An expansion valve (10) for heat transfer systems such as an air conditioning system, includes a control element (20) for controlling the flow rate of working fluid through the valve. The control element has a stem member (28) and movable member (42) movable on the stem member. Openings (38) to an internal passage (30) in the stem member are regulated by positioning the movable member to achieve regulated flow rate of refrigerant material through the valve. The movable member of the control element is moved by a plunger (24) of a proportional solenoid (22). The proportional solenoid has a magnetic flux circuit including a low permeance isolation tube (62) surrounding the plunger, which enables removal of the coil (76) and frame (78) of the solenoid from the valve. The solenoid further includes a variable permeance flux washer (69) the flux through which varies with plunger position, which is disposed from a gap (82) which provides an area of magnetic saturation. The proportional solenoid produces force/displacement characteristics which enable precise control of the control element and accurate regulation of the refrigerant flow rate through the valve.
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