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Subelement G7
Practical Circuits
Section G7A
Power supplies; schematic symbols
What useful feature does a power supply bleeder resistor provide?
• It acts as a fuse for excess voltage
It ensures that the filter capacitors are discharged when power is removed
• It removes shock hazards from the induction coils
• It eliminates ground loop current

Hint: a Bleeder causes a Discharge blood

The bleeder resister of a power supply performs the safety feature of discharging the filter capacitors. The resistor is arranged in parrallel with the power supply. The use of the resistor is very important in this application, as it lowers the risk of electric shock from stored electrical energy in the filter capacitors by allowing the voltage level to decay quickly when the power supply is shut off.

The bleeder resistor is shown as symbol 9 on the G7-1 schematic for the general class exam.

Link to circuit symbol guide: [Wikipedia Gallery of Common Electronic Symbols] (http://en.wikipedia.org/wiki/Electronic_symbol#Gallery_of_common_electronic_s ymbols)

Hint: The question and answer both have power.

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Which of the following components are used in a power supply filter network?
• Diodes
• Transformers and transducers
• Quartz crystals
Capacitors and inductors

Both capacitors and inductors are FILTERS

and are used in a power-supply filter network. The combination of these two components is often referred to as an LC circuit (L is the Inductor, C is the Capacitor). Together they act to "tune" the circuit to smooth out voltage and current pulses from the power supply to provide a constant DC output voltage.

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Which type of rectifier circuit uses two diodes and a center-tapped transformer?
Full-wave
• Full-wave bridge
• Half-wave
• Synchronous

In a full wave [NOT bridge] rectifier, the return currents go to the center tap on the transformer, while each diode conducts for half of the AC cycle, capturing the full wave and converting it to DC.

https://en.wikipedia.org/wiki/Rectifier#Full-wave_rectification

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What is an advantage of a half-wave rectifier in a power supply?
Only one diode is required
• The ripple frequency is twice that of a full-wave rectifier
• More current can be drawn from the half-wave rectifier
• The output voltage is two times the peak output voltage of the transformer

In this circuit we only wish to capture half of the incoming AC waveform, thus giving the name "Half-wave rectifier". Only one diode is used since it will conduct during the positive portion of the cycle, and that is all that is required.

Half-wave rectifier

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What portion of the AC cycle is converted to DC by a half-wave rectifier?
• 90 degrees
180 degrees
• 270 degrees
• 360 degrees

(B). A half-wave rectifier will convert 1/2 of the full AC cycle of 360 degrees, so it will convert 180 degrees of the wave to DC.

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What portion of the AC cycle is converted to DC by a full-wave rectifier?
• 90 degrees
• 180 degrees
• 270 degrees
360 degrees

A FUll-wave = $360°$

As its name indicates, a full-wave rectifier will convert the full wave or 360 degrees of the AC waveform to DC.

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What is the output waveform of an unfiltered full-wave rectifier connected to a resistive load?
A series of DC pulses at twice the frequency of the AC input
• A series of DC pulses at the same frequency as the AC input
• A sine wave at half the frequency of the AC input

Hint: a full wave rectifier is TWICE the fun.

The output waveform of an unfiltered full-wave rectifier connected to a resistive load will be a series of DC pulses at twice the frequency of the AC input. The rectifier will change each half cycle of the AC current waveform into pulses of DC current. This results in one pulse for the positive 1/2 cycle and one pulse for the negative 1/2 cycle for a total of 2 pulses DC output for each wavelength of AC converted.

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Which of the following is an advantage of a switchmode power supply as compared to a linear power supply?
• Faster switching time makes higher output voltage possible
• Fewer circuit components are required
High-frequency operation allows the use of smaller components
• All these choices are correct

Hint: a Switchmode uses SMALLER components, NOT FEWER.

An advantage of a switch mode power supply over a linear power supply, is that the high frequency operation allows the use of smaller components. This type of power supply is highly efficient and switches very quickly. The high operating frequencies (typically 50 kHz - 1 MHz instead of much lower 60 Hz AC) means that compact and lightweight transformers can be used. This makes a great difference in the size and weight of the equipment.

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Which symbol in figure G7-1 represents a field effect transistor?
• Symbol 2
• Symbol 5
Symbol 1
• Symbol 4

Symbol 1 in figure G7-1 represents a field effect transistor.

The figure shows the arrow coming IN from the GATE to meet the vertical bar, representing the transistor, at center left. At the top of the bar the upper terminal has a perpendicular line to the right indicating the DRAIN. While at the bottom of the bar the lower terminal has a perpendicular line to the right indicating the SOURCE.

Link to circuit symbol guide: Wikipedia Gallery of Common Electronic Symbols

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Which symbol in figure G7-1 represents a Zener diode?
• Symbol 4
• Symbol 1
• Symbol 11
Symbol 5

(D). Symbol 5 represents a Zener diode.

The figure is represented by a horizonal bar across the vertical circuit line. The left end of this bar tilts down, and the right end tilts up. These tilts represent the change in voltage beween the input and output. These tilts make it look a little like a Z on its side - as a reminder for "Zener." Pointing into the bottom center of the horizontal bar is a triangle, located on the circuit line and the tip hitting the bar. This triangle symbol represents the voltage drop, D, of the diode.

Note for the visually impaired: If possible, notify your VE in advance of any accommodation you may need. They may be able to give you an exam without questions on figure G7-1. A worded description of the symbol is given here for both your own knowledge and to be able to describe the symbol to the VE if necessary.

Refer to the following links for the official exam circuit figure and symbol guide:

Link to circuit symbol guide: Wikipedia Gallery of Common Electronic Symbols

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Which symbol in figure G7-1 represents an NPN junction transistor?
• Symbol 1
Symbol 2
• Symbol 7
• Symbol 11

Hint; Arrow Not Pointing iN

Symbol 2 represents an NPN junction transistor

The transistor is represented by a vertical bar with a line coming in to the center of the bar from the left (Base). The upper terminal has a line coming into the right at an upward 45 degree angle (Collector). The lower terminal has a line to the right downward 45 degree angle (Emitter) with an arrow pointing away from the transistor.

A way to remember the symbol for an NPN transistor is that the arrow is Not Pointing iN.

Note for the visually impaired:
If possible, notify your VE in advance of any accommodation you may need. They may be able to give you an exam without questions on figure G7-1. A worded description of the symbol is given here for both your own knowledge and to be able to describe the symbol to the VE if necessary.

Refer to the following links for the official exam circuit figure and symbol guide:

Link to circuit symbol guide: Wikipedia Gallery of Common Electronic Symbols

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Which symbol in Figure G7-1 represents a solid core transformer?
• Symbol 4
• Symbol 7
Symbol 6
• Symbol 1

(C). Symbol 6 represents a multiple-winding transformer.

The core of the transformer is represented by two long vertical parallel lines close together (if there were no lines, the core would be air). On the left side of the core is the primary winding represented as vertical line with multiple connected semi-circular dips or waves, with the bottoms of the waves towards the core, and the pointed wave peaks pointing away from the core. On the right side is the secondary winding represented by a similar wavy line (bottoms of the waves toward the core). The number of windings on the two sides can be the same or different (which would indicate a step-up or step-down transformer). The symbol shown in figure G7-1 shows 4 windings on the left and 2 on the right.)

Refer to the following links for the official exam circuit figure and symbol guide:

Figure G7-1 (.jpg format)

Figure G7-1 (.pdf format)

Link to circuit symbol guide: Wikipedia Gallery of Common Electronic Symbols

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Which symbol in Figure G7-1 represents a tapped inductor?
Symbol 7
• Symbol 11
• Symbol 6
• Symbol 1

An inductor is a coil; symbol 7 is mostly the same symbol as a normal inductor, which is intended to represent that coil. (note symbol 6 as well which is a transformer, which consists of two coils).

A tapped inductor is one in which it's possible to "tap" a point in the coil to effectively shorten the coil. Sometimes that tap can be moved. Thus the symbol shows a wire coming off of the side of the coil in the symbol, indicating the tap.