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Which of the following statements is NOT true about DC signal?
A DC voltage is always negative or always positive.
A DC voltage has both positive and negative values with change in time.
Cell, batteries and regulated power supplies provide a steady DC that is ideal for an electronic circuit.
Pure Direct Current always has a constant value.
A DC voltage has both positive and negative values with change in time.
A DC (Direct Current) signal is defined by a flow of charge that does not change direction over time. It maintains a constant polarity (either always positive or always negative) relative to a reference point.
IEC 60050-131
A DC (Direct Current) signal is defined by a flow of charge that does not change direction over time. It maintains a constant polarity (either always positive or always negative) relative to a reference point.
VDCтАЛ=T1тАЛтИл0TтАЛv(t)dt тАФ Average value calculation for a periodic signal over period T
In a DC system, the voltage V remains unidirectional. While the magnitude may vary in a pulsating DC signal (like the output of a rectifier), the polarity remains constant because the current does not reverse its direction of flow through the load. AC (Alternating Current), by contrast, changes its polarity periodically with time.
DC polarity remains constant in time (v(t)тЙе0 or v(t)тЙд0 for all t)
Pure DC has zero ripple and a constant amplitude
Pulsating DC varies in magnitude but maintains the same direction
Batteries and solar cells are typical sources of stable DC
Constant polarity is essential for electrolytic chemical processes
Ideal for powering semiconductor and sensitive electronic components
Harder to transform voltage levels compared to AC using simple transformers
High-voltage DC (HVDC) requires expensive converter stations
Battery charging and storage systems
Electronic circuits and microprocessor power supply units
Option B is incorrect because a signal that changes polarity (positive to negative and back) is by definition an Alternating Current (AC) signal.
A DC voltage is often characterized by its average value rather than its instantaneous peak value.
B is correct тАФ A DC voltage cannot change polarity over time as that is the fundamental characteristic of an AC signal.
Always remember that DC means 'Direct' flow, implying no direction change, whereas AC means 'Alternating', which implies periodic direction reversal.
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