Sentence "The resistor must be large enough so that it will never supply the high valley current IP" ... Should read ... "The resistor RA must be large enough so that it will never supply more than the low valley current Iv". One might add . . . RA forms the Load Line that intersects the -ve resistance region of the characteristic, intersects the vertical VA axis above VP and intersects the horizontal IA axis such that the intersection with the negative resistance part of the characteristic is to the left of IV. Thus, the PUT switches down when VC < VV and switches on when VC > VP. Les L.