Difference Between Thyristor and Thyratrons:
Before the discovery of thyristors, thyratrons were used for industrial control, but now they have been replaced by the thyristors because of the following drawbacks of thyratrons.
1. Thyratron needs a large anode to cathode supply voltage and a separate filament supply whereas the thyristor needs only one main supply and a control signal.
2. Thyratrons takes long time in ionising and deionising process and, therefore, is unsuitable for higher frequencies (beyond 1 kHz). A thyristor can operate over a wide range of frequencies because it can withstand as high as 800 A/μs rate of rise of current.
3. A thyristor has much smaller turn-on and turn-off times in comparison to those of a thyratron.
4. Because of the filament and heating of anode by accelerated electrons, the life of thyratron is much smaller than that of a thyristor.
5. Thyratron is less reliable than thyristor.
6. Thyratrons have internal losses much larger than those of a thyristor.
7. Thyratron is bulkier than thyristor because in thyratron large spacing is required to be provided to avoid any arc-backs and unwanted flash-overs.
8. The voltage drop across a gas tube is much higher (as much as 20 to 30 ) whereas voltage drop across a thyristor is quite small (about 1.5 V).
9. Thyratrons are costlier than thyristors.
10. Thyratrons are less efficient than thyristors.
11. Thyratrons are non-semiconductor devices whereas thyristors are semiconductor devices.
12. Thyratrons are less accurate than thyristors.
A thyristor is a current controlled device, while the thyratron is a voltage controlled device. This results in different design of gate-control circuits for them.