There are two main types of proportional valves.

I/P converters convert current (I) input, typically as 4 ~20mA, into pneumatic pressure (P) output, whereas E/P converters converts voltage (E) input, usually 0 ~ 10VDC into pneumatic pressure (P) output.

There are a variety of different configurations for these valves. But essentially, a varying electromagnetic force is created, that precisely moves an internal mechanism such as a spool or diaphragm, which allows for a proportionate amount of flow or pressure.