Thanks Stenzer, Fvm and Easy for reply.
As Stenzer saym the zero current is max 10uA, that correspond to
1500/10uA= 150Mohm
when mos is open, and, this is trascurable when parallel with a R isolation to measure..
I will be more clear about how we will do the measure (in case this helps).
In attach you have the same schematic of last time (FIG 1), but in red i higlighted the 2 resistor we need to measure; this 2 resistor are the isolation resistor due to air, case ecc.
This resistor seems to be tipical around some Mohm.
We have 2 incognit, RG1, RG2, so we need 2 equation for solve them.
1 Equation comes from opening M1, closing M2, and measure the Vout
2 Equation comes from opening M2 ,closing M1 ,and measure the Vout (that will be different)
So, solving the equation , we have RH1 and RH2
As I sayd, the NMOS i find for 1700V, but the PMOS not.
Relay seems the solution ,or In alternative I find a IGBT .
Now, Im thinking, that Rele or IGBT ,both need a tipical -open collector- driver circuit (FIG 2 , to put Rele or IGBT on /off), that is ,another NPN (o NMOS) with a Rc on collector,(or Drain) ,and then Rele (IGBT) and then to 1500V.
This Rc (from 1500V generator to gnd) ,disturb my equation because Rc could compete with RH1 RH2
Working at 1000V or less, we have no this problem; The PMOS or IGBT was substitute by a PhotoMos, so,
driving him with some mA to 3.3V, we put On/Off the Photomos ..NO driver circuit needed connected to 1000V..No distrurb of the measure.
Now,at 1500V, no Photomos.
Thanks for support and excuse for long mail
Roberto