Thyristor - a Type of Diode

Thyristor is a type of diode that allows current to flow if and only if a control voltage is applied to its gate terminal. This kind of diode has three electrodes namely anode, cathode and gate. Thyristors 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.

Some of the significant points on this characteristic talks about the Holding Current, Latching Current, Reverse Current, and Forward Break-Over Voltage. Latching Current (IL) is the amount of the anode current required to constantly maintain the operation of a thyristor immediately after turning it on. On the other hand, Holding Current (IH) is the current required to maintain a thyristor into its on-state. In order for us to turn off a thyristor, the forward anode current must be less than compared to its IH in a particular period of time. If it is not maintained properly, the thyristor will not return to its state of blocking when the voltage across anode-to-cathode increases again. In other words, if there is no IG applied externally, there is a chance or possibility to return to its conducting state. Reverse Current (IR) will only be present and conduct through a device if and only if it is in a reverse-biased condition. Most of the time, current flows once the circuit is in a forward-biased condition.

However, there are instances that there is a presence of a reverse current that conducts 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 is a break-over type transistor. It consists of 3 terminals namely Base 1, Base 2 and Emitter. UJT is said to be a transistor but it has a different characteristics, properties and operation compared to conventional BJT or FET because it is only used as a switch unlike to some transistors such as BJT and FET, it also allows the input signal to be amplified. Waveform generators, thyristor gate control, timers and of oscillators are some of its application. UJT is used in a relaxation oscillator because if you’re going to see its characteristics, it has a negative resistance region which can be easily used and employed in relaxation oscillator.

As technology is keep on improving and developing, PUT has been invented. PUT stands for Programmable Unijunction Transistor. From the word itself, its structure and operation is the same as UJT. It is said to be programmable because it can be adjusted to a desired VP through external resistance and its intrinsic standoff ratio. The resistor connected in the cathode terminal of the transistor limits the cathode current of PUT. When VBB (Supply Voltage) is applied, the capacitor starts charging with the help of the resistor R. Once the voltage across the capacitor exceeds the given VP, the PUT goes into its negative resistance and it creates a low resistance path from the terminal of the transistor which makes the capacitor discharges. Once the voltage across the capacitor is less than VV (Valley Point Voltage), the PUT comes back to its initial condition and the discharging path for the capacitor is not available. Again, the capacitor starts to charge with the help of the resistor and the cycle is repeated.