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Basic AC-DC Power Supplies
31 questions By Tony R. Kuphaldt
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Question 4 of 31
Suppose a technician measures the voltage output by an AC-DC power supply circuit:
The waveform shown by the oscilloscope is mostly DC, with just a little bit of AC “ripple” voltage appearing as a ripple pattern on what would otherwise be a straight, horizontal line. This is quite normal for the output of an AC-DC power supply.
Suppose we wished to take a closer view of this “ripple” voltage. We want to make the ripples more pronounced on the screen, so that we may better discern their shape. Unfortunately, though, when we decrease the number of volts per division on the “vertical” control knob to magnify the vertical amplification of the oscilloscope, the pattern completely disappears from the screen!
Explain what the problem is, and how we might correct it so as to be able to magnify the ripple voltage waveform without having it disappear off the oscilloscope screen.
Reveal answerThe problem is that the vertical axis input is DC-coupled.
Follow-up question: predict the frequency of the ripple voltage in this power supply circuit.
Notes:As usual, what I’m looking for in an answer here is an explanation for what is happening. If a student simply tells you, “the vertical input is DC-coupled,” press them for more detail. What does it mean for the input to be “DC-coupled,” and why does this cause the line to disappear from the screen when we increase the vertical sensitivity? What alternative do we have to “DC coupling” on an oscilloscope?
One nice thing about oscilloscopes is that they cannot be damaged by “pegging” the display, as can analog multimeters. The same concept applies, though, and is useful in explaining to students why waveforms disappear from the screen when the vertical sensitivity is too great.
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Question 5 of 31
AC-DC power supply circuits are one of the most common circuit configurations in electronic systems. Though designs may vary, the task of converting AC power to DC power is vital in the functioning of a great many electronic devices.
Why is this? What is it about this kind of circuit that makes it such a necessary part of many electronic systems?
Reveal answerMost electric power distribution systems are AC, yet most electronic circuits function on DC power.
Notes:One factor not mentioned in the answer is circuit operating voltage. How do the operating voltages of a typical AC power system and a typical electronic circuit (radio, alarm clock, computer) compare? Ask your students what purpose a power supply has with regard to voltage.
Ask your students if the word “supply” is truly appropriate for this type of circuit. Does it really supply energy, or does it just convert energy from one form into another?
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Question 6 of 31
Although not a popular design, some power supply circuits are transformerless. Direct rectification of AC line power is a viable option in some applications:
However, this form of AC-to-DC power conversion has some significant limits. Explain why most power supply circuits utilize a transformer instead of directly rectifying the line power as this circuit does.
Reveal answerTransformers provide voltage/current ratio transformation, and also electrical isolation between the AC line circuit and the DC circuit. The issue of isolation is a safety concern, as neither of the output conductors in a non-isolated (direct) rectifier circuit is at the same potential as either of the line conductors.
Follow-up question: explain in detail how the issue of non-isolation could create a safety hazard if this rectifier circuit were energized by an earth-grounded AC line circuit.
Notes:Many old television sets used such transformerless rectifier circuits to save money, but this meant the metal circuit chassis inside the plastic cover was energized rather than being at ground potential! Very dangerous for technicians to work on.
i m using power supply for CCTV PTZ Camera with his adaptor rating of 230v AC input & 24V AC Output ,2A load current. what would be the power consumption is their from the main supply line? means is it taking 2 amp from main line of it would be from the adaptor itself?