# The anode, cathode and gate. The symbol of

The activity aims to give some basic knowledge
and principles on how thyristors operate through the utilization of different
thyristor circuits.    The students shall be able
to:

1)
Know how thyristors
differ from each other depending on its operation.

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2)
Analyze the output
characteristic curves.

3)
Compare and Contrast
the calculated, simulated and actual values and determine the factors that
affect the discrepancy.

4)
Understand some of
the equivalent circuits of thyristors depending on its availability.A
Thyristor is a special type of diode that only allows current to flow if and
only if a control voltage is applied to its gate terminal. This kind of diode
consists of three electrodes namely anode, cathode and gate. The symbol of
thyristor is shown in Figure 1.

Figure
1: Thyristor Symbol and
P-N JunctionsThyristors have different working
principle depending on its classification. Generally, the thyristor is switched off and no current flows between the
anode and the cathode when there is no current flowing into the gate. On the
other hand, when there is a flow of current into the gate, it effectively flows
into the base of the n-p-n transistor, which makes the thyristor operates.

Figure
2: The circuit and its V-I CharacteristicsFigure 2 shows the representation of
the circuit (a) used to obtain the V-I Characteristics (b). Some of the
significant points on this characteristic talks about the Latching Current,
Holding Current, Reverse Current, and Forward Break-Over Voltage. Latching
Current (IL) is the amount of the anode current required to maintain
the operation of a thyristor immediately after it has been turned on. Holding
Current (IH) is the minimum anode current required to maintain a
thyristor in the on-state. In order for us to turn off the resistor, the
forward anode current must be less than to its holding current in a certain
period of time to vacate the junction. If it is not maintained long enough, the
thyristor will not have returned to its fully blocking state when the voltage
across anode-to-cathode increases again. In other words, if there is no gate
current applied externally, there is a possibility to return to its conducting
state. Reverse Current (IR) will flow through the device if and only
if it is in a reverse-biased condition. Once the thyristor is turned on by a
gate signal and its anode current is greater than the holding current, the
device continues to conduct due to positive feedback even if the gate signal is
removed. This is because the thyristor is a latching device and it has been
latched to the on state.

Relaxation Oscillator can be
constructed through UJT. UJT or Unijunction Transistor