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Electrician - Fourth Period > Controlled Rectifiers > Flashcards

Controlled Rectifiers Flashcards

1
Q

What are the three leads of an SCR called?

A
  1. Anode (A)
  2. Cathode (K)
  3. Gate (G)
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2
Q

What family of electronic devices does a silicon-controlled rectifier (SCR) belong to?

A

The Thyristor family of electronic devices

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3
Q

What configurations are SCR’s available in?

A
  1. Flat Pac or Tab style - typically bolted to a heat sink and used in low-current applications from 2A to 20A
  2. Stud Mount - the anode is usually a threaded stud that is threaded into a heat sink. The large flexible lead is the Cathode (K) and the smaller wire is the Gate (G) connection. These are designed for medium-power circuits
  3. Hockey Puk - used in very large power circuits. Easily distinguishable because of its small terminal or pin connector for the Gate (G) circuit
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4
Q

What does triggering an SCR mean?

A
  • Triggering an SCR means that a current pulse is applied to the gate of the SCR to make it start conducting
  • Sometimes called gating or firing
  • Once the SCR is made to conduct, it will continue to conduct as long as the curent from Anode to Cathode is equal to or greater than its holding current (IH)
  • The holding current is about one mA per ampere of the rated anode current (IT(AV))
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5
Q

Explain how an SCR works as two PN-junctions or two switches in series

A
  1. The lower PN junction/switch closes when it receives an appropriate gate pulse (+VGK)
  2. Once the lower switch is closed, the neegative potential of the cathode (K) is able to reach the N material of the top PN Junction/switch
  3. This causes the top junction/switch to close since it is now forward biased
  4. Current is now able to flow through the SCR nad the Gate connection can be removed
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6
Q

How much current is required for a Gate Pulse?

A

About five times the minimum gate current (IGT) rating of the SCR

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7
Q

How could you accidentally turn on an SCR?

A
  1. Exceeding Breakover Voltage - If the peak applied circuit voltage exceeds the breakover voltage of the SCR, the leakage current will increase until it is equal to the amount of gate current required to turn the SCR on, and then the SCR is gated into conduction
  2. Fluctuations in Voltage - When an SCR is subjected to sudden high rates of voltage change from anode to cathode, the capacitive charging current of the PN-junctions within the SCR may be enough to accidentally start the SCR. To prevent this, a Resistance-Capacitive circuit is connected across the SCR to limit the rate at which the voltage can increase. A resistor may also be etched/connected from Gate to Cathode to prevent charge build up across the junction. If this charge is allowed to exceed 0.7V it could falsely Gate the SCR
  3. Temperature change - Leakage current doubles for every 10°C of temperature rise. If the temperature rises to a point where the leakage current is equal to gate current the SCR will turn on
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8
Q

What is Commutating an SCR?

A
  • Commutating an SCR is turning an SCR off
  • In order for the SCR to turn off, the anode current must drop to less than the SCR holding current (IH)
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9
Q

What are some different ways of commutating an SCR?

A
  1. Natural Commutating - happens when a switch is operated in series or parallel with the SCR. When this switch is opened momentarily, it interrupts the current in the circuit and commutates the SCR
  2. Frequency/AC Commutation/Class F Commutation - In an AC circuit, such as a controlled rectifier circuit, every alternate half cycle of the applied voltage reverse-biases the SCR, and it is commutated.
  3. Forced Commutation - applies only to SCR’s in DC circuits where current flows and must be forced to turn off. This method attempts to reverse the direction of current flow through the SCR which results in commutating the SCR
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10
Q

What is Forced Commutation?

A
  • Applies primarily to SCR’s in DC circuits where current continues to flow and must be turned off
  • There are two methods that share the same principle: Separate Source Commutation and Capacitor Discharge Commutation
  • In Separate Source Commutation, pressing the OFF button momentarily connects the commutation source across the SCR from Anode to Cathode. Current attempts to flow reverse-bias through the SCR. The moment this is connected, the SCR turns off
  • In Capacitor Discharge Commutation, the SCR is conducting, the load is on, and the capacitor charges to the supply voltage through the SCR. When the OFF button is pressed, it connects the charged capacitor across the conducting SCR. The charged Capacitor attempts to discharge through the SCR. The discharge current reverse-biases the SCR and instantly commutates it and turns the load off
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11
Q

How could you bench test and SCR?

A
  • The SCR can be turned on by momentarily connecting the Anode to the Gate
  • The SCR can be turned off by momentarily connecting the Anode to the Cathode
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12
Q

When checking an SCR with a digital multimeter, you should always isolate it from the circuit to avoid any false readings

True or False?

A

True

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13
Q

When testing an SCR with a digital multimeter, why might you get a low voltage reading when testing between the Gate (G) and the Cathode (K)?

A
  • Because some SCR’s have an internal snubber circuit built into it by the manufacturer, this circuit conducts in both directions
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14
Q

What ratings of an SCR must you consider when replacing an SCR?

A
  1. Voltage Rating - VRRM ratings are never stated because they are much higher than the VDRM, the forward voltage rating. To ensure reliability and prevent falsely turning on due to high transient voltages, the VDRM of SCR’s is usually selected in excess (usually double) the peak voltage of the supply
  2. Power Dissipating - although SCR’s can deliver high currents to a load, but they also dissipate a lot of heat. And for this reason, they require large efficient heat sinks
  3. dv/dt rating - the rate of change of voltage with respect to time, listed in volts per microsecond. This is the highest rate of change of voltage across the SCR, from Anode (A) to Cathode (K), forward-biased, that it can sustain without falsely turning on. Exceeding this value causes the SCR to turn on because of the current flow that charges the interjunction capacitance becoming equal to the gate current
  4. Gate Current Rating - the minimum current required to turn an SCR on. Manufacturers usually provide for Hard Gate Drive which is usually five times more than the listed Gate Current. The Gate Current usually lasts between 10 and 15 microseconds. Hard Gate Drive provides reliable, precise turn-on regardless of temperature or load condition
  5. Application - some SCR’s may have to withstand high rates of current change while others do not. The physical dimensions and polarity (If the wrong polarity is used, a faulty condition such as a short-circuit may occur) of an SCR are important as well , new replacement SCR’s must have leads or terminals that fit the existing assembly
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15
Q

What are the functions of an SCR?

A
  1. Control the average voltage applied to a DC load from an AC source - phase control
  2. Control the ouput voltage of an Alternator
  3. Act as a switch in an electronic control circuit
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16
Q

How does an SCR control the average voltage applied to a DC load from an AC source?

A
  • Called Phase Control
  • An SCR can only be turned on when it is forward-biased, which happens in the positive half cycle of the AC supplywaveform
  • Every time a Gate Pulse triggers an SCR, the SCR turns on
  • The SCR will conduct until the end of that positive half cycle of the AC supply, after which it is commutated by either frequency or AC commutation
  • The Firing Angle plus the Conduction Angle always equal 180°
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17
Q

How do you operate a DC Motor above base speed?

A
  • Decrease the current through the field winding
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18
Q

How do you operate a DC motor below base speed?

A
  • Decrease the current through the armature
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19
Q

What is Base Speed-Down?

A
  • When the DC drive for a DC motor operates by controlling the average voltage to only the armature of the motor
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20
Q

How does Base Speed-Down work?

A
  • Field connection for the motor may have only one rated voltage and one value of current for the shunt field, some DC motors offer two values of of voltage for the shunt field; a higher voltage for slower speed range, and a lower voltage for fully weakened field higher speed range
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21
Q

What is a common method used to regulate or maintain a specific output voltage of an Alternator?

A
  • A common method used to regulate or maintain a specific value of output voltage is to control the alternator field current
  • This function is carried out be an Automatic Voltage Regulator (AVR)
  • An exciter is equipment that supplies current to the field of an alternator
22
Q

Describe the operation of an Alternator with brushless excitation and AVR

A
  • The exciter is equipment that supplies current to the field of an alternator
  • Most alternators or AC gen-sets, are equipped with rotating exciters, usually of the brushless type, which are housed in their own enclosure and mounted on the main shaft of the alternator
  • The Automatic Voltage Regulator (AVR) receives AC voltage from the stator of the alternator, rectifies the AC voltage and applies varying amounts of it to the exciter field
  • The AVR controls how much current flows in the exciter field
  • The output of the exciter is usually three-phase AC that is rectified by a three-phase full-wave rectifier mounted on the shaft of the alternator
  • By controlling the field exciter, it controls the current in the field of the alternator
  • The system operates as a magnetic amplifier, a small current change in the exciter field produces a larger change in the current in the alternator field
23
Q

Describe the principle of operation for Automatic Voltage Regulators (AVR)

A
  1. The AVR receives voltage from the residual magnetism of the stator or a voltage is applied from a pre-exciter to the AVR input terminals
  2. The AVR rectifies this AC voltage
  3. The AVR applies this rectified coltage to the exciter field
  4. Once rated voltage is reached, the AVR controls the exciter field current to maintain the terminal voltage
24
Q

What is a Zener Diode?

A
  • A Zener Diode safely conducts in reverse bias under certain conditions
  • The voltage the Zener diode conducts at is called its Zener Voltage (VZ), below this value it will not conduct
  • If the rated Zener voltage is applied, the Zener Diode conducts and maintains an almost constant VZ from Cathode (K) to Anode (A)
25
Q

What is the name of a rectifier that is made by replacing two diodes in a bridge rectifier with two SCR’s?

A
  • Controlled Bridge Rectifier
26
Q

What is the name of a circuit that prevents the undesirable triggering of an SCR due to dv/dt?

A
  • A Snubber Circuit
27
Q

In a Resistive-Capacitive Timing Trigger circuit, what controls the capacitor charge rate?

A

A variable resistor

28
Q

Why do additional gate pulses within a cycle not have an effect on the SCR?

A
  • Because the SCR is already conducting
29
Q

What is the purpose of the isolation transformer in a pulse-trigger circuit?

A
  • It prevents high voltages from the SCR’s power circuit from affecting the low-voltage componets in the triggering circuit
30
Q

Where would an SCR often be used as a switch?

A
  1. SCR used as a switch in an electronic control circuit such as a Battery Charger and,
  2. As the switch that completes the low-impedance path to the fire alarm control panel in conventional fire alarm control panels
31
Q

What is a smoke detector?

A
  • An initiating device in a fire alarm system that detects the presence of smoke
  • Once smoke has been detected or is exceeding an unacceptable concentration, the detector sends a signal to the fire alarm control panel
32
Q

What are the three levels of current that the initiating circuits of a fire alarm system operate with and what do they indicate?

A
  1. 10-13mA through an end-of-line resistor indicating a normal condition
  2. Zero current indicatinga broken wire and a trouble condition
  3. High current through the low impedance path of a detector in an alarm condition
33
Q

Why must a conventional smoke detector be connected with the correct polarity across the SCR?

A
  • The initiating circuits of a fire alarm system operate with a DC voltage, and the smoke detector containing the SCR is polarity sensitive and must be connected properly
34
Q

What is a Triac?

A
  • A member of the Thyristor family of electronic devices
  • A Triac is an AC solid-state switch that conducts in both directions and requires fate trigger current like an SCR to conduct
  • The Gate Trigger current controls the turn on for both positive and negative alternations of the AC cycle
  • Terminals of Triac’s are marked
    • T1 - Main Terminal
    • T2 - Main Terminal 2
    • G - Gate
  • A triac is equivalent to two SCR’s connected in inverse parallel
35
Q

What are the differences between an SCR and a Triac?

A
  • The number of junctions in a triac results in greater junction capacitance than in an SCR, and this tends to increase the turn off time. If a a triac is subjected to high rates of voltage change, the junction capacitance may cause the the triac to turn on accidentally
  • High current rated triac’s are not available, they’re manufactured in current ratings up to 40A and higher current triac’s (over 40A) are stud-mount type. Hockey Puk style triacs are not available. For largegr current applications manufacturers use two SCR’s in inverse parallel instead of one triac
  • Triac’s conduct in both alternations of the AC supply so they require the same amount of cooling as two SCR’s where we would only be carrying half the current. When triac’s are mounted on heat sinks, the mating surface must be treated with a layer of silicon grease, this aids in the transfer of heat from the triac’s case to the heat sink
36
Q

How should a Triac be connected?

A
  1. The Gate should be positive when T2 is positive
  2. The Gate should be negative when T2 is negative
37
Q

What is a Diac?

A
  • A Diac is a semiconductor device that belongs to the Thyristor family
  • A Triac is generally controlled by a Diac
  • A Diac conducts when the voltage across it reaches its breakover voltage rating in either direction and does not require a gate
  • Although they come in a variety of breakover voltages, the typical rating is 30V
38
Q

What is the relationship between the firing angle, the conduction angle, and the average current that passes through a load in terms of a Resistive Capacitive Time Delay circuit for firing a Triac?

A
  • The longer it takes to charge the capacitor, the larger the firing angle
  • The larger the firing angle, the smaller the conduction angle
  • The smaller the conduction angle and less average current passes through the load
39
Q

Why might you get a voltage reading when testing a Triac with a digital multimeter?

A
  • Because a snubber circuit is often etched into the triac between the Gate and T1 termals
  • This is an internal resistor connected between the two terminals so it will conduct in both directions
40
Q

An SCR conducts in only one direction

True or False?

A

True

41
Q

What do Alternator voltage regulators supply direct current to?

A

The field

42
Q

What does a Pulse Transformer provide when used in the Trigger Circuit of an SCR?

A

Electrical Isolation

43
Q

When the SCR inside a smoke detector is triggered into conduction, how does it behave?

A

Like a closed switch

44
Q

What kind of supply voltage can control the initiating circuits of a fire alarm system?

A

DC voltage

45
Q

When in its opoeration, what is a Diac?

A

A trigger device

46
Q

What electronic device can trigger a Triac in both the positive and negative alternation?

A

A Diac

47
Q

Puk style Triac’s are a available for high currents

True or False?

A

False

48
Q

A Diac is a three terminal device that is often used in the trigger circuit of Triac’s

True or False?

A

False, a Diac is a two terminal device

49
Q

Why would an inductor be used in a lighting load controller?

A

To filter current

50
Q
A

Electrician - Fourth Period (24 decks)

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