Archive for March, 2016

March ’16 Meeting – Power Supplies, Linear and Switching

March 3rd, 2016 No comments

The March 2016 Meeting was at 7:30 on March 14 at HCESD Fire Station #4, 22855 Franz Rd, Katy, TX 77449.

Summary of Presentation by Brian AF5IT:

Sooner or later every ham wants or needs to set up a mobile radio in a base station configuration.  Doing so will require a power supply which converts the 120 volts AC power to 12 to 14 volts DC power.  There are primarily two types of regulation used in today’s commercially available power supplies:  Linear and Switch Mode.  The linear regulation supply constantly draws AC current and regulates on the DC side of the step down transformer by using a transistor as a variable resistor.  Output current draw adjusts base bias on the transistor allowing for constant voltage output.  Any unused power is dissipated off as heat.  Advantages:  less parts, easier to troubleshoot.  Disadvantage:  not too efficient

The switch mode supply regulates by rectifying AC power, using that filtered, but unregulated, voltage to power an oscillator IC chip whose output drives a switching transistor.  In the transistor collector circuit is the primary of a transformer.  The on and off action of the transistor causes current to go and stop through the transformer, resulting in a magnetic field which is felt by the secondary and creates current flow in the secondary circuit.  This waveform is a square wave running at around 15Khz.  The wave is rectified and filtered and sent to the device needing power.  Output voltage fluctuation is detected and feedback to the IC chip which adjusts its output frequency and/or duty cycle, adjusting the voltage across the transformer primary.  Power is only drawn for the AC supply when needed.  Advantages:  Highly efficient, cooler running.  Disadvantage:  many parts and may be victim of domino effect failure.

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