Thursday reading/thoughts

Went to see Adam Perkins today about an inductance bridge / digital oscilloscope. He had neither one of them. The digital o-scopes are guarded by professors and kept in certain labs. Richard Messner's communications lab has a few that could be used.

THE THYRISTOR:

• 3 Terminal Device

1. Anode - touches p-type
   
2. Cathode - touches n-type
   
3. Gate

• 2 pn junctions (2 BJTs that share the innermost p and n type layers)
  

Thyristors:
ON state - Thyristor behaves like a pin diode and is able to carry very high currents with an extremely low voltage drop
Flow of current cannot be interrupted at any arbitrary instant. Current commutation must be used to revert back to blocking state.
The thyristor can be forced to switch off by reversing the direction of current flow (reversing voltage)

A special type of thyristor, called a Gate Turn Off (GTO) thyristor, can stop the load current at any time by applying a negative current to the gate. This will allow the thyristor to act as a normal on/off switch. A pulse width modulator could be used to supply the current to the gate, allowing for turn on/turn off for given periods of time depending on how much power is requested to be delivered to the motor.

The current requirement to turn off the GTO thyristor is a minimum value proportional to the load current through a ratio known was the turn-off gain. GTO thyristors must be equipped with protection circuitry. GCT thyristors are similar to GTOs but are gate commutated and do not need as much protection circuitry.

Two such examples of protection circuitry are the dv/dt snubber and di/dt snubber, which reduces the back emf stress on the power electronics.