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May/June 2010

ISA Certified Control Systems Technician (CCST) program

Certified Control System Technicians (CCSTs) calibrate, document, troubleshoot, and repair/replace instrumentation for systems that measure and control level, temperature, pressure, flow, and other process variables.

CCST question

In a full-wave bridge rectifier circuit used in most modern power supplies, as shown above, which statement below is true?

A. The input current between terminals A and B is a sinusoidal AC current, and the output voltage between terminals X and Y is an inverted AC voltage.

B. The output voltage between terminals A and B is an AC voltage, and the input current between terminals X and Y is a square-wave DC current.

C. The output voltage between terminals A and B is a constant DC voltage, and the input current between terminals X and Y is a sinusoidal AC current.

D. The input current between terminals A and B is a sinusoidal AC current, and the output voltage between terminals X and Y is a constant DC voltage.

The correct answer is D. The input current between terminals A and B is a sinusoidal AC current, and the output voltage between terminals X and Y is a constant DC voltage. The transformer accepts an AC current. The diode bridge rectifies the sinusoidal waveform of the AC current and produces a DC ripple voltage, but the RC filter circuit smoothes out the voltage pattern to give the constant DC voltage shown in the figure below.

Answers B and C cannot be correct, since Terminals A and B are used for the input current to the transformer circuit. Answer A is not correct since the output from the diode bridge and RC filter circuit is a DC voltage, not an inverted AC voltage.

Reference:  Hughes, T.A., Measurement and Control Basics, Fourth Edition, ISA, 2007.