Thyristors are solid state electronic components usually made from silicon. They're use for switching and control in circuits.
A thyristor has the same characteristics of a transistor and a diode, the only difference is thyristors can't be use for amplification.
To apply this components successfully in circuits one needs to understand it working principle and their potential functions in circuits.
It consists of three terminals namely, cathode, anode and gate. The cathode has a negative terminal, the anode has a positive terminal while the gate is the control terminal.
Likend to a transistor, a thyristor terminal is just like the gate acting as the base, the cathode acting as a collector and the anode as the emitter.
•Anode: This is the positive terminal of the thyristor, when a positive voltage is applied to the pin current flows from it to the cathode, however if a positive voltage is applied it blocks the flow of current.
•Cathode: This is the negative terminal of the thyristor, current flows from the anode to this terminal.
•Gate: this terminal is use for controlling the thyristor depending on the amount of current that flows into it. It can make the component switch on or switch of.
Thyristors are use in high voltage circuits up to 5000v, 400A depending on how it's is manufactured.
They are use in simple circuits like burglar and fire alarms.
Working principle of a thyristor.
A thyristor has a basic structure of P-N-P-N just like two diodes connected in series in one direction.
The P-N junction is in forward bias and the other P-N is also in forward bias. The N-P junction in reverse bias at the middle of the two forward bias junctions blocks current from flowing from the anode to the cathode, at this stage it's said to be in forward blocking state, also if current is applied to the thyristor in reverse bias the first and last semiconductors
(N-type and P-Type) blocks the flow of current from the anode to cathode, it's said to be in the reverse blocking state.
Current will flow from the anode to cathode if current is applied to the gate thus in this mode it's in the forward conducting state.
Operating modes of thyristors.
• Thyristor turned ON.
When a current is applied to a thyristor with voltage applied to the gate, it's switched on letting current to flow from the anode to cathode and will continue to do so even when the controlling voltage is removed from the gate, in this mode it's said to be "latched on".
To switch off or unlatch the thyristor, the anode to cathode current needs to be reduced below it threshold value also known as the holding current or current is totally disconnected from it.
•Thyristor turned OFF.
For the component to be turned off or lachted off, the current flowing through it must be reduced below it holding current or threshold value, a simple way of doing this in circuit is to apply a variable resistor between the anode of the thyristor and the supplying current, so by this way the current can be reduced variably below the threshold current. Common vairable resistors are thermistors, force sensors, LDR and humistor.
Types of thyristors.
There are many types of thyristors depending on the way they are lacthed on or off and the way their conductivity is controlled, examples are listed below.
- Silicon controlled rectifier(SCR)
- Light activated silicon controlled rectifier (LASCR)
- Reverse conducting thyristors (RCT)
- Gate turn off thyristors (GTO)
- MOS turn off thyristor (MTO)
- Emitter turned off thyristor(ETO)
- Triode for alternating current.(TRIAC)
- Diode for alternating current.(DIAC)
- Silicon diode for alternating current (SIDAC).
• Silicon controlled rectifier.
Circuit symbol of SCR.
As Shown in the schematic diagram above, an SCR is use to control the buzzer when a switch is switched on by someone at the back of a door.
This is the most common type of thyristor, it's has three terminals namely, anode, gate and cathode.
Current flows from anode to cathode when connected in forward bias in circuits based on the controlling terminal called the gate and blocks current from flowing when connected in reverse bias.
When an SCR is connected in forward bias with no current applied to the gate current can't flow from the anode to cathode thus acting as an insulator in this state. However if a little current is applied to the gate enough to trigger it on, current flows smoothly from the anode to cathode, even when the gate current is removed, so the SCR is said to be latched on.
To unlatch the SCR, the current flowing through the anode is reduced or removed.
Circuit applications of SCR.
SCR is use in motors drivers, static switches for AC and DC circuits, fire alarms and burglar systems.
As a simple door alarm.
As Shown in the schematic diagram above, an SCR is use to control the buzzer when a switch is switched on by someone at the back of a door.
When the switch is off, current can't flow in the circuit because the voltage at the gate of the thyristor is at 0v. When the switch is put ON the current flows through the gate which then controls current to flow from the anode to the cathode thus the buzzer rings with a loud sound.
When the switch is put off the buzzer will still keep ringing until the battery is disconnected from the SCR.
•light activated Silicon controlled rectifier.
Circuit Symbol of an LASCR
This type of thyristor with an acronym LASCR blocks current when no current flows into it gate just like a silicon controlled rectifier.
The gate of the LASCR is intentionally opened to light, when light or photons strikes the sensitive area of the gate, charges are generated in it. When the much photons or light is exposed to the most sensistive area of the gate much current is generated much than the threshold current of the thyristor and this trigger the thyristor and it is latched on.
When the LASCR is lacthed or turned on by the gate current, current flows from the anode to the cathode, but if the LASCR is the hindered from light it's still remained latched on.
Circuit applications of LASCR.
•Reverse conducting thyristors.
Normal thyristors conducts electricity only in forward bias with a controlling gate current, an RCT is an exception to this.
Reverse conducting thyristors conducts electricity in the forward and reverse bias.
It consists of an SCR coupled with a diodes in the reverse bias which eliminates undesired loop inductance and reduces reverse voltage transient.
Circuit applications of RCT.
•Gate turn-of thyristors.
Gate turn of thyristors also known as GTO are quite different from other types of thyristors which when turned on by the gate current still conducts electricity even when the gate current is removed and needed to be latched off by disrupting the supply current. However gate turn of thyristors can be latched of by applying a negative voltage to the gate without requiring the removal of the current from the anode to cathode.
Circuit applications of GTO.
•MOS turn off thyristor.
MOS turn off thyristor also known as MTO is formed from then combination or fabrication of a Metalic oxide field effect transistor (MOSFET) and a gate turn of thyristors to improve the gate turn of ability.
Gate turn off thyristor requires a an high gate negative volatge to turn of the thyristor this made the MOT which requires a little gate turn off volatage to be developed.
Unlike other thyristor, MOT has two control terminals which are turn on gate and turn off gate also known as the MOSFET gate.
To latch on the thyristor, voltage of high magnitude is applied to the turn on gate, this makes current to flow from the anode to cathode.
Latching of the thyristor, a voltage pulse needs to be applied to the turn of gate also known as the MOSFET gate. The MOSFET gate is turned on when current is applied to it which then short the anode and gate of the thyristor.