Describe the operation of a basic thyristor (SCR).
1 Answer
A thyristor, also known as a Silicon Controlled Rectifier (SCR), is a semiconductor device that acts as a controlled switch. It allows current to flow in one direction only, similar to a diode, but can also be triggered to conduct by an external signal or control voltage. The operation of a basic thyristor involves three main states: off-state, forward blocking state, and forward conducting state.
Off-State:
In the off-state, the thyristor behaves like an open switch, and there is no current flowing between its anode (A) and cathode (K) terminals. The gate (G) terminal, which is the control terminal of the thyristor, is not conducting any current, and the device remains non-conductive.
Forward Blocking State:
To turn on the thyristor, a positive voltage is applied to the gate terminal relative to the cathode (G-K voltage). This voltage triggers a small current to flow into the gate, which then creates a voltage drop across the gate-cathode junction. This forward-biases the gate-cathode junction and lowers the potential barrier between the N-type and P-type semiconductor regions inside the thyristor.
Forward Conducting State:
Once the G-K voltage reaches a certain threshold level, typically called the "trigger voltage" or "gate trigger current," the thyristor enters the forward conducting state. In this state, the thyristor starts to conduct current from the anode to the cathode. Importantly, once the thyristor is triggered into conduction, the gate control loses its influence, and the thyristor remains conducting even if the G-K voltage is removed.
The thyristor will continue to conduct until the current flowing through it drops below a certain value known as the "holding current" or if the supply voltage is removed. At this point, the thyristor returns to the off-state, and the whole process can be initiated again by applying the trigger voltage to the gate terminal.
It's important to note that once a thyristor is in the forward conducting state, it cannot be turned off directly by reducing the gate current to zero. Instead, special circuitry or reverse voltage across the anode and cathode terminals (reversed bias) is typically used to turn off the thyristor, and this is called the "reverse recovery" process.
Thyristors find applications in various power control and switching circuits, including motor control, lighting control, power supplies, and other high-power electronic systems.
Off-State:
In the off-state, the thyristor behaves like an open switch, and there is no current flowing between its anode (A) and cathode (K) terminals. The gate (G) terminal, which is the control terminal of the thyristor, is not conducting any current, and the device remains non-conductive.
Forward Blocking State:
To turn on the thyristor, a positive voltage is applied to the gate terminal relative to the cathode (G-K voltage). This voltage triggers a small current to flow into the gate, which then creates a voltage drop across the gate-cathode junction. This forward-biases the gate-cathode junction and lowers the potential barrier between the N-type and P-type semiconductor regions inside the thyristor.
Forward Conducting State:
Once the G-K voltage reaches a certain threshold level, typically called the "trigger voltage" or "gate trigger current," the thyristor enters the forward conducting state. In this state, the thyristor starts to conduct current from the anode to the cathode. Importantly, once the thyristor is triggered into conduction, the gate control loses its influence, and the thyristor remains conducting even if the G-K voltage is removed.
The thyristor will continue to conduct until the current flowing through it drops below a certain value known as the "holding current" or if the supply voltage is removed. At this point, the thyristor returns to the off-state, and the whole process can be initiated again by applying the trigger voltage to the gate terminal.
It's important to note that once a thyristor is in the forward conducting state, it cannot be turned off directly by reducing the gate current to zero. Instead, special circuitry or reverse voltage across the anode and cathode terminals (reversed bias) is typically used to turn off the thyristor, and this is called the "reverse recovery" process.
Thyristors find applications in various power control and switching circuits, including motor control, lighting control, power supplies, and other high-power electronic systems.