True proportional power controller

A newer type of power controller is the true proportional power controller—also known as current proportioning controller or phase-angle-fired proportional con-... 

True proportional control is obtained with solid-state switching of loads through SCRs, triacs, and similar solid-state devices. This allows a reduction of cycle time to the millisecond level. If the cycle time is reduced to one-half the power line period (8.3 ms for 60 Hz), then the proportioning action is referred to as stepless control or phase-fired control. A 50% phase-fired output is shown in Fig. 4.33. 

In a true proportional controller, the SCR switches extremely fast—in the order of half a microsecond. This rapid switching causes radio frequency interference (RFl) to be generated on the power lines. This can cause interference with nearby computer and communications equipment. A further problem is that the power drawn from the power lines has a severely distorted wave shape. This can result in extra charges from the power company. 

For many years it was believed that hydrogen atoms could not be seen" in the electron density maps produced by X-ray diffraction. The reason for this is that the atomic X-ray scattering power is proportional to the square of the atomic number. This statement was generally, but not invariably, true until the demise of film-recording methods in the mid-1960 s and the advent of the computer-controlled X-ray diffractometers which could provide very accurate X-ray diffraction intensities. 

---