PPT #5/6 Notes Flashcards
The zener diode acts as a ________-________ by itself or in conjunction with other semi-conductor devices
voltage-regulator
Zener diode is often referred to as a?
Avalanche diode
The zener diode normally operates in ______-_______ which is why it is referred to as an avalanche diode
reverse- breakdown
The overall forward and reverse characteristics of the zener diode are similar to a standard _____ diode.
silicon
When source voltage is applied to the zener in the reverse direction, the resistance of the PN junction is _____, as voltage increases, what occurs?
high
critical voltage is reached and avalanche occurs
On a zener diode, the PN junction should only produce leaky current in the micro-ampere range. True or False?
True
As Zener avalanche current is reached, PN junction resistance becomes ____ and the current increases rapidly. How is the current controlled/limited?
low
By a circuit resistor or resistance RL
Is a Zener diode capable of limiting the positive or negative segments of an AC cycle? How?
Yes, when it is supplied with AC and connected as a shunt.
A zener diode installed as a voltage regulator is installed in series with a current-limiting _______ and in parallel with the ________
resistor (RL)
load (RL)
When the zener is installed in parallel it is called a _____-________
shunt-regulator
Total current through the resistor (R1) is the sum of the zener and load (RL) current. True or False?
True
As source voltage increases, the current does not increase through R1 and RL because the zener diode acts as a _______-_________
voltage regulator
When voltage across the zener diode increases, the resistance of the diode ________
decreases
Reducing resistance at the zener diode causes zener current to rise, increasing the _______-_____ across the resistor (r1)
voltage drop
The larger the voltage drop across the resistor (R1), lowers the output voltage but keeps the current across RL ________
constant
DECREASING voltage across the zener diode causes the current through it to decrease and R1 drops less voltage. This, in turn, raises the output voltage to normal. True or False?
True
Lowering the voltage drop creates ________ output voltage
Higher voltage drop creates _______ output
steady
lower
As load increases or decreases, the zener diode, acting as a shunt, will draw more or less _________ respectively
current
Main purpose of a zener diode is to regulate and maintain a constant output
voltage
Zener voltage should not vary more than ___
0.1v
Zener diodes are designed to deliver constant voltage ratings ranging from ____ volts to _________ volts
few - several-hundred
What is an SCR?
Silicon controlled rectifier
An SCR is a member of the ________ family of semi- conductor devices
Thyristor
An SCR uses ________-_________
regenerative - feedback
When part of the output is returned to the input to reinforce the
input power?
Regenerative feedback
How many layers does an SCR have?
4
The ____ is un-directional and operates based on __________
SCR
current
An SCR is used to control ___ and ___ power and can also rectify AC
Ac and Dc
Ac
______-_______ is the current that must be supplied to initiate the SCR
latching current
________-________ the current that must be maintained for the SCR to remain in conduction
holding current
________ _____-______ is the forward voltage that will put the SCR into conduction
Voltage break-over
SCR operates much like a regular diode in _______-____. In forward bias, a certain value of _______ break-over voltage must be reached before the SCR will conduct
reverse bias
forward break-over
The internal structure of an SCR consists of two PN junctions joined together to form a _____ layer. With reverse bias voltage applied to this layered structure, there is no noticeable current flow due to the formation of _______ ______
PNPN
depletion regions
Forward blocking state is the normal state for ____
SCR
The SCR will only conduct if one of the two following conditions apply?
- Exceed forward VBO
- Apply a gate voltage
With forward bias voltage applied to the layered structure, there is no noticeable current flow due to the formation of the _______ depletion region in the center
larger
Conditions required to “gate on” the SCR
- Must be positive on the ________
- Must be negative on the ________
- Gate must be + compared to the cathode and have a knee voltage of at least
anode
cathode
1.4v
When gate of the SCR is triggered with a forward bias. The SCR will conduct and continue to conduct so long as ______ is supplied
current
S1 switch is momentarily closed in order to supply enough gate current through the current-limiting resistor. Once the SCR is conducting, the S1 switch can be open and the gate current can be removed. True or False
True
How to turn off there SCR?
- install switch on the ________
- install shorting wire from __ to __
- _______ signal on gate (not rec)
- drop golfing current below ______
- anode
- A to C
- negative (not rec)
- L.C.D
Shorting wire between anode and cathode momentarily shorts out the SCR bringing volts to __ which brings forward current below holding value
0
a _______ is essentially a bidirectional version of the SCR (Silicon Controlled Rectifier)
Triac
Difference between Triacs and SCR?
Triacs allow current flow in both directions
Triacs can be triggered by a positive or negative voltage to the gate. True or False?
True
How to turn on the triac?
- __ on T1
- ___ on T2
+ on the ____
at least ___v potential between t1 and t2
neg
pos
gate
1.5v
How to turn off the triac?
drop load current below holding current
The characteristics of the triac are based on terminal __ as the voltage reference point. The triac can be triggered into conduction in either direction by a __________ (IG) in either
direction.
1
gate current
What is the voltage reference point in a triac?
terminal 1
A ________ ___-___ may be replaced with a triac.
mechanical cut-out
Can a pair of triacs replace reversing contactors on a PSC motor?
Yes
Continuity should be seen across a triac when the gate is energized. true or false?
true
A diac is a _________ device with ___ gate
bi-directional
no gate
Diac will not conduct until _____ is exceeded and it turns off by dropping load current below ________ current
VBO
holding
Diacs ensure the Triacs are not turned on by ______ in the line
spikes
Diac is a triggering device used with ________
Triacs
When a positive or negative voltage reaches the break over voltage, the diac rapidly switches from a _____ resistive state to a _____ resistive state
high
low
Testing a Diac should have high resistance in both directions. true or false?
true
Photocells have _____ resistance when no light is present
high
photo cells are _______ biased
forward
Photocells are used when time response is not critical. true or false?
true
Photocells are commonly used with?
pilot lights
How do photocells work with gas valves and pilots?
No light = high resistance and no current flow to valve
Light = low resistance and current flow to gold gas valve open
Photo DIODES are operated in the ______ biased mode and are _______ with a _____ response time
reverse
rugged
fast
Photo diode is lens added to a semi-conductor that allows light through on the PN junction which excites electrons and allows current to flow. True or False?
True
