Simple n crude method to measure mosfet rds(on), will this work ?

[BravoV]

I guess the picture tells how crude/simple it is, will this work ?

[digsys]

Short answer, NO. Easiest setup is to apply say 5-12VDC with a load around your expected current (5-10A ?).
Drive the gate past it's linear region, 9v should be fine, and quickly check Vloss across Drain Source. If you heatsink the FET to a slab of
Aluminium (no insulation required), you can then check Rds-on against current, Vgate etc

[BravoV]

Short answer, NO. Easiest setup is to apply say 5-12VDC with a load around your expected current (5-10A ?).
Drive the gate past it's linear region, 9v should be fine, and quickly check Vloss across Drain Source. If you heatsink the FET to a slab of
Aluminium (no insulation required), you can then check Rds-on against current, Vgate etc

Sigh .. as I guessed, thanks Digsys.

Actually I was asked by a friend who lives far away as he wants to sort out bunch of old never used power mosfets to find the lowest RDS on among them.

Looks like I'm going to ask him to do much more complicated method  , which is to wire the fet's DS with a car lamp, hook up to the 12 volt car battery, and start to probe the voltage drop at various points to determine the ball park number on the fet's DS resistance.

He doesn't have any electronic experience before hand. 

[T3sl4co1l]

Yes, if you have a milliohmeter, or the device has quite high resistance to begin with.

As mentioned, the voltage drop under load is probably a more repeatable figure; it also accounts for the somewhat nonlinear Rds vs. Id curve, so you know what voltage drop to expect in place.

An aside: if the voltage drop, at the test current, is less than a diode drop (< 0.5V?), it doesn't matter which direction you measure it in.

Tim