Power Supplies.
Subelement L04
Section L04
For the same transformer secondary voltage, which rectifier has the highest average output voltage?
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Half-wave
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Quarter-wave
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Full-wave centre-tap
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Correct AnswerBridge
The half-wave configuration is not as efficient at rectification as the full-wave. The real choice lies between the two full-wave alternatives: bridge or full-wave with centre-tap. The bridge configuration rectifies the full secondary winding voltage. In contrast, the two-diode with centre-tapped transformer secondary only rectifies half the secondary voltage on each half-cycle.
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In a half-wave power supply with a capacitor input filter and a load drawing little or no current, the peak inverse voltage (PIV) across the diode can reach _____ times the RMS voltage.
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Correct Answer2.8
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0.45
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5.6
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1.4
During conduction, the capacitor charges up to the peak value of the waveform (1.4 times the RMS value). On the opposite half-cycle, and from the diode's standpoint, the transformer winding reaching opposite peak value (1.4 times the RMS value) adds to the capacitor charge for a total of 2.8 times the RMS value.
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In a full-wave centre-tap power supply, regardless of load conditions, the peak inverse voltage (PIV) will be _____ times the RMS voltage:
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1.4
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Correct Answer2.8
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0.636
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0.707
With the RMS voltage defined in this example as the voltage from one extremity of the secondary winding to the centre-tap, each diode is subjected to 2.8 times the RMS voltage, peak reverse voltage from half the transformer winding adds to the peak DC output as a reverse bias on the diode during non-conduction ( each diode serves as a half-wave rectifier ).
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A full-wave bridge rectifier circuit makes use of both halves of the AC cycle, but unlike the full-wave centre-tap rectifier circuit it does not require:
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Correct Answera centre-tapped secondary on the transformer
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any output filtering
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a centre-tapped primary on the transformer
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diodes across each leg of the transformer
For a given transformer the maximum output voltage available from a full-wave bridge rectifier circuit will be:
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Correct Answerdouble that of the full-wave centre-tap rectifier
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half that of the full-wave centre-tap rectifier
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the same as the full-wave centre-tap rectifier
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the same as the half-wave rectifier
The discussion relates to rectification, thus DC voltage output is the criteria. Imagine a 12 volts AC secondary with a centre tap. A bridge rectifier across the full secondary will obviously provide twice the voltage of a full-wave centre-tap rectifier where each diode draws from half the secondary. The bridge rectifier will also output slightly more DC voltage after filtering than a half-wave rectifier across the same full secondary.
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The ripple frequency produced by a full-wave power supply connected to a normal household circuit is:
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30 Hz
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Correct Answer120 Hz
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60 Hz
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90 Hz
Key word: FULL-WAVE. The two half-cycles are put to contribution. The output goes from zero to peak and back 120 times per second. Half-wave would be 60 hertz.
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The ripple frequency produced by a half-wave power supply connected to a normal household circuit is:
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Correct Answer60 Hz
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90 Hz
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120 Hz
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30 Hz
Key word: HALF-WAVE. One half-cycle only is put to contribution. The output goes from zero to peak and back 60 times per second. Full-wave would be 120 hertz.
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Full-wave voltage doublers:
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Correct Answeruse both halves of an AC wave
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create four times the output voltage of half-wave doublers
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use less power than half-wave doublers
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are used only in high-frequency power supplies
A voltage doubler returns a DC voltage approximately twice the supplied AC voltage. Through combinations of diodes and capacitors, both half-cycles are rectified and added together. Two ubiquitous configurations are respectively designated as "half-wave" doubler and "full-wave" doubler. The designation has more to do with the ripple frequency than how energy is transferred to the output. Ripple frequency in the "full-wave" doubler is twice the supply frequency. They can be implemented at normal line frequency or in switching power supplies.
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What are the two major ratings that must not be exceeded for silicon-diode rectifiers used in power-supply circuits?
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Average power; average voltage
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Capacitive reactance; avalanche voltage
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Peak load impedance; peak voltage
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Correct AnswerPeak inverse voltage; average forward current
During conduction, the diode must support the average forward current. Under reverse bias, the diode must support the peak inverse voltage present across it.
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In a high voltage power supply, why should a resistor and capacitor be wired in parallel with the power-supply rectifier diodes?
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To ensure that the current through each diode is about the same
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Correct AnswerTo equalize voltage drops and guard against transient voltage spikes
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To smooth the output waveform
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To decrease the output voltage
Parallel capacitors are used to bypass voltage spikes. Parallel resistors across each diode in a chain of diodes equalize reverse voltage.
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What is the output waveform of an unfiltered full-wave rectifier connected to a resistive load?
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A steady DC voltage
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A sine wave at half the frequency of the AC input
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A series of pulses at the same frequency as the AC input
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Correct AnswerA series of pulses at twice the frequency of the AC input
A full-wave rectifier puts both half-cycles to contribution: pulsating direct current with 120 zero-to-peak transitions per second is produced.
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Filter chokes are rated according to:
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power loss
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breakdown voltage
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Correct Answerinductance and current-handling capacity
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reactance at 1000 Hz
Filter chokes are wired in series with the rectifier output. The choke must support the current drawn by the load. Its inductance influences the reduction in ripple.
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Which of the following circuits gives the best regulation, under similar load conditions?
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A half-wave bridge rectifier with a capacitor input filter
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A half-wave rectifier with a choke input filter
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A full-wave rectifier with a capacitor input filter
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Correct AnswerA full-wave rectifier with a choke input filter
Regulation is the change in voltage from no-load to full-load. The first filter element determines the classification. Capacitor-input filters ensure high output voltage but poor regulation: voltage soars to the peak AC value under no load and drops under load. Capacitor-input leads to high peak rectifier current. Choke-input filters limit the soar in voltage through counter-EMF and by opposing capacitor charge current. Peak rectifier current is constrained but output voltage approximates the average value of the AC waveform. Half-wave circuits have the poorest regulation.
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The advantage of the capacitor input filter over the choke input filter is:
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better filtering action or smaller ripple voltage
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improved voltage regulation
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lower peak rectifier currents
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Correct Answera higher terminal voltage output
Regulation is the change in voltage from no-load to full-load. The first filter element determines the classification. Capacitor-input filters ensure high output voltage but poor regulation: voltage soars to the peak AC value under no load and drops under load. Capacitor-input leads to high peak rectifier current. Choke-input filters limit the soar in voltage through counter-EMF and by opposing capacitor charge current. Peak rectifier current is constrained but output voltage approximates the average value of the AC waveform. Half-wave circuits have the poorest regulation.
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With a normal load, the choke input filter will give the:
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highest output voltage
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Correct Answerbest regulated output
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greatest percentage of ripple
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greatest ripple frequency
Regulation is the change in voltage from no-load to full-load. The first filter element determines the classification. Capacitor-input filters ensure high output voltage but poor regulation: voltage soars to the peak AC value under no load and drops under load. Capacitor-input leads to high peak rectifier current. Choke-input filters limit the soar in voltage through counter-EMF and by opposing capacitor charge current. Peak rectifier current is constrained but output voltage approximates the average value of the AC waveform. Half-wave circuits have the poorest regulation.
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There are two types of filters in general use in a power supply. They are called:
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choke input and capacitor output
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choke output and capacitor input
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Correct Answerchoke input and capacitor input
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choke output and capacitor output
Regulation is the change in voltage from no-load to full-load. The first filter element determines the classification. Capacitor-input filters ensure high output voltage but poor regulation: voltage soars to the peak AC value under no load and drops under load. Capacitor-input leads to high peak rectifier current. Choke-input filters limit the soar in voltage through counter-EMF and by opposing capacitor charge current. Peak rectifier current is constrained but output voltage approximates the average value of the AC waveform. Half-wave circuits have the poorest regulation.
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The main function of the bleeder resistor in a power supply is to provide a discharge path for the capacitor in the power supply. But it may also be used for a secondary function, which is to:
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act as a secondary smoothing device in conjunction with the filter
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Correct Answerimprove voltage regulation
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provide a ground return for the transformer
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inhibit the flow of current through the supply
Regulation is the change in voltage from no-load to full-load. By ensuring a certain minimum current draw on the supply, the bleeder prevents the capacitors from fully charging up to peak AC values when no external load is connected.
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In a power supply, series chokes will:
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impede the passage of DC but will pass the AC component
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impede both DC and AC
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Correct Answerreadily pass the DC but will impede the flow of the AC component
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readily pass the DC and the AC component
When using a choke input filter, a minimum current should be drawn all the time when the device is switched on. This can be accomplished by:
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placing an ammeter in the output circuit
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increasing the value of the output capacitor
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Correct Answerincluding a suitable bleeder resistance
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utilizing a full-wave bridge rectifier circuit
In the design of a power supply, the designer must be careful of resonance effects because the ripple voltage could build up to a high value. The components that must be carefully selected are:
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first capacitor and second capacitor
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first choke and second capacitor
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Correct Answerfirst choke and first capacitor
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the bleeder resistor and the first choke
Series resonance in the first choke and first capacitor across the rectifier may cause excessive rectifier peak current and abnormally high peak reverse voltages on the diodes.
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Excessive rectifier peak current and abnormally high peak inverse voltages can be caused in a power supply by the filter forming a:
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tuned inductance in the filter choke
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Correct Answerseries resonant circuit with the first choke and first capacitor
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short circuit across the bleeder
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parallel resonant circuit with the first choke and second capacitor
Series resonance in the first choke and first capacitor across the rectifier may cause excessive rectifier peak current and abnormally high peak reverse voltages on the diodes.
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In a properly designed choke input filter power supply, the no-load voltage across the filter capacitor will be about nine-tenths of the AC RMS voltage; yet it is advisable to use capacitors rated at the peak transformer voltage. Why is this large safety margin suggested?
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Resonance can be set up in the filter producing high voltages
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Under heavy load, high currents and voltages are produced
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Under no-load conditions, the current could reach a high level
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Correct AnswerUnder no-load conditions and a burned-out bleeder, voltages could reach the peak transformer voltage
Inductors oppose changes in current. If no current at all is drawn from a choke-input filter, the effect of the inductor vanishes: no more counter-EMF or opposition to peak capacitor charging current. Subsequent capacitors are allowed to fully charge to peak AC values. [ nine tenths the RMS: RMS is 0.707 times peak, average is 0.637 times peak, 0.637 is nine tenths of 0.707 ]
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What is one characteristic of a linear electronic voltage regulator?
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A pass transistor switches from its "on" state to its "off" state
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The control device is switched on or off, with the duty cycle proportional to the line or load conditions
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Correct AnswerThe conduction of a control element is varied in direct proportion to the line voltage or load current
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It has a ramp voltage at its output
In a 'linear' voltage regulator, a voltage higher than necessary is first produced; this voltage is brought down through a voltage dropping component. A regulator circuit (e.g., a Zener in a shunt configuration) may draw more or less current through a passive resistor to compensate for external changes. The dropping element, in a series configuration, may be a tube or transistor whose conduction may be varied. In a switching regulator, the incoming DC is switched on and off; the on time is varied so that the average DC output is maintained regardless of current draw.
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What is one characteristic of a switching voltage regulator?
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It gives a ramp voltage at its output
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Correct AnswerThe control device is switched on and off, with the duty cycle proportional to the line or load conditions
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The conduction of a control element is varied in direct proportion to the line voltage or load current
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It provides more than one output voltage
In a 'linear' voltage regulator, a voltage higher than necessary is first produced; this voltage is brought down through a voltage dropping component. A regulator circuit (e.g., a Zener in a shunt configuration) may draw more or less current through a passive resistor to compensate for external changes. The dropping element, in a series configuration, may be a tube or transistor whose conduction may be varied. In a switching regulator, the incoming DC is switched on and off; the on time is varied so that the average DC output is maintained regardless of current draw.
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What device is typically used as a stable reference voltage in a linear voltage regulator?
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A varactor diode
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A junction diode
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Correct AnswerA Zener diode
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An SCR
What type of linear regulator is used in applications requiring efficient utilization of the primary power source?
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A constant current source
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A shunt current source
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Correct AnswerA series regulator
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A shunt regulator
Key word: EFFICIENT. A linear regulator with an active series dropping device (tube or transistor) wastes less energy as dropping resistance is adjusted to whatever current is drawn. A linear regulator with a passive dropping resistor and a control device in a shunt configuration (Zener, tube or transistor) is wasteful because a fixed amount of current is needed to maintain a given drop in voltage regardless of load current; the device draws less when load current increases and vice-versa. The shunt configuration may be needed if the unregulated source demands a constant load.
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What type of linear voltage regulator is used in applications requiring a constant load on the unregulated voltage source?
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A shunt current source
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A series regulator
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Correct AnswerA shunt regulator
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A constant current source
Key words: CONSTANT LOAD. A linear regulator with an active series dropping device (tube or transistor) wastes less energy as dropping resistance is adjusted to whatever current is drawn. A linear regulator with a passive dropping resistor and a control device in a shunt configuration (Zener, tube or transistor) is wasteful because a fixed amount of current is needed to maintain a given drop in voltage regardless of load current; the device draws less when load current increases and vice-versa. The shunt configuration may be needed if the unregulated source demands a constant load.
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How is remote sensing accomplished in a linear voltage regulator?
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A load connection is made outside the feedback loop
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By wireless inductive loops
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Correct AnswerA feedback connection to an error amplifier is made directly to the load
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An error amplifier compares the input voltage to the reference voltage
Key word: REMOTE. Voltage regulation relies on comparing the output voltage to a set reference and using the error to adjust conduction in the control element of the regulator. Sensing the voltage at the load rather than at the output terminals of the power supply compensates for losses all the way out to the load.
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What is a three-terminal regulator?
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A regulator containing three error amplifiers and sensing transistors
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A regulator that supplies three voltages with variable current
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Correct AnswerA regulator containing a voltage reference, error amplifier, sensing resistors and transistors, and a pass element
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A regulator that supplies three voltages at a constant current
A three-terminal regulator is a single integrated circuit comprising a voltage reference, a comparator, an error amplifier, sensing resistors and a pass transistor. Some include thermal shutdown, current foldback and over-voltage protection. The three terminals are: unregulated DC input, regulated DC output and ground. Specifications include: maximum output current, maximum output voltage, maximum input voltage and minimum input voltage (because a minimum voltage differential is needed to maintain regulation, the drop-out voltage).
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In addition to an input voltage range what are the important characteristics of a three-terminal regulator?
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Output voltage and minimum output current
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Correct AnswerOutput voltage and maximum output current
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Maximum output voltage and minimum output current
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Minimum output voltage and maximum output current
A three-terminal regulator is a single integrated circuit comprising a voltage reference, a comparator, an error amplifier, sensing resistors and a pass transistor. Some include thermal shutdown, current foldback and over-voltage protection. The three terminals are: unregulated DC input, regulated DC output and ground. Specifications include: maximum output current, maximum output voltage, maximum input voltage and minimum input voltage (because a minimum voltage differential is needed to maintain regulation, the drop-out voltage).
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What type of voltage regulator contains a voltage reference, error amplifier, sensing resistors and transistors, and a pass element in one package?
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Correct AnswerA three-terminal regulator
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An op-amp regulator
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A switching regulator
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A Zener regulator
A three-terminal regulator is a single integrated circuit comprising a voltage reference, a comparator, an error amplifier, sensing resistors and a pass transistor. Some include thermal shutdown, current foldback and over-voltage protection. The three terminals are: unregulated DC input, regulated DC output and ground. Specifications include: maximum output current, maximum output voltage, maximum input voltage and minimum input voltage (because a minimum voltage differential is needed to maintain regulation, the drop-out voltage).
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When extremely low ripple is required, or when the voltage supplied to the load must remain constant under conditions of large fluctuations of current and line voltage, a closed-loop amplifier is used to regulate the power supply. There are two main categories of electronic regulators. They are:
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non-linear and switching
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linear and non-linear
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stiff and switching
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Correct Answerlinear and switching
In a 'linear' voltage regulator, a voltage higher than necessary is first produced; this voltage is brought down through a voltage dropping component. A regulator circuit (e.g., a Zener in a shunt configuration) may draw more or less current through a passive resistor to compensate for external changes. The dropping element, in a series configuration, may be a tube or transistor whose conduction may be varied. In a switching regulator, the incoming DC is switched on and off; the on time is varied so that the average DC output is maintained regardless of current draw.
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A modern type of regulator, which features a reference, high-gain amplifier, temperature-compensated voltage sensing resistors and transistors as well as a pass element is commonly referred to as a:
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Correct Answerthree-terminal regulator
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nine-pin terminal regulator
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twenty-four pin terminal
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regulator six-terminal regulator
A three-terminal regulator is a single integrated circuit comprising a voltage reference, a comparator, an error amplifier, sensing resistors and a pass transistor. Some include thermal shutdown, current foldback and over-voltage protection. The three terminals are: unregulated DC input, regulated DC output and ground. Specifications include: maximum output current, maximum output voltage, maximum input voltage and minimum input voltage (because a minimum voltage differential is needed to maintain regulation, the drop-out voltage).
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In a series-regulated power supply, the power dissipation of the pass transistor is:
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the inverse of the load current and the input/output voltage differential
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dependent upon the peak inverse voltage appearing across the Zener diode
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indirectly proportional to the load voltage and the input/output voltage differential
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Correct Answerdirectly proportional to the load current and the input/output voltage differential
The pass transistor is the device acting as a variable resistor to drop the unregulated DC source down to the regulated output. Power is voltage times current: in this case, the difference in voltage from input to output times the current drawn by the load.
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In any regulated power supply, the output is cleanest and the regulation is best:
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across the secondary of the pass transistor
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across the load
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at the output of the pass transistor
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Correct Answerat the point where the sampling network or error amplifier is connected
A voltage regulator maintains a stable output by comparing a sample of the output voltage with a reference and adjusting conduction in the pass transistor accordingly. The corrective action is only accurate for the precise point where the measurement is taken. Because of losses, the load itself may find itself at a lower voltage: this is the reason for 'remote sensing' in certain applications.
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When discussing a power supply the_______ resistance is equal to the output voltage divided by the total current drawn, including the current drawn by the bleeder resistor:
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Correct Answerload
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ideal
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rectifier
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differential
Per Ohm's Law, resistance is voltage divided by current. Output voltage and total current drawn describe the load placed on the power supply.
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The regulation of long-term changes in the load resistance of a power supply is called:
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Correct Answerstatic regulation
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active regulation
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analog regulation
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dynamic regulation
Key words: LONG-TERM. Regulation is the change in voltage from no-load to full-load. Static regulation relates to the supply's performance in relation with long-term changes in load resistance or line variations (AC source). Dynamic regulation is required when the current draw varies as a Morse key is pressed (CW) or with each syllable (SSB) in a final amplifier. A large output capacitor, the last in the filter configuration, can improve dynamic regulation.
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The regulation of short-term changes in the load resistance of a power supply is called:
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Correct Answerdynamic regulation
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static regulation
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analog regulation
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active regulation
Key words: SHORT-TERM. Regulation is the change in voltage from no-load to full-load. Static regulation relates to the supply's performance in relation with long-term changes in load resistance or line variations (AC source). Dynamic regulation is required when the current draw varies as a Morse key is pressed (CW) or with each syllable (SSB) in a final amplifier. A large output capacitor, the last in the filter configuration, can improve dynamic regulation.
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The dynamic regulation of a power supply is improved by increasing the value of:
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the input capacitor
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the bleeder resistor
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Correct Answerthe output capacitor
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the choke
Regulation is the change in voltage from no-load to full-load. Static regulation relates to the supply's performance in relation with long-term changes in load resistance or line variations (AC source). Dynamic regulation is required when the current draw varies as a Morse key is pressed (CW) or with each syllable (SSB) in a final amplifier. A large output capacitor, the last in the filter configuration, can improve dynamic regulation.
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The output capacitor, in a power supply filter used to provide power for an SSB or CW transmitter, will give better dynamic regulation if:
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the negative terminal of the electrolytic capacitor is connected to the positive and the positive terminal to ground
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a battery is placed in series with the output capacitor
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it is placed in series with other capacitors
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Correct Answerthe output capacitance is increased
Regulation is the change in voltage from no-load to full-load. Static regulation relates to the supply's performance in relation with long-term changes in load resistance or line variations (AC source). Dynamic regulation is required when the current draw varies as a Morse key is pressed (CW) or with each syllable (SSB) in a final amplifier. A large output capacitor, the last in the filter configuration, can improve dynamic regulation.
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In a regulated power supply, four diodes connected together in a BRIDGE act as:
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matching between the secondary of the power transformer and the filter
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a tuning network
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Correct Answera rectifier
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equalization across the transformer
In a regulated power supply, components that conduct alternating current at the input before the transformer and direct current before the output are:
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chokes
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Correct Answerfuses
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capacitors
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diodes
In a regulated power supply, the output of the electrolytic filter capacitor is connected to the:
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solid-state by-pass circuit
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matching circuit for the load
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Correct Answervoltage regulator
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pi filter
Remember your Basic Qualification? The Regulated Power Supply comprises: the input, the transformer, the rectifier, the filter, the regulator and the output.
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In a regulated power supply, a diode connected across the input and output terminals of a regulator is used to:
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Correct Answerprotect the regulator from reverse voltages
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provide an RF by-pass for the voltage control
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provide additional capacity
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protect the regulator from voltage fluctuations in the primary of the transformer
A fast reverse-biased diode between the output and input terminals prevents a large capacitor following the three-terminal regulator from discharging through the regulator if the input was ever short-circuited.
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