Th e following knowledge is required regarding the SCR:
• Construction
• Th e symbol
• Current versus voltage characteristic curve
• Values and coding
• Principle of operation (eff ect of a voltage on the gate)
• Input versus output waveforms
• How to switch the component on and off . Construction:
Th e SCR is a 4-layer PNPN device.
Figure 8.37: Construction of the SCR
Th yristors are semiconductors that can have a high voltage connected to them and which allow large currents to fl ow through them.
Symbol:
Th e IEC symbol for the SCR is shown below. One can see that it is basically a diode with a gate. It is important to draw the symbol correctly and to label it fully.
Figure 8.38: The IEC symbol for the SCR
Figure 8.39: Physical sizes and shapes of types of thyristors Th e size of the SCR is dependent on the supply voltage it will be connected to, as well as the maximum current that will fl ow through the device. Th ese components handle a lot of current, and should one be selected that is not capable of handling the currents fl owing through it, it may overheat and burn out, and possibly cause damage to other parts of the circuit as well.
SCRs are categorised into three sections regarding their ratings, and when choosing an SCR, one would have to take these ratings into account.
Low current Th ey switch up to 1 A and up to 100 V.
Medium current Up to 10 A and about 800 V
Solid-state switching for automotive engines High current Up to 2,5 kA and 10 kV
For control of motors, lights and appliances
Characteristic curves:
Th e current versus voltage characteristic curve (or I/V curve) for the SCR is shown below.
Take note When you draw the symbol for the SCR, it must be clear, neat and the terminals must be labelled.
Principle of operation:
In the reverse region, the SCR acts just like a normal diode. In other words, it will not conduct, unless the manufacturer’s specifi cations are exceeded, in which case the SCR will do one of two things: It might explode, or just passively stop working.
It could now conduct in both directions, or it may not conduct at all in any direction. Th is point is called the reverse-breakdown voltage (VREVBREAKDOWN).
How it reacts in the forward region is quite a diff erent story. Without anything connected to the gate, no current will fl ow. Only when the supply voltage exceeds what is known as the forward-breakover voltage (VFWDBREAKOVER) will a reaction take place. At this point the SCR breaks into conduction and current will fl ow through the component. Th e interesting thing is, if the supply voltage is reduced to below the forward-breakover voltage, current will continue to fl ow though the SCR. It will only stop conducting if the current falls below the holding current (IH). Th at is the minimum current that is required to keep the depletion layers small enough so that electrons can move across the gaps. Th is is not the preferred way of switching on the SCR, as it may lead to damaging the component. Th ere are other ways of switching on the SCR which will be investigated later.
Values (coding)
Some characteristics from datasheets for SCRs look as follows. Th is is merely included to give an idea of the values and information.
Silicon Controlled Rectifi er (SCR) 25 Amp, TO48 Maximum ratings and characteristics:
Surge current (at 60 Hz), ITSM ½ Cycle (150 A) VGT (3.0 V) IGT (40 mA)
Peak forward gate current (5 A) Peak reverse gate voltage (5 V)
Type VFWDBREAKOVER IHOLDING
NTE5520 25 V 6.5 mA
NTE5521 50 V 6.5 mA
NTE5522 100 V 6.5 mA
NTE5523 150 V 6.5 mA
NTE5528 500 V 3 mA
Take note Th e SCR can only conduct in one direction.
Applications:
If the supply voltage is 240 V and the SCR has a forward-breakover voltage of 250 V, it will never be triggered. But if the SCR is 240 V, the output will look as follows:
Figure 8.41: A concept circuit for an SCR and the corresponding output wave Th e SCR will only start conducting at 240 V, because that is the highest the AC supply goes, and the SCR requires 240 V to break into conduction. However, this is not the ideal way to switch on an SCR. In fact, one never does that.
During the negative half cycle, the component is reverse-biased and no conduction can take place.
One needs to look closely at the eff ect of the gate.
A small voltage of between 0 V to 3 V between the cathode and the gate has a dramatic eff ect in the forward-breakover voltage of the SCR.
Figure 8.42: IV curve of SCR with various gate voltages
If one assumes that a circuit has a supply voltage of 240 V and the SCR has a forward-breakover voltage of 240 V, this would mean the SCR would switch on at position A (or 90°) and conduct from 90° to 180°. At this point the SCR would switch off and not conduct during the negative half cycle as the SCR is reverse-biased.
If one assumes a gate voltage of 0,5 V, this would make the depletion layers inside the SCR smaller (narrower) and conduction could take place sooner. In eff ect it means that the forward-breakover voltage would be reduced from 240 V to perhaps Did you know?
Figure 8.43: Eff ect of the diff erent gate voltages on the fi ring angle of the SCR If one changes the gate voltage to 1 V, the forward-breakover voltage would be further reduced. Th e SCR would switch on at position C, and conduct for a longer time for the remainder of the positive half cycle.
With 1,5 V on the gate, the SCR will switch on at position D, and conduct for an even longer time for the remainder of the positive half cycle.
From about 2 V to 3 V (depending on the type of SCR), the component will switch on immediately and conduct for just under 180°.
So each time the gate voltage is increased by a fraction, the SCR conducts for a longer time.
An SCR can conduct for a maximum of 180°, and the longer the time that current fl ows through the load, the brighter the lamp will be or the faster the motor will turn.
Before looking at some practical circuits, knowledge of the following terminology is required:
Figure 8.44: The diff erence between fi ring angle and conduction angle
Take note Th e later the SCR switches on, the less time for current to fl ow.
Conduction angle Th e number of degrees for which the SCR is turned on and is actually conducting.
Firing or phase angle Th e angle at which the SCR fi res and actually start its conduction (but up to this point the component is off ).
240 V 200 V
Th is section now looks at how the thyristor can be used as a control element for lamp dimming or motor speed control.
Practical applications for thyristors
It is now possible to start looking at how the SCR actually works in a circuit. Before one can explain how a circuit works, one has to determine in which direction current will be fl owing through the load, because that will determine how the output waveforms are drawn.
For the circuit shown below, the supply voltage is taken as 240 V and all the SCRs have a fi ring angle of 80°. (Th at means they will switch on at 80°.)
Figure 8.45: A bridge-type circuit containing SCRs
During the positive half cycle (assume the top is positive and the bottom negative)
Th is circuit is similar to a bridge rectifi er, so the current will fl ow from the top, through SCR4, down through the load, through SCR2 and to the negative at the bottom. So during the positive half cycle only SCR2 and SCR4 will be conducting, while SCR1 and SCR3 will be off . (Th ey would be reverse biased – current cannot fl ow against the arrows.)
During the negative half cycle (now the top is negative and the bottom positive) Current will fl ow from the bottom through SCR3, down through the load (in the same direction), through SCR1 and to the negative at the top. So during the negative half cycle only SCR1 and SCR3 will be conducting, while SCR2 and SCR4 will be off . Because current fl ows from the top to the bottom of the load during both half cycles, it creates a pulsating rectifi ed wave, but each cycle will only switch on at 80°.
It is important to show where each cycle will switch on.
Another circuit that uses thyristors is shown below. Th e circuits may look similar at fi rst, but be careful. Remember, one MUST determine in which direction current will be fl owing through the load in both positive and negative cycles, because that will determine how the output waveforms are drawn.
Figure 8.47: SCRs can be used to control the speed of AC loads
When asked to draw the output waves across the load for two complete cycles of the input, if each SCR has a conduction angle of 120°, before rushing off to start drawing waves, one must fi gure out the direction of the current fl ow through the load.
During the positive half cycle (assume the top is positive and the bottom negative)
Current will fl ow from the top through SCR1, down the middle and through D2. From there it will fl ow DOWN through the load and back to the negative.
During the negative half cycle (assume the top is negative and the bottom positive)
Current will fl ow from the bottom UP through the load (in the opposite direction) and now through SCR2, down the middle and through D1. From here it will fl ow back to the negative.
It has thus been established that the current fl ows DOWN through the load during the positive half cycle, but UP during the negative half cycle. Th is means the waves will be at the top and the bottom of the sketch.
Figure 8.48: Output waves for the AC load
Take note First fi gure out in which direction the current fl ows through the load in both the positive and negative half cycles. Th en fi ll in all the required information on your sketch.
Take note A conduction angle of 120° means it fi res at 60°
(180° – 120° = 60°).
Lamp-dimming circuit
A circuit that controls the brightness of a lamp by means of a SCR control is shown below.
Figure 8.49: An SCR-controlled lamp dimmer
Should one want to build it on a breadboard, a possible layout will look as follows:
Figure 8.50: The breadboard planning for the circuit
Figure 8.51: The actual layout of the circuit
Purpose of the components:
R1 – to prevent a short circuit across the supply if R2 is set to minimum R2 – to control the size of the gate voltage, hence determining the fi ring angle D1 – to prevent a signal to the gate of the SCR during the negative half cycle D2 – only allows a positive signal to the gate of the SCR
SCR – controls for how long current fl ows through the load, and ultimately the brightness of the lamp.
Changing the value of R2
How does changing the value of R2 change the brightness of the lamp if R2 is set to position A (at the top)?
Keep in mind that R2 together with D1 is in parallel with the cathode gate of the SCR (and voltages across parallel branches are equal). Th e full voltage drop across R2 would be across the cathode–gate, therefore, Vcg would be maximum. Th is will cause the SCR to switch on sooner, allowing current to fl ow through the lamp for a longer time. Th e lamp will be bright.
Figure 8.52: The output waves with R2 set to position A is displayed on the oscilloscope. We can see that the fi ring angle is small, so the SCR switches on sooner, and conducts for a relatively long time.
If R2 is set to position B (at the bottom), this would mean that only the voltage across D1 will now be across the cathode-gate, and Vcg is at minimum. Th e SCR will switch on later, and current will fl ow through the lamp for a shorter time, which will make it burn dimly.
Figure 8.53: The output waves with R2 set to position B are displayed on the oscilloscope. We can see that the fi ring angle is big, so the SCR switches on later, and conducts for a shorter time.
As R2 is adjusted from the top to bottom, the lamp will gradually burn less and less brightly.
How to switch the SCR on:
• Forward-bias the SCR (positive on anode, negative on cathode).
• Allow the supply voltage to exceed the forward-breakover voltage. (Th is is not the preferred way of switching on an SCR.)
• With a supply connected, apply a small voltage to the gate of the SCR.
• Apply a pulse to the gate of the SCR.
How to switch the SCR off :
• Remove the supply.
• Let the current through the SCR drop to below the holding current.
• Short-circuit the cathode and the gate.
TRIACS
TRI ACs can be seen as bi-directional SCRs, so they operate exactly like SCRs, but they conduct in both directions.
Th e following knowledge regarding the TRIAC is required:
• Construction
• Th e symbol
• Current versus voltage characteristic curve
• Values and coding
• Principle of operation (eff ect of a voltage on the gate)
• Input versus output waveforms
• How to switch the component on and off Construction:
It is not necessary to understand the actual construction of the TRIAC, except how the PNPN layers are put together. For the purposes of this study, only the application and operation of the component will be examined, as it is not possible to assemble a TRIAC in a school context.
Symbol:
Th e IEC symbol for the TRIAC refers to the terminals as anode 1 and 2. Th is is because each side basically has an anode and a cathode, so they are simply referred to as terminal 1 and terminal 2. (MT1 and MT2)
Figure 8.54: IEC symbol for a TRIAC
2
1
Characteristic curve for a TRIAC:
Figure 8.55: The I/V characteristic curve for a TRIAC is shown above Principle of operation:
Th e TRIAC can conduct in both the forward and the reverse regions. When the supply is connected, no current will fl ow, be it in either region, until the supply voltage exceeds what is known as the forward-breakover voltage (VFWDBREAKOVER) or the reverse-breakover voltage (VREVBREAKOVER). Once again this is not the preferred way of switching on a TRIAC.
Note that there is no breakdown voltage for the TRIAC. At this point the TRIAC breaks into conduction and current will fl ow through the component. Th e supply voltage can then be reduced to below the break-overvoltage values and it will continue to conduct. It will only stop conducting if the current falls below the holding current (IH). Th at is the minimum current that is required to keep the depletion layers small enough so electrons can move across the gaps.
How to switch the TRIAC on:
• Apply a supply voltage across the TRIAC. Th is supply could be connected either way around.
• Allow the supply voltage to exceed the forward-breakover voltage. (Th is is not the preferred way of switching on an TRIAC.)
• With a supply connected apply a small voltage to the gate of the TRIAC.
• Apply a pulse to the gate of the TRIAC.
How to switch the TRIAC off :
• Remove the supply.
• Let the current through the TRIAC fall below the holding current.
• Short-circuit terminal 1 and terminal 2.
Take note
DIAC
A DIAC is a two-terminal thyristor. Th e terminals are also labelled anode 1 and anode 2 (or terminal 1 and terminal 2), for the same reasons as for the TRIAC.
Construction:
It is not necessary to understand the actual construction of the DIAC, except how the PNPN layers are put together. For the purposes of this study, only the application and operation of the component will be examined, as it is not possible to assemble a DIAC in a school context.
Symbol:
Th e symbol for the DIAC is shown below.
Figure 8.56: IEC symbol for a DIAC
It has no gate and needs a certain forward-breakover voltage before it will fi re.
Characteristic curve:
Figure 8.57: The I/V characteristic curve for the DIAC
Principle of operation: Th e DIAC can conduct in both the forward and the reverse regions. When the supply is connected, no current will fl ow, be it in either region, until the supply voltage exceeds what is known as the forward-breakover voltage (VFWDBREAKOVER) or the reverse breakover voltage (VREVBREAKOVER). Typical breakover voltages usually vary between 30 to 50 V. Th is is the only way of switching on the DIAC.
It will continue to conduct and only stop if the current falls below the holding I Min Holding
VRev Breakover
Forward Conduction
VFwd Breakover I Min Holding Reverse
Conduction
A DIAC only has one job, and that is to fi re a TRIAC. Th is is to ensure that the TRIAC is fi red at exactly the right time, and to eliminate transient signals on the gate of the TRIAC from triggering it.
How to switch the DIAC on:
• Apply a supply voltage across the DIAC.
• Allow the supply voltage to exceed the forward-breakover voltage. (usually between 30 to 50 V).
• Th e supply polarity could be either way around.
How to switch the DIAC off :
• Remove the supply
• Let the current through the DIAC fall below the holding current.
• Short-circuit terminal 1 and terminal 2.