Controlled Rectifiers

Question 1

What are the three leads of an SCR?

Answer 1

Cathode (K)
Anode (A)
Gate (G)

Question 2

Name some of the different types of an SCR:

Answer 2

Flat Pac or Tab Style
Stud Mount
Puk Style

Question 3

What are heat sinks generally made of?

Answer 3

Aluminium

Question 4

An RC snubber circuit is connected across the SCR to limit the rate at which the voltage can increase. T or F?

Answer 4

True

Question 5

What does Vdrm stand for?

Answer 5

The dynamic repetitive maximum (Vdrm)

Question 6

When replacing SCRS, you need to consider what five factors?

Answer 6

Voltage rating
Power dissipation
Dv/Dt rating
Gate current rating
Application

Question 7

Controlling the average voltage is called what?

Answer 7

Phase Control

Question 8

To operate a dc motor above base speed, the current through the field winding must be reduced. T or F?

Answer 8

True

Question 9

Most alternators or ac gen-sets, are equipped with rotating exciters, usually a brushless type. T or F?

Answer 9

True

Question 10

The voltage a zener diode conducts is called the Zener voltage and it only conducts in forward bias under certain conditions. T or F?

Answer 10

False, the zener voltage conducts in reverse bias under certain conditions.

Question 11

A saturated charge is desired when charging lead-acid batteries. T or F?

Answer 11

False. A tapered charge is desired.

Question 12

An RC time delay circuit is an effective gate for an SCR. T or F?

Answer 12

True

Question 13

How does a pulse trigger circuit control an SCR?

Answer 13

By providing a current pulse of sufficient magnitude and duration to trigger the gate of the SCR, turning it on

Question 14

What components are required for a controlled-bridge rectifier?

Answer 14

Two diodes and two SCRS

Question 15

Name some of the examples of circuits that are used to provide trigger pulses to SCRS?

Answer 15

Integrated circuits
Digital circuits
Ramp generator
Comparator circuits

Question 16

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

Answer 16

To isolate the low-voltage trigger circuit from the higher-voltage SCR circuit

Question 17

What are the 3 current levels that may be measured in a convention fire alarm initiating circuit?

Answer 17

10-13mA through an end of line resistor- normal
Zero current - broken wire/trouble
High current - through low Z path of a detector alarm condition

Question 18

A triac is an AC solid state switch that conducts in both directions. T or F?

Answer 18

True

Question 19

Since the triac is used in ac circuits, what is it commutated with?

Answer 19

Class F commutation

Question 20

How is phase control of triacs accomplished?

Answer 20

By connecting a variable triggering circuit to the gate

Question 21

Two common applications of Triacs are

Answer 21

Lighting dimmers
Small ac motor speed control

Question 22

What single-phase motors are capable of speed control using the phase-control method?

Answer 22

Shaded-pole
Permanent-split capacitor motors

Question 23

In order to have consistent triggering, what must the polarity of the gate be when T2 is negative?

Answer 23

Negative

Question 24

What ratings determine the size of a Triac?

Answer 24

The current IT and voltage rating (Vdrm)

Question 25

What does SCR stand for?

Answer 25

Silicon Controlled Rectifier

Question 26

What material is the Gate made out of?

Answer 26

P-Type material

Question 27

What are two differences you can see when looking at an SCR and a TRIAC?

Answer 27

They look the same.

Except for…
- Their Part Number
- Their Ratings

TRIAC’s have smaller ratings.