Controlled Rectifiers Flashcards
What are the names of the three (3) terminals of an SCR?
- anode
- cathode
- gate
What are the two (2) requirements for an SCR to turn on and conduct?
Forward Bias
Positive gate pulse
What is the name of a circuit used to prevent the undesirable triggering of an SCR due to dv/dt?
Snubber circuit
What is commutation of an SCR?
Turning off an SCR
What is the name of the process of controlling the average voltage with an SCR?
Phase control
What is the name of a rectifier that is made by replacing two diodes in a bridge rectifier with two SCR’s?
Controlled-Bridge Rectifier
What does an AVR do?
Controls the output voltage of an alternator
Three configurations of SCRs are
- Flat pac or tab style (low current 2-20A)
- stud mounted (medium power)
- put style (large power)
Three ways of accidentally triggering an SCR
- exceeding break over voltage
- fluctuations in voltage
- temperature changes
In an RC timing trigger circuit, what controls the capacitor charge rate?
A variable resistor
Why do additional pulses within a cycle not have an effect on the SCR?
The SCR is already triggered and is conducting
What is the purpose of the isolation transformer in a pulse-trigger circuit?
To isolate the low-voltage trigger circuit from the higher voltage SCR circuit.
What are the three (3) current levels that may be measured in a conventional fire alarm initiating circuit?
- normal (10-13mA)
- trouble (zero current)
- alarm (high current through low impudence path)
Why must a conventional smoke detector be connected with the correct polarity across the SCR?
If reverse-bias, it will not go into conduction.
In what kind of circuits are triacs used?
AC
What two (2) devices form an equivalent of a triac when they are connected in inverse parallel?
Two SCRs
What member of the thyristor family of electronic devices is used in the triggering circuit for a triac?
A diac
In order to have consistent triggering, what must the polarity of the gate be when T2 is negative?
Negative
What are two (2) common applications of circuits containing triacs?
- light dimmers
- small ac motor speed control
a SCR conducts current in ______ direction
One
In order to provide base speed down control of a DC motor an SCR must be placed in the _________ circuit.
Armature
To ensure proper triggering of a TRIAC, the gate polarity must be the same as __________.
T2
A TRIAC conducts current in _________ __________.
Two directions
When more load is added to a cap filtered circuit the percent ripple would ____________.
Increase
A P-Type material has a net _________ charge.
Neutral
The current that flows when a photocell is not exposed to any light is known as ______ _________
Dark current
A single phase diode conducts for ______ degrees.
180
What conditions must exist in order to properly trigger a silicon-controlled rectifier?
Forward Bias
Positive Gate pulse
What is the recommended method of triggering an SCR?
Gate Pulse
What are three undesirable methods of triggering an SCR?
- Exceed Breakover Voltage
- Fluctuations in voltage
- Temperature changes
What are three ways to commutate an SCR?
Natural
Frequency
Forced
Is the SCR considered a normally on device?
No; Normally off
In a class B fire alarm system, what are the three conditions of a smoke detector?
- normal
- alarm
- trouble
What is the purpose of the FF coil?
Field Failure Coil; prevents motor runaway
What is the current limit of a triac?
40 A
What is the typical breakover voltage of a diac?
+- 30V
What does the potentiometer set?
Output voltage to alternator
What is the purpose of the external snubber circuit?
Limit the rate at which voltage can increase
