am not very good with transistors, but what i seem to be looking for is a npn with a Vceo of 70 ish volts, with a beta of 100 am i right?
but what i dont understand is how exactly are we tricking the zener??
wvmarle:
However you may be right, but as you didn't give the actual part number we can't check the data sheet.
The actual PMOS is the FDS8935 the NMOS on my schematic is just some genereic NMOS.