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in this circuit:

schematic

simulate this circuit – Schematic created using CircuitLab

is it possible to measure resistance between drain and source if no motor(inductance) is connected or do the diode/capacitor falsify results?

assuming the gate still carries a charge after being connected to a power supply, do i need to connect it to a power supply ground to get rid of the charge or just shorten source and gate? can there be a charge at all or will R1 (1MOhm) take care of that?

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  • \$\begingroup\$ Do you need to measure while the 12V source is applied or disconnected? One can use ohm's law, Rds(on) = Vds / I. Measure current while applying a known voltage and you have the drain to source resistance. I assume looking up the data sheet isn't sufficient. \$\endgroup\$ – tokamak Mar 9 '16 at 23:32
  • \$\begingroup\$ Can't answer your last 2 questions without knowing the capacitance on the gate, stray and designed. \$\endgroup\$ – tokamak Mar 9 '16 at 23:34
  • \$\begingroup\$ when in the circuit with no motor attached at 0V and outside a circuit, just the mosfet at 0V \$\endgroup\$ – James Baker Mar 10 '16 at 13:51
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A MOSFET does not have resistance in the same sense that R1 and R2 do. There is no single number which characterizes the behavior of the drain-source path. Instead, the equivalent resistance (drain-source voltage divided by drain-source current) will depend on 3 things: gate-source voltage, drain-source voltage or current (take your pick) and temperature. For data sheet switching use, gate-source voltage is generally held quite high, like 10 volts, while drain-source current is specified at some high level, where "high" depends on the transistor rating. Than the drain-source voltage is measured and the ratio of voltage to current is calculated to produce a resistance figure, referred to as Rds(on).

So it's perfectly possible to measure the effective resistance of the MOSFET in your drawing if you replace the motor with a resistor or other element which produces the same current as the motor would draw. But it is not (reasonably) possible to accurately determine the effective resistance without actually drawing current. Among other things, it's necessary to take into account of whatever heatsink has been attached to the MOSFET.

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The resistance of the mosfet should be low, in the 10mΩ's or even lower (check your datasheet) when its on and you can check the resistance with a meter (but the result might not be entirely accurate.

If the mosfet is not switching on, the gate voltage needs to be higher than the drain to for the mosfet to switch on, so adding a 1MΩ resistor to ground isn't going to change that, it will not protect the gate from inrush current. Put a resistor in between the driver and the gate if the driver doesn't have protection. The gate is a thin layer of silicon and it's easy to "blow out" with ESD or inrush current, it needs protection. A resistor of 100Ω should do the trick.

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  • \$\begingroup\$ changed the position of R2 \$\endgroup\$ – James Baker Mar 10 '16 at 0:08

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