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Subelement E7
PRACTICAL CIRCUITS
Section E7D
Power supplies and voltage regulators; Solar array charge controllers
What is one characteristic of a linear electronic voltage regulator?
  • It has a ramp voltage as its output
  • It eliminates the need for a pass transistor
  • The control element duty cycle is proportional to the line or load conditions
  • Correct Answer
    The conduction of a control element is varied to maintain a constant output voltage

In electronics, voltage regulators maintain a constant output voltage.

So, whether the question mentions Linear or Switching voltage regulators, choose the answer that mentions maintaining a constant output voltage.

The resistance of the regulator varies in accordance with the load resulting in a constant output voltage.

https://en.wikipedia.org/wiki/Linear_regulator

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What is one characteristic of a switching electronic voltage regulator?
  • The resistance of a control element is varied in direct proportion to the line voltage or load current
  • It is generally less efficient than a linear regulator
  • Correct Answer
    The controlled device's duty cycle is changed to produce a constant average output voltage
  • It gives a ramp voltage at its output

In electronics, voltage regulators maintain a constant output voltage.

So, whether the question mentions Linear or Switching voltage regulators, choose the answer that mentions maintaining a constant output voltage.


A switching electronic voltage regulator turns the device on and off multiple times per cycle in order to lower the voltage. The design variables are the number of times it turns on and off, and the length of time it is on or off.

If the device were on 100% of the time, the voltage would be 100%. To reduce the voltage, the switching electronic voltage regulator turns the device off for some percentage of time and on for some percentage of time. That percentage depends on how many times per cycle the device is off or on, and how long it is off or on.

-k6yxh

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What device is typically used as a stable reference voltage in a linear voltage regulator?
  • Correct Answer
    A Zener diode
  • A tunnel diode
  • An SCR
  • A varactor diode

A Zener diode is typically used as a stable reference voltage in a linear voltage regulator. When reverse biased, the diode breaks down and allows current to flow in the reverse direction when the voltage is above the avalanche point. The Zener voltage is the voltage necessary to cause the avalanche to occur and allow current to flow. As long as the reverse bias voltage remains above the avalanche point, the voltage drop across the Zener diode remains constant to provide a stable reference for regulating power supply output voltage.


Hint: this is a stretch but it worked for me...
stables are for horses
Zorro had a horse
Z is for Zener

A Zener Diode is the simplest type of voltage regulator.


Hint 2: a ZEE-ner is LEEnyer (linear). Weird, but helps me remember.

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Which of the following types of linear voltage regulator usually make the most efficient use of the primary power source?
  • A series current source
  • Correct Answer
    A series regulator
  • A shunt regulator
  • A shunt current source

We can narrow the answer down to two potential answers right off the bat to "A series regulator" and "A shunt regulator".

A shunt regulator is exclusively used in low-power circuits as it is far less efficient than a series regulator.

REF: https://en.wikipedia.org/wiki/Linear_regulator#Simple_shunt_regulator

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Which of the following types of linear voltage regulator places a constant load on the unregulated voltage source?
  • A constant current source
  • A series regulator
  • A shunt current source
  • Correct Answer
    A shunt regulator

An example of a shunt regulator is using a Zener diode in series with a resistor between supply and ground. The Zener diode establishes a constant voltage drop and the resistor sets a constant current.

Note: You can immediately dismiss two of the possible answers. "Regulator" appears in both the question and the answer.

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What is the purpose of Q1 in the circuit shown in Figure E7-3?
  • It provides negative feedback to improve regulation
  • It provides a constant load for the voltage source
  • Correct Answer
    It increases the current-handling capability of the regulator
  • It provides D1 with current

Realize that the transistor Q1, a bipolar junction transistor (BJT), is the valve for current flow and is controlled by the Base current. The Zener diode D1 cannot supply the load with enough current and operate as a reference point.

Q1 increase or amplifies the current-handling capability of the diode supplying current to the Base.

Hint: Arrow is pointing up or increasing. Only 1 answer with "increases"

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What is the purpose of C2 in the circuit shown in Figure E7-3?
  • Correct Answer
    It bypasses hum around D1
  • It is a brute force filter for the output
  • To self-resonate at the hum frequency
  • To provide fixed DC bias for Q1

If you see closely, C2 is connected in parallel to D1. This will basically act as a filter across D1 and prevent oscillations across D1 and hence prevent the hum.

Remember the term bypass capacitor and that a bypass capacitor will pass above certain wavelengths of AC current while blocking DC current. Bypass capacitors are very common for eliminating undesirable AC current such as noise so it is useful to remember this term.

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What type of circuit is shown in Figure E7-3?
  • Switching voltage regulator
  • Grounded emitter amplifier
  • Correct Answer
    Linear voltage regulator
  • Emitter follower

In this question, it is most important to choose the most correct answer-- a linear regulator-- even though an emitter follower is present in the circuit.

D1 is a zener diode; it begins to conduct in reverse when the voltage exceeds a breakdown value. R1 provides current to D1 until this happens. Therefore, the voltage at Q1's base is approximately D1's zener voltage (probably somewhere around 12.7V in this case).

Q1 is an emitter follower that "amplifies" the voltage on its base on its emitter. Together, this entire circuit accepts a high voltage on its input (on the left, 25V) and produces 12V on its output (at right) and thus forms a linear regulator.

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What is the main reason to use a charge controller with a solar power system?
  • Prevention of battery undercharge
  • Control of electrolyte levels during battery discharge
  • Correct Answer
    Prevention of battery damage due to overcharge
  • Matching of day and night charge rates

Batteries are best charged by constant current, not constant voltage. However, solar cells put out a constant voltage (unless they're being over-drawn), and the solar cell voltage is chosen to be a higher voltage than the battery voltage.

The ideal charge current is dependent on the battery size and technology (Lead acid vs lithium vs metal hydride, etc) and beyond the scope of this question.

But what a charge controller does is regulate the voltage output to be whatever voltage that causes the proper current to flow into the battery, and disconnects the input current from the battery when the battery is full.

If you were to hook a 12V solar panel to a 12V battery, it would charge VERY slowly, especially as the battery neared it's capacity. Technically, you could never fully charge the battery.

But if you were to hook a 25V solar cell directly to a 12V battery, you could very quickly overcharge the battery damaging it and possibly causing an explosion. A charge controller will safely take the 25V from the panel, safely charge the battery to capacity, and stop charging the battery when it is full.

Thus, the main reason for using a charge controller is to prevent an over-charge condition.

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What is the primary reason that a high-frequency switching type high voltage power supply can be both less expensive and lighter in weight than a conventional power supply?
  • The inverter design does not require any output filtering
  • It uses a diode bridge rectifier for increased output
  • Correct Answer
    The high frequency inverter design uses much smaller transformers and filter components for an equivalent power output
  • It uses a large power factor compensation capacitor to create free power from the unused portion of the AC cycle

Hint: smaller transformer and components = lighter and more often less expensive.

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What circuit element is controlled by a series analog voltage regulator to maintain a constant output voltage?
  • Reference voltage
  • Switching inductance
  • Error amplifier
  • Correct Answer
    Pass transistor

A Pass transistor is a circuit element that can be used to maintain a constant output voltage. When the voltage goes over a set level, the transistor is turned off, keeping the voltage from going too high on the output.

(Note that this is a somewhat simplistic explanation for ease of understanding; in actuality the transistor is adjusted to keep a constant output voltage across the load resister. For more information see http://www.circuitstoday.com/voltage-regulators) Hint. Transistor is the only CIRCUIT ELEMENT. voltage and inductance are properties of circuit elements, amplifier is a circuit.

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What is the drop-out voltage of an analog voltage regulator?
  • Minimum input voltage for rated power dissipation
  • Maximum amount that the output voltage drops when the input voltage is varied over its specified range
  • Correct Answer
    Minimum input-to-output voltage required to maintain regulation
  • Maximum amount that the output voltage may decrease at rated load

Linear voltage regulators maintain a constant output voltage in a circuit by using a control element that responds to current. If the "pass element" in a series regulator doesn't receive enough input voltage it can't perform that control function. Consequently output voltage begins to drop below the regulated value.

The minimum voltage a regulator requires before drop out occurs is called the "drop-out voltage."


Hint: A voltage regulator maintains regulation. Only one answer choice has this word.

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What is the equation for calculating power dissipation by a series connected linear voltage regulator?
  • Input voltage multiplied by input current
  • Input voltage divided by output current
  • Correct Answer
    Voltage difference from input to output multiplied by output current
  • Output voltage multiplied by output current

Electrical power is measured in Watts and calculated by multiplying the voltage by the current. In an electrical circuit, every device will have either a voltage "drop", i.e. consumes power, such as a resistor, or a "voltage gain", i.e. produces power such as a battery.

A linear voltage regulator is no different in this respect. A linear voltage regulator is explicitly designed to do this in a reliable fashion, by taking a higher (usually unregulated) voltage and producing a lower, regulated voltage.

A power calculation across a regulator can be difficult if the input is highly unregulated, or the current demand of the regulated circuit changes frequently but that's not the point of the question. Power consumption, and in most cases "dissipation", meaning wasted power, is always the voltage drop multiplied by the current through the device.

An example could be a 9-volt battery connected to a 5 volt linear regulator, powering a circuit that consumes 2 amps. Since current is always conserved, the 2 amps at 5 volts going into the powered circuit comes from 2 amps at 9 volts coming from the battery. While the powered circuit would consume 10 Watts

\[P=IV=(2 \text{ A})(5\text{ V})=10 \text{ W}\]

the regulator has to dissipate the excess energy from the 18 Watts

\[P=IV=(9\text{ V})(2 \text{ A})=18 \text{ W}\]

coming from the battery. In this case, you would see 2 amps of current moving through the regulator, with 9 V as its input and 5 V as its output. The regulator would be dissipating 8 Watts.

\[(9 \text{ V} - 5 \text{ V}) \times 2 \text{ A} = 4 \text{ V} \times 2 \text{ A} = 8\text{ W}\]

Linear regulators are specifically designed to do this, but care has to be taken that the heat generated by the regulator is properly dissipated and within the design limits of the regulator.

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What is one purpose of a "bleeder" resistor in a conventional unregulated power supply?
  • To cut down on waste heat generated by the power supply
  • To balance the low-voltage filament windings
  • Correct Answer
    To improve output voltage regulation
  • To boost the amount of output current

Hint: If you give blood and you're a "Bleeder" you have "improved output"

kg5kou

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What is the purpose of a "step-start" circuit in a high voltage power supply?
  • To provide a dual-voltage output for reduced power applications
  • To compensate for variations of the incoming line voltage
  • To allow for remote control of the power supply
  • Correct Answer
    To allow the filter capacitors to charge gradually

A "step start" circuit gradually increases the voltage applied to filter capacitors. The reason we use this circuit, is to avoid putting the full voltage on the capacitor all at once. If we were to put the full voltage on the capacitor all at once, it might fail (literally blow up), or at least result in a decrease in the capacitor's useful life.

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When several electrolytic filter capacitors are connected in series to increase the operating voltage of a power supply filter circuit, why should resistors be connected across each capacitor?
  • To equalize, as much as possible, the voltage drop across each capacitor
  • To provide a safety bleeder to discharge the capacitors when the supply is off
  • To provide a minimum load current to reduce voltage excursions at light loads
  • Correct Answer
    All of these choices are correct

Answer: All of these choices are correct.

Think of a simple capacitor: it is two plates separated by an insulator. When you connect capacitors in series, you are charging up all of the capacitors with the same number of electrons (the same current flows through each because they are in series).

If the capacitors have any imperfections, they will each end up with a different voltage, and resistors can help balance the capacitors because the fuller ones will have a higher voltage, and therefore get discharged faster by the resistor.

The value of the resistors chosen is high such that the resistance is at least 10 times the highest value of \(X_C\), where

\[X_C = \frac{1}{2πfC}\]

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