hi
i was looking for a .5a n chanel mosfet and stumbled upon a bs170 and noticed the rds on value, was quoted @ 5 ohms, ive never seen it quoted this way, can someone please explain why its quoted this way and how to work out the +v figure rds on using the 5 ohms figure?

thanks very much

 

Rds is its On-Resistance. ALL Mosfets have this spec.
For the BS170 it is typically only 1.2 ohms but the maximum for some of them is 5 ohms when its gate-source voltage is 10V.

The datasheet has graphs that show only typical devices but you cannot buy a typical one. You get whatever they have.