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Before the discovery of thyristors, thyratrons were used for in­dustrial control, but now they have been replaced by the thyristors because of the following drawbacks of thyratrons.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.