Week 4

We're already know about PIC16F877A. In this week, I have make some research about thyristor and I would like to introduce you about Thyristor, a device that I want to use in my project circuit for replacing the relay.

Thyristors

Thyristors or silicon controlled rectifiers (SCR) are find many uses in electronics, and in particular for power control. Thyristors or silicon controlled rectifiers, SCRs have even been called the workhorse of high power electronics.

Thyristors are able to switch large levels of power are accordingly they used in a wide variety of different applications. Thyristors even finds uses in low power electronics where they are used in many circuits from light dimmers to power supply over voltage protection.

The term SCR or silicon controlled rectifier is often used synonymously with that of thyristor - the SCR or silicon controlled rectifier is actually a trade name used by General Electric for a thyristor.

Thyristor discovery

The idea for the thyristor was first described by Shockley in 1950. It was referred to as a bipolar transistor with a p-n hook-collector. The mechanism for the operation of the thyristor was analysed further in 1952 by Ebers.

Then in 1956 Moll investigated the switching mechanism of the thyristor. Development continued and more was learned about the device such that the first silicon controlled rectifiers became available in the early 1960s where it started to gain a significant level of popularity for power switching.

Thyristor applications

Thyristors, SCRs are used in many areas of electronics where they find uses in a variety of different applications. some of the more common applications for thyristors are outlined below:

• AC power control (including lights, motors,etc).
• Overvoltage protection crowbar for power supplies.
• AC power switching.
• Control elements in phase angle triggered controllers.
• Within photographic flash lights where they act as the switch to discharge a stored voltage through the flash lamp, and then cut it off at the required time.

Thyristors are able to switch high voltages and withstand reverse voltages making them ideal for switching applications, especially within AC scenarios.

Thyristor Basic

The thyristor is a device that has a number of unusual characteristics. The thyristor device has three terminals: Anode, cathode and gate, reflecting thermionic valve / vacuum tube technology. As might be expected the gate is the control terminal while the main current flows between the anode and cathode.

As can be imagined from the thyristor symbol shown below, the device is a "one way device" giving rise to the GE name for it the silicon controlled rectifier. Therefore when the device is used with AC, it will only conduct for a maximum of half the cycle.

In operation, the thyristor will not conduct initially. It requires a certain level of current to flow in the gate to "fire" the thyristor. Once fired, the thyristor will remain in conduction until the voltage across the anode and cathode is removed - this obviously happens at the end of the half cycle over which the thyristor conducts. The next half cycle will be blocked as a result of the rectifier action. It will then require current in the gate circuit to fire the thyristor again.

 Thyristor Symbol

The thyristor symbol used for circuit diagrams or circuit seeks to emphasis its rectifier characteristics while also showing the control gate. As a result the thyristor symbol shows the traditional diode symbol with a control gate entering near the junction.

The schematic of Thyristor