Power

Electronics
For Students, Professionals
and Beyond
eBook 15

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 Power Elec tronic s

TABLE OF
Our Terms of Use
CONTENTS
This Basic Electronics Tutorials eBook is focused on power electronics devices with
the information presented within this ebook provided “as-is” for general information
purposes only.
1. Introduction to Power Electronics . . . . . . . . . . . . . . . . . . 1
2. The Silicon Controlled Rectifier . . . . . . . . . . . . . . . . . . . 1 All the information and material published and presented herein including the text,
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3. Using The SCR With A DC Supply . . . . . . . . . . . . . . . . . . . 3 in part or in whole the supporting website: www.electronics-tutorials.ws, unless
4. Using The SCR With An Alternating AC Supply . . . . . . . . . . . 4 otherwise expressly stated.

5. SCR Phase Control . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 This free e-book is presented as general information and study reference guide for the
education of its readers who wish to learn Electronics. While every effort and reasonable
6. Resistor-Capacitor Phase Control . . . . . . . . . . . . . . . . . . 6 care has been taken with respect to the accuracy of the information given herein, the
7. The Triode AC Switch . . . . . . . . . . . . . . . . . . . . . . . . . . 6 author makes no representations or warranties of any kind, expressed or implied, about
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9. The Diode AC Switch . . . . . . . . . . . . . . . . . . . . . . . . . . 8
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12. The Solid State Relay . . . . . . . . . . . . . . . . . . . . . . . . 11
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 Power Elec tronic s

1. Introduction to Power Electronics 2. The Silicon Controlled Rectifier

Electronic components and devices began with the invention of the semiconductor The Silicon Controlled Rectifier, (SCR) is the main switching device of the thyristor
diode and bipolar transistor. Every device we use today has evolved over the years from family, and as such is known more commonly as simply, a Thyristor. The SCR is a three-
these two basic semiconductor silicon devices. But as well as diodes and transistors, terminal solid state semiconductor device, hence the “silicon” part of its name. It requires
other types of semiconductor devices were invented to start the beginning of a new age a positive gate signal or pulse to turn it “ON”, the “controlled” part of the name and once
of power electronics. These new devices belong to a family of thyristors used for the “ON” it behaves like a rectifying diode, the “rectifier” part of its name.
switching and control of electrical power and energy, either as an input or an output.
While the junction diode is constructed as a two layer (p-n) rectifying device, and the
As its name implies, power electronics is a branch of electrical engineering that combines bipolar transistor as a three layer (n-p-n or p-n-p) amplifying device. The silicon controlled
together the fields of solid-state electronics, electrical power, and control theory. Power rectifier is constructed as a four layer (p-n-p-n) ON/OFF switching device that contains
electronics systems, can be sub-divided into two basic types of electronic elements. three p-n junctions within its construction.
Power semiconductors, which do the actual switching of voltages and currents, and
microelectronic control which monitors external conditions and tells the power Figure 1. Silicon Controlled Rectifier Symbol
semiconductors when to operate. Anode (A) As shown in Figure 1. the silicon controlled rectifier (thyristor) has
There are now several types of power semiconductor devices, such as Silicon Controlled three connecting terminals labelled as: “Anode”, “Cathode” and
Rectifiers (SCR), Triacs (Triode AC’s), Diacs (Diode AC’s) and UJT’s (Unijunction Transistor). “Gate”. It operates as a unidirectional device, similar to the diode,
These electronic devices are all capable of operating as very fast solid state switches for by passing current through itself from Anode to Cathode in one
the control of high voltages and currents into the kiloamperes and kilovolts range. Gate direction only. Hence its schematic symbol is similar to that of a
(G) conventional diode.
Today, power electronic systems and components are used everywhere with typical
applications including: AC and DC motor drives, heating and lighting controls, phase Cathode (K)
But unlike the diode, the thyristor can be made to operate as
control, electric vehicles, solid-state power supplies, battery chargers, controlled either an open-circuited switch when reverse biased, or as a
rectifiers, inverters, power switching devices, IOT, etc, etc, the list is endless with the rectifying diode when the thyristors gate is triggered. In other words, thyristors operate as
silicon controlled rectifier being the most popular for such applications. on/off switching devices, but they cannot be used for amplification as a single gate pulse
is only required to turn it fully-on into saturation.
But it is also important to note that all the power Power Electronics uses
semiconductor devices when used for power electronics solid state semiconductor Since the silicon controlled rectifier is a 4-layer device, it can be thought of as an n-p-n
and in power control applications are always operated devices for high-power bipolar transistor with an extra p-layer added on to the end. The operation of the silicon
as fast acting on/off switches to convert energy from one switching applications controlled rectifier can best be explained by assuming that it is made up of a pair of
form to another, they are not amplifying devices. tightly couples transistor layers connected back-to-back as a pair of complementary
regenerative switches to produce a self-latching action. This simple back-to-back two-
transistor analogy is shown in Figure 2.

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Figure 2. Simple Two-transistor Model of an SCR If the Anode terminal is made more positive with respect to the Cathode and the Gate
Anode A Anode
terminal is open, the two outer p-n junctions (j1 and j3) are now forward biased but the
ILOAD centre n-p junction (j2) is reverse biased but any forward current is also blocked as the
TR1 IA device is acting as an open-circuit switch.
P P PNP
TR2
j1 However, if a positive current (IG) is injected into the Gate terminal which is effectively
N N N the base of the NPN transistor TR2, the resulting collector current flows in the base of
j2 transistor TR1. This in turn causes a collector current to flow in the PNP transistor, TR1
P G P P IG
Gate j3
which increases the base current of TR2 and so on.
Gate
N TR1 N TR2
Very rapidly the two transistors force each other The SCR has four layers of
NPN
IA+IG into a permanent conducting state since they are alternating P-type and N-type
connected as a regenerative feedback loop which semiconductor materials with
Cathode K Cathode
cannot stop once started. silicon used as the intrinsic
semiconductor
The two transistor analogy of Figure 2. shows a PNP and an NPN transistor interconnected The electric current now flowing through the device
together to form a regenerative feedback pair. The n-type emitter of TR2 becomes the between the Anode and the Cathode terminals is limited only by the impedance of the
device’s Cathode (K), while the p-type emitter of TR1 is the device Anode (A). external circuit so long as current through the device remains above the holding current.
The collector current of the NPN transistor TR2 feeds directly into the base of the PNP This regenerative switching action forcing the silicon controlled rectifier into conduction
transistor TR1, while the collector current of TR1 feeds into the base of TR2. That is, always occurs even if the gate voltage is applied permanently, or only momentarily as a
the collector current of transistor TR1 becomes the base drive for TR2, and vice versa. gate pulse. Once triggered into conduction, and passing current in the forward direction
The combined loop gain of this configuration will be equal to the product of the two (anode positive), the gate loses all control of the device due to the regenerative latching
individual transistors gains. action of the two interconnected transistors.
Clearly this two transistor analogy will not allow current to flow from the cathode to Note that the application of any additional gate signals or pulses after regeneration has
the anode terminal, but only when the anode is positive with respect to the cathode. been initiated will have no effect because the SCR is already conducting and fully-on.
Then these two theoretically inter-connected transistors rely heavily upon each other
for conduction as each transistor gets its base-emitter current from the other’s collector- The only way to turn a silicon controlled rectifier “OFF” (commutation) again is to reduce
emitter current. So until the NPN transistor TR2 is given some base current to start the anode-to-cathode voltage across it to zero (0) or its minimum holding current, (IH)
conduction, nothing can happen even if an Anode-to-Cathode voltage is present. voltage value. This will cause both back-to-back transistors to turn off and remain in an off
state until another Gate voltage is applied to the Gate terminal. The term ‘turn-off’ implies
When the anode terminal is negative with respect to the Cathode and the Gate terminal that a fully conducting device has returned back to its non-conducting forward blocking
is open, the central n-p junction (j2) becomes forward biased, but the two outer p-n state.
junctions (j1 and j3) are reversed biased, thus the device behaves very much like an
ordinary diode blocking reverse current flow.
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Then we can see that a silicon controlled rectifier (SCR) has two possible states: a high From the I-V characteristic curve of Figure 3. we can see that in the forward-biased (anode
impedance “off” state and a low-impedance “on” state. Under normal steady-state positive, cathode negative) region, the device remains in a non-conducting state, until
conditions an SCR can only be triggered on by the application of a positive drive signal, either it is triggered “on” by a suitable gate pulse or the forward voltage reaches a high
pulse, or supply of sufficient direction and amplitude to its “Gate” terminal. enough value to cause a forward breakover condition to occur. Once the breakover
voltage is reached, the SCR conducts permanently.
The operating voltage-current I-V characteristics curves for the operation of a Silicon
Controlled Rectifier are given in Figure 3. In the reverse-biased (anode negative, cathode positive) region, the SCR blocks the
flow of reverse current until at some high enough reverse voltage value, called the peak
Figure 3. I-V Characteristics of a Silicon Controlled Rectifier inverse voltage (PIV) rating, avalanche breakdown occurs and the SCR conducts without
the application of any Gate signal.
P N P N
This is an important negative characteristic of the silicon controlled rectifier, as SCR’s can
Anode (A) Cathode (K) be unintentionally triggered into conduction by a high reverse over-voltage as well as high
Forward
Current temperature or a rapidly rising dv/dt voltage such as a voltage transient or spike.
+ - Conducting
Forward
+I State
Biased One of the main advantages of using a silicon controlled rectifier as a switch is that it is
+ Gate (G) Region a current operated device with a very high current gain, so a small Gate current pulse or
Conventional Current Flow
signal can control a much larger Anode load current.
Holding
Current IH Breakover
Voltage
3. Using The SCR With A DC Supply
PIV
-V +V The operation of an SCR makes them suitable for use in medium to high-voltage AC
Reverse Forward power control applications, such as lamp dimming, regulators and motor control. But
Voltage Gate Voltage they can also be used in DC circuits as a crowbar device for overvoltage protection.
Triggered ON
Reverse Voltage (Conduction) However, for SCR’s used in circuits fed from DC supplies, natural commutation condition
Reverse Blocking
OFF State cannot occur as the DC supply voltage is continuous and steady-state, so some other way
Breakdown Forward Voltage
Voltage
to turn “off” the SCR must be provided.
Blocking
Reverse
Biased -I There are two common methods for turning an SCR off at the appropriate time, current
Region interruption and forced commutation. Both methods require reducing the anode-to-
Reverse
Current
cathode voltage to zero, or by reducing the forward current to below the holding value
and turning off the SCR because once triggered it will remain forever conducting.

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Figure 4. Gate Triggering by a DC Signal or Pulse Note also that when an SCR is in its forward-conducting state, all three p-n junctions
OFF become forward biased and so it presents a minimum resistance to the forward Anode
Figure 4. shows a simple “on-off” firing current flowing through it. For example, the 2N650x series I2R voltage drop across the
S2
Load
circuit which uses resistive triggering and device when fully conducting is approximately 1.8 volts.
the SCR as a switch to control a lamp load.
ON S1 It could also be used as an on-off control
IA
circuit for a motor, heater or some other
4. Using The SCR With An Alternating AC Supply
+
A such DC load.
VDC RG
SCR Since the silicon controlled rectifier has the ability to turn itself “off” whenever its forward
- IG The SCR is forward biased and triggered into current is reduced to zero or to below its minimum holding current value IH, it therefore
G K
conduction by briefly closing the normally- follows that when used on a sinusoidal AC supply the SCR will turn itself off at some value
open “on” push button, S1. This briefly near to the crossover point of each and every half-cycle of the AC waveform, and as we
RGK
connects the SCR’s Gate terminal to the DC now know, will remain in an off state until the application of the next positive Gate pulse.
supply via the resistor, RG.
Since an AC sinusoidal voltage supply is continually reversing in polarity from positive to
Once the SCR has been triggered into conduction, it self-latches as before and stays negative and back again at every half-cycle. This periodic action allows the SCR to turn off
conducting even when the push button is released due to the presence of the DC supply. automatically every 180o zero cross-over point of the positive half of the waveform. This
The SCR will remain in its “on” state so long as load current through it remains above its effect is known as “natural commutation” and is a very important characteristic of the
holding current. silicon controlled rectifier.
Additional operations of push button, S1 will have no effect on the circuits state as Figure 5. Gate Triggering Using an AC Signal
once “latched” the Gate loses all control. The SCR is now turned fully “on” (conducting)
allowing full load current to flow through the device in the forward direction and back to Lamp The resistive SCR firing circuit of figure
the battery supply. 5. is similar in design to the previous
ON/OFF
DC SCR circuit of Figure 4.
S1
As the SCR has self-latched into conduction, the circuit can only be reset (off) by
interrupting the DC supply and reducing the SCR’s Anode current to zero or below its IA The difference here is that there is no
minimum holding current (IH) value. D1 additional “off” switch as it is generally
VAC
A
not required with AC supplies.
Operating the normally-closed “off” push button, S2 breaks the circuit, reducing the circuit RG
SCR
current flowing through the silicon controlled rectifier to zero, thus forcing it to turn “off” 2N6507 The inclusion of diode D1 helps prevent
IG
until the next application of another Gate signal. For example, the 2N650x series SCR, the G K reverse bias being applied to the Gate
typical holding current (IH) value is about 18 mA. terminal during the negative half-cycle
RGK VGK from π to 2π.

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During the positive half-cycle of the sinusoidal waveform of Figure 5, the SCR is forward
biased but with switch S1 open, zero gate current (IG) is applied, so both the SCR and the 5. SCR Phase Control
lamp are off. On the negative half-cycle, the SCR is reverse biased as before so will remain
We now know that a silicon controlled rectifier only conducts in one direction of a
off regardless of the switching condition of S1.
sinusoidal waveform under the control of the gate signal. This feature makes it ideally
If switch S1 of Figure 5. is now closed, at the beginning of each positive half-cycle the SCR suited for use in phase control applications.
is off. The positive going supply voltage increases upwards from zero at a rate determined
by the supply frequency. If it were possible to rapidly turn switch S1 ON and OFF, so that the thyristor received its
Gate signal at the “peak” (90o) point of each positive half-cycle, the device would only
Diode D1 becomes forward biased, and after a short time period there will be sufficient conduct for one half of the positive half-cycle. In other words, conduction would only take
positive trigger voltage dropped across the resistive network of RG and RGK at the Gate place during one-half of one-half of a sine wave. Thus this condition would result in the
terminal to turn the SCR and therefore the lamp “on”. lamp receiving “one-fourth” or a quarter of the total power available from the AC source.
The SCR is now latched fully-on for the remaining duration of the positive half-cycle but By accurately varying the timing relationship between the Gate pulse and the positive
will automatically turn “off” again due to its natural commutation when the positive half- half of the sinusoidal waveform, the SCR can be made to turn on at any electrical angle,
cycle ends and the Anode current falls below the SCR’s holding current (IH) value. α (alpha) between 0o and 180o (0 to π) with respect to the positive half-cycle of the source
During the following negative half-cycle, the SCR remains fully-off and non-conducting voltage. This triggering angle α is commonly called the SCR’s firing angle.
until the next positive half-cycle is reached when the switching process repeats itself. The Figure 6. Phase Angle Triggering
SCR is triggered into conduction once again as long as switch S1 remains closed.
Conduction
In the resistive triggering circuit of Figure 5. the fixed V
value resistor RG effectively sets the firing angle of the Increasing the gate to cathode
voltage above a certain α1
SCR. Increasing RG decreases the gate drive current IL
and increases the firing angle. threshold turns the SCR “on”
and conducts current Lamp
Thus we can vary the angle of firing between 0o and 0
90o using this method by changing the resistive value of RG, or by changing it to a suitable VAC
SCR
variable resistance. Resistor RGK provides the Gate terminal with a negative bias during the α2
negative half-cycle and keeps the SCR in a stable off state when there is no Gate drive.
0o < α< 90o
So we can see then that the lamp (or any connected load) receives only half of the
available power from the AC source as the SCR only conducts current during the positive
half-cycle when it becomes forward biased. α3
Then the SCR acts like a rectifying diode during the negative half-cycle blocking current
flow. The SCR will continue to supply half power to the lamp while switch S1 is closed. Gate Trigger Pulse

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Figure 6. shows the effect of varying the firing angle of an SCR. At the start of each positive At this point the capacitor discharges into the Gate of the SCR through the forward-biased
half-cycle the SCR is off due to natural commutation. If the SCR is triggered at the very diode turning it on. The time duration of the positive half of the cycle at which conduction
beginning of the half-cycle (α1 = 0o), the load (a lamp) will be “on” for the full duration of starts is controlled by time constant of the passive RC combination.
the positive half of the cycle.
Since the capacitor voltage VC lags the supply voltage, VAC by an angle α, this value is
As the application of the gate trigger pulse increases along the half cycle (α = 0o to 90o ), set by the variable resistor, VR1. Increasing the value of VR1 has the effect of delaying the
the lamp will be illuminated for less time as the average voltage delivered to the lamp will triggering voltage and current supplied to the Gate which in turn causes more lag in the
be proportionally less, thus reducing its brightness. conduction time increasing the RC time constant.
Then phase control is achieved by varying the electrical angle at which the SCR triggered. As a result, the fraction of the half-cycle over which the device conducts can be controlled
That is by varying the firing angle αF. For a half-wave circuit, the firing angle, αF plus the between 0 and 180o, which means that the average power dissipated by the lamp can be
conduction angle, αC will be equal to 180 electrical degrees. adjusted. However, as we know, the SCR is a unidirectional device, only a maximum of
50% power can be supplied during each positive half-cycle, because it cannot conduct
6. Resistor-Capacitor Phase Control during the negative half-cycles when it is reverse biased.
There are a variety of ways to achieve 100% full-wave AC control using “thyristors”. One
While the resistive triggering circuit of Figure 5. would work, the maximum firing angle is way is to include a single thyristor within a diode bridge rectifier circuit which converts
only 90o as the gate current is in-phase with the applied voltage. A larger variation to the AC to a unidirectional current through the thyristor while the more common method is to
firing angle can be obtained by changing the phase and the amplitude of the gate current use two thyristors connected back-to-back in inverse parallel. A more practical approach
using an RC triggering network. is to use a single Triac as this device can be triggered in both directions, therefore making
Figure 7. RC Phase Angle Triggering Circuit them suitable for AC switching applications.
Conduction The single-phase SCR circuit of Figure 7. uses 7. The Triode AC Switch
a combination of a resistor and a capacitor to
Lamp
trigger the SCR into conduction.
The “Triode AC Switch” or Triac for short, is another solid state semiconductor power
ON S1 IA During the positive half-cycle, the SCR is switching device and which is also part of the thyristor family. The triac can be triggered
forward biased. Capacitor, C charges up via into conduction using either a positive or a negative voltage applied to its anode terminal.
VAC VR1
the variable resistor VR1 following the AC Also, both positive and negative trigger pulses can be applied to its Gate terminal making
SCR
supply voltage waveform. it a bidirectional switching device.
D1
A The Gate terminal triggers into conduction Electrically, the triac behaves just like two silicon controlled rectifiers connected back-to-
only when the voltage at point A has risen back in inverse parallel with respect to each other. Being a three-terminal device, these
C VC high enough to cause diode D1, to conduct. two back-to-back SCR’s share a common Gate terminal.

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Since the triac conducts in both directions of a sinusoidal waveform, the concept of an Figure 9. Triac I-V Characteristics Curves
Anode terminal and a Cathode terminal used to identify the main power terminals of Forward
an SCR are replaced with identifications of: MT1, for Main Terminal 1 and MT2 for Main +I Current
Terminal 2. The Gate terminal, G is referenced the same. Mode I+ Mode I-
ON state MT2
The triac’s Gate terminal is associated with the MT1 terminal, similar to the gate-cathode Conducting
+ +

relationship of the silicon controlled rectifier. Thus current flow in either direction OFF State + G
-
-
-

between MT2 and MT1 is initiated by applying a small current signal between MT1 and Reverse Voltage MT1
- +
Blocking
the Gate. The construction and schematic symbol of a triac is shown in Figure 8. Quadrant II Quadrant I

Figure 8. Triac Symbol and Construction -V +V

MT2 MT2 MT2 Reverse Forward
Voltage Gate Voltage
Quadrant III Triggered ON
P N Triac (Conduction)
Symbol - - OFF State
N P + + ON state
Forward Voltage
Gate - + Blocking
Conducting
P N G + -
G Reverse
Mode III- Mode III+ Quadrant IV
N P -I Current

For standard AC phase control circuits such as lamp or motor control, the triac is usually
MT1 MT1 MT1
triggered into conduction in Quadrant Ι by a positive gate current, labelled above as
The triac is a 4-layer, p-n-p-n device in the positive direction and an n-p-n-p device in mode Ι+ as the Gate and MT2 polarities are always the same. This results in a more
the negative direction. Unlike the silicon controlled rectifier, a triac can conduct current symmetrical switching action because the Gate is at its most sensitive. But it can also be
in either direction when triggered by a single gate pulse. Thus a triac has four possible triggered by a negative gate current, mode Ι-.
triggering modes of operation as follows. Similarly, in Quadrant ΙΙΙ triggering the Gate with a negative gate current, (-ΙG) is
+
Ι   Mode = MT2 current positive (+ve), Gate current positive (+ve) also common. Resulting in mode ΙΙΙ-. Modes Ι- and ΙΙΙ+ are, however, less sensitive
Ι-  Mode = MT2 current positive (+ve), Gate current negative (-ve) configurations requiring a greater gate drive than the two common modes of Ι+ and ΙΙΙ-.
Hence, triac operation is commonly preferred in Quadrants Ι+ and ΙΙΙ- only.
ΙΙΙ+  Mode = MT2 current negative (-ve), Gate current positive (+ve)
ΙΙΙ-  Mode = MT2 current negative (-ve), Gate current negative (-ve) Also, just like SCR’s, triac’s also require a minimum holding current IH to maintain
conduction. Triac’s still exhibit individual electrical characteristics such as different
These four modes in which a bidirectional triac can be triggered into conduction in two breakdown voltages, holding currents and trigger voltage levels exactly the same as we
quadrants is shown using the I-V characteristics curves of Figure 9. would expect from a single SCR device.
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As we have seen above, the triac turns-off automatically at the zero crossing point of each
8. Triac Phase Control Applications half-cycle until the VR1 – C1 triggering process starts again for the next half cycle.
As with the silicon controller rectifier, the triac can also be used as a simple static AC However, because the triac requires differing amounts of Gate current in each switching
power switch providing a simple on/off zero-crossing switching function, or phase-control mode of operation, for example Ι+ and ΙΙΙ-, the switching action of the triac is therefore
by varying its firing angle. In on/off power-control applications, a triac must change state asymmetrical. This means that it may not trigger at the exact same phase angle or voltage
from a conducting state to a blocking state twice per cycle, at each zero-crossing point. point for each positive and negative half cycles as the non-polarised capacitor changes in
different directions.
Since the Gate trigger voltage is derived from the same AC power source, there is
an inherent synchronisation of the Gate triggering pulse and the corresponding AC Also as with the silicon controlled rectifier, the triac’s gate should be protected against
waveform. Thus the firing angle of the triac can be varied in both half-cycles using the transient over voltages and currents which may trigger it into conduction when off.
previous RC phase-shifting network. Therefore the disadvantage of using this simple RC triggering network can be overcome
by using a semiconductor triggering device such as the Diac.
The basic phase triggering circuit of Figure 10. uses the triac in series with the lamp (or
any load) across an AC sinusoidal supply. The variable resistor, VR1 controls the amount of
phase shift on the Gate of the triac which in turn, controls the amount of voltage applied 9. The Diode AC Switch
to the lamp load by turning it on at different times during the AC cycle.
The Diode AC Switch, or Diac for short, is another solid state bidirectional switching
Figure 10. Triac Phase Control device. Unlike the SCR and the Triac, the diac has no Gate connection making it a two
Lamp
The triggering voltage value of the terminal device, labelled A1 and A2. The diac is purely an electronic trigger diode which
BTA06 triac is derived from the series RC offers no control or amplification but produces a controlled breakdown switching action
combination of VR1 and C1. At the start from either polarity of a suitable AC voltage supply.
VR1 MT2
of each half-cycle, C1 charges up via the
VAC BTA06- variable resistor, VR1. Internally the diac has a three-layer semiconductor structure similar in construction to
600B the bipolar transistor. The difference being that the doping concentrations of the two pn-
A
This continues until the voltage at point junctions are approximately the same resulting in a more symmetrical negative resistance
G MT1 “A” is sufficiently high enough to trigger characteristic in both directions when it begins conduction.
C
the triac into full conduction. This allows
capacitor, C1 to quickly discharge back We can see from the diac I-V characteristics curves of Figure 11. that the diac blocks the
into the gate of the triac ready for the flow of current in both directions until the applied voltage is greater than the device
next half cycle. breakdown voltage VBR.

Note that once the triac is triggered into conduction and saturated, the triac effectively At this point, instant voltage breakdown of the device occurs and the diac conducts
shorts out the gate triggering RC phase control circuit connected in parallel across it, and heavily, in a similar way to a zener diode, passing a sudden pulse of voltage to the
the triac takes control for the remainder of the half-cycle. connected device.

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This VBR point in either the first quadrant or the third quadrant is called the breakdown When the charge across capacitors plates has reached the breakdown voltage level of
voltage or breakover voltage. the diac, the diac conducts and the capacitor discharges into the gate through the diac.
This sudden discharge produces a pulse of current, which fires the triac into conduction.
Figure 11. Diac Switching Characteristics and Symbol The phase angle at which the triac is triggered can be varied using VR1, which controls the
Forward charging rate of the capacitor.
+I Quadrant I
A1 Current
(mA) The Diac is a gate le ss de vice
Forward
Once the triac has been fired into conduction, it is
Symbol +ON
Breakdown maintained in its “ON” state by the load current flowing designed to breakdown at
Reverse State
Voltage through it. The voltage across the resistor–capacitor a fixed voltage
Breakdown Voltage
combination is limited by the “ON” voltage of the triac.
A2
-VBR -VON ION
-V +V This condition is maintained until the end of the present half-cycle of the AC supply. When
Reverse VON VBR Forward the supply voltage enters its next half-cycle, the capacitor voltage again begins to rise (this
Voltage -ION
-ON Voltage time in the opposite direction) and the cycle of firing the triac repeats over again.
State Negative
Resistance
Characteristics
Then we have seen that when an increasing positive or negative half-cycle applied across
(mA) the terminals of the diac reaches its trigger point, it undergoes avalanche breakdown
Quadrant III Reverse
-I Current
exhibiting a negative-resistance characteristic which extends over a large range of current.
However, this means that whenever we want to use an SCR or triac for AC power control
Commercial diac’s such as the DB3 or DB4 are designed with fixed breakdown voltage as a we may need a separate diac as well. Fortunately for us, a diac and a triac have already
low as about 30 to 40 volts and a high saturation voltage. Thus it can be used as a trigger been combined together into a single switching device called a Quadrac.
device for SCR’s and triac’s in phase-triggering and variable power control applications.

Figure 12. AC Power Switch with Diac Triggering 10. The Quadrac
The advantage of using a diac as The Quadrac is basically a single semiconductor package with the Diac fabricated into
Lamp shown in Figure 12. is that they the gate terminal of a Triac device. The advantage here is that it is one device and not two.
MT2 provide a sharper, more instant gate
VR1 trigger pulse, as opposed to a steadily Figure 13. The Quadrac Symbol
BTA06- VS
600B
rising ramp switching voltage.
MT2 The quadrac device is an internally triggered bidirectional AC switch
With reference to Figure 12. as the and is gate controlled for either polarity of main terminal voltage which
G MT1
Diac
AC supply voltage increases at the G means it can be used in full-wave phase-control applications such as
C VC beginning of each cycle, the capacitor heater controls, lamp dimmers, and AC motor speed control, etc.
charges up via the potentiometer, VR1 MT1
Quadrac
as before.
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 Power Elec tronic s

Like the triac, quadracs are a three-terminal semiconductor switching device labelled An arrow is used in the UJT symbol which points towards the base indicating that the
MT2 for main terminal one (usually the anode), MT1 for main terminal two (usually the Emitter terminal is positive and the silicon bar is negative material. Figure 14. shows the
cathode) and G for the gate terminal as shown in Figure 13. symbol, construction, and equivalent circuit of the UJT.
The quadrac is available in a variety of package types depending upon their voltage and Figure 14. The Unijunction Transistor and Symbol
current switching requirements with the TO-220 package being the most common. The B2
quadrac is designed to be an exact replacement for most triac devices. PN
B2
B2 +
Junction
(Base2) RB2
11. The Unijunction Transistor E
Channel
D
P
(Emitter) Emitter E VBB VBB
The Unijunction Transistor, or UJT for short, is another solid state three-terminal device
that can be used in gate pulse, timing circuits and trigger generator applications to switch N
RB1
and control either thyristors and triac’s for AC power control type applications. B1
(Base1) -
Although it uses the name of “transistor”, its switching characteristics are very different B1 B1
from those of a conventional bipolar junction or field effect transistor. UJT Simplified
Construction
Symbol Equivalent Circuit
UJT’s have unidirectional conductivity and negative The Unijunction Transistor
impedance characteristics acting more like a variable is a solid state triggering The symbol for the unijunction transistor is very similar to that of the junction field effect
voltage divider during breakdown so cannot be used device with only one single transistor or JFET, except that it has a bent arrow representing the Emitter (E) input. While
for amplification but instead is as an on/off switching pn-junction similar in respect of their ohmic channels, JFET’s and UJT’s operate very differently and
transistor. should not be confused.
Like n-channel FET’s, the UJT consists of a single solid piece of n-type semiconductor Resistance RB1 is given between the Emitter, E and terminal B1, while resistance RB2 is
material forming the main current carrying channel with its two outer connections given between the Emitter, E and terminal B2. As the physical position of the pn-junction
marked as Base 2 (B2) and Base 1 (B1). Between Base 1 and Base 2 the unijunction is closer to terminal B2 than B1 the resistive value of RB2 will be less than RB1. The static
transistor has the characteristics of an ordinary resistive device. resistance of the channel is typically between about 4kΩ and 10kΩ’s for most common
UJT’s such as the 2N1671, 2N2646 or the 2N2647.
The third connection, confusingly named as the Emitter (E) is placed asymmetrically
along the conducting channel and is formed by fusing the p-type material into the n-type When used in a circuit, terminal B1 is connected to ground and the Emitter serves as the
silicon channel. The Emitter junction is positioned along the channel so that it is closer to input to the device. Suppose a voltage VBB is applied across the UJT between B2 and B1 so
terminal B2 than B1. that B2 is biased positive relative to B1.

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 Power Elec tronic s

The most common application of a unijunction transistor is as a triggering device for and VOFF is constantly repeated producing a series of pulses to the SCR’s Gate acting as an
SCR’s and triacs but other UJT applications include sawtoothed generators, simple oscillator circuit.
oscillators, phase control, and timing circuits. The simplest of all UJT circuits is the
relaxation oscillator producing non-sinusoidal waveforms as shown in Figure 15. The SCR of Figure 15. turns off due to the commutation action of the motor load. For AC
operation using a triac the UJT triggering circuit can be fed from a suitable rectifier and
Figure 15. Unijunction Transistor Circuit smoothing circuit. This allows the UJT oscillator to be synchronised with the incoming
IL
mains supply.
Vcc
The free-running oscillator circuit can be designed to produce 10’s of trigger pulses
R1
per each half-cycle of the waveform. The variable resistance, VR1 controls the operating
VR1 Motor
10k-100k Ω 100Ω M Load frequency of the UJT relaxation oscillator, which in turn controls the conduction angle of
the triac. Clearly the triac will be fired into conduction on the first or second gate trigger
E B2 pulse only, but this may be within a few degrees of the half-cycle giving full power to the
A
connected load.
UJT SCR
C106
B1 G K
12. The Solid State Relay
+
C 10uF-100uF IG
- R2
100 Ω Solid State Relay’s, or SSR’s, are semiconductor equivalents of the electromechanical
0V relay. Solid state relays are designed to switch and control both AC or DC currents without
the use of moving parts by means of an SCR, TRIAC, or power switching transistor output,
Initially, the capacitor is fully discharged but charges up exponentially through the instead of the usual mechanical normally-open (NO) contacts.
variable resistor, VR1. As the Emitter terminal of the UJT is connected to the capacitor,
when the charging voltage VC across the capacitor becomes greater than the forward Similar to an electro-mechanical relay, a small input voltage, typically 3 to 32 volts DC, can
voltage drop of the p-n junction, it behaves as a normal diode becoming forward biased be used to control a much large output voltage, or current. For example, 240V, 10Amps.
triggering the UJT into conduction. The unijunction transistor is “on”. This makes them ideal for microcontroller, PIC and Arduino interfacing as a low-current,
3.3 or 5-volt signal from say a micro-controller or logic gate can be used to control a
At this point the Emitter to B1 impedance collapses as the Emitter goes into a low mains connected load.
impedance saturated state with the flow of Emitter current through resistor R2 taking Solid State Re lays use
place. As the ohmic value of resistor R2 is very low, the capacitor discharges rapidly One of the main components of a solid state relay (SSR) light to isolate their input
through the UJT and a fast rising voltage pulse appears across R2. is the Opto-isolator (also called an optocoupler) which and output with thyristors,
contains one (or more) infra-red light emitting diode (IR triacs and transistors as
When the voltage across the capacitor decreases below the holding point of the UJTs pn LED) light source, and a photo sensitive device within a switching elements
junction (VOFF), it turns off. No current flows into the Emitter junction so once again the single package.
capacitor charges up through VR1. This charging and discharging process between VON

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 Power Elec tronic s

As the only connection between the input and output is a beam of light, high voltage For most DC SSR’s the output switching device commonly used are power transistors,
isolation (usually several thousand volts) is easily achieved by means of this internal opto- Darlington’s, IGBT´s or MOSFETs. For an AC SSR, the switching device is either a triac
isolation. Thus the output device of an opto-isolated solid state relay is turned “on” by for low current applications or back-to-back SCRs for controlling very large amounts
energising this input LED with very low-voltage signal. of power. As power switching devices, SCRs are preferred due to their high voltage and
current capabilities.
Figure 16 shows different ways to activate or turn “on” a typical solid state relay. A voltage
greater than its minimum value (usually 3 volts DC) must be applied to its input terminals Figure 17. Solid State Relay Output Circuit Types
(equivalent to the electro-mechanical relay coil). This DC signal may be derived from a
+
mechanical switch, digital logic gate or micro-controller port. DC Transistor or Thyristor Bridge
MOSFET Output Configuration
Figure 16. Solid State Relay DC Input Circuits Output Output
Output Output
+Vcc Drive Drive
Circuitry Circuitry
+ S + -
SSR SSR
R R
Switching
DC Input Output Output
Transistor Back-to-Back
IR - IR Thyristors Triac Output
- LED NPN LED
Output Output Output
Direct Switching Control Transistor Control Output
Drive
Drive
Circuitry
Circuitry
+Vcc +Vcc
+ SSR + SSR The most common application of solid state relays is in the switching of an AC load,
R R whether that is to control the AC power for ON/OFF switching, light dimming, motor
Micro-controller
Logic Gate Output (sink mode) Output speed control or other such applications.
- IR - IR
TTL
LED LED
One of the biggest advantages of solid state relays over an electromechanical relay is
its ability to switch “OFF” AC loads at the point of zero load current, thereby completely
Open-collector Logic Gate Control Micro-controller Control
Sink mode eliminating the arcing, electrical noise and contact bounce associated with conventional
mechanical relays and inductive loads.
The output switching capabilities of a solid state relay can be either AC or DC similar
to its input voltage requirements. The output circuit of most standard solid state relays While there is a wide variety of commercially available solid state relays for switching
are configured to perform only one type of switching action giving the equivalent of a high voltages and currents, we can also make our own inexpensive and simple version
normally-open, single-pole, single-throw (SPST-NO) operation of an electro-mechanical for switching lower AC loads such as a heater, lamp or solenoid using our previous
relay. knowledge about the triac as shown in Figure 18.

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 Power Elec tronic s

Figure 18. AC Switching SSR End of this Power Electronics eBook

600W
Last revision: November 2022
L Copyright © 2022 Aspencore
R2 IL 5A Heating https://www.electronics-tutorials.ws
MOC 3020 Element
Opto-isolator
180 Ω Free for non-commercial educational use and not for resale
+5V R1 0.5W
+ MT2
1
240Ω 6 Triac 120V(RMS) With the completion of this Power Electronics eBook you should have gained a basic
To Micro-
controller D1 G understanding and knowledge of rectifiers. The information provided here should give you
4 MT1
2 a firm foundation for continuing your study of electronics and electrical engineering as
- R3 BTA06-600B
well as the study of power electronics systems.
56 Ω
MOC 3020 random switching
MOC 3041 zero-switching
N For more information about any of the topics covered here please visit our website at:

www.electronics-tutorials.ws
Here in Figure 18. the heating element is turned ON and OFF by turning the opto-isolator
ON and OFF in response to a voltage output signal from a temperature controller or other
Main Headquarters Central Europe/EMEA
such device. The load current of the heating element is switched using a triac.
245 Main Street Frankfurter Strasse 211
As an opto-isolator only needs a small amount of input/control power to operate, the Cambridge, MA 02142 63263 Neu-Isenburg, Germany
control signal could also be from a PIC, Arduino, Raspberry PI, or any other such micro- www.aspencore.com info-europe@aspencore.com
controller. The opto-isolator forms the basis of a very simple solid state relay application
which can be used to control any AC mains powered load such as lamps and motors.
Here we have used the MOC 3020 which is a random switching opto-triac isolator. The
MOC 3041 opto-triac isolator has the same characteristics but with built-in zero-crossing
detection allowing the load to receive full power without the heavy inrush currents when
switching inductive loads.
Diode D1 prevents damage due to accidental reverse connection of the input voltage,
while the 56-ohm resistor (R3) shunts any di/dt currents when the triac is OFF eliminating
false triggering. It also ties the gate terminal to MT1 ensuring the triac turns-off fully.

w w w.e l e c tro nic s- tu to r ials .ws 13