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Switch mode power supplies


Most computers and television sets derive their power from a switch mode source. In industry the pattern is the same with varying levels of complexity. The main driving forces behind this revolution are reductions in price and size. To achieve these goals the switching frequencies are getting higher which results in smaller components. Unfortunately, this does create another set of challenges because higher switching frequencies mean greater distortion of the mains supply and higher losses due to the "skin effect" in conductors. However, industry is gradually providing the technology to overcome these problems.

Regulators based on the switch mode principle are now available. The main component is the IGBT (insulated gate bipolar transistor) and this is used to switch a DC power source on and off very quickly (100kHz being typical). When the power supply is applied to a load the resulting current waveform is sinusoidal. The voltage is adjusted by changing the "mark space ratio" or put simply the time that the pulses are on and off.

Field Bus Interfaces

Most REO units can be supplied with INTERBUS or PROFIBUS interfaces so that several units and other devices can be controlled from a central PLC.

The pdf below contains two diagrams, one showing phase angle control, the other showing pulse width modulation (switch mode) control.

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