Numerical Control

Numerical control is is a flexible method of automatically controlling machines through a series of number, letters, and symbols that can be applied to wide variety of operations like drafting, assembly, inspection, sheet metal working, etc. But it is more prominently used for various metal machining processes like turning, drilling, milling, shaping etc.

 

Unijunction Transistor (UJT)

Transistors are semiconductor components that can be used as amplifiers of electronic signals, variable resistors, or as a switches. It regulates current or voltage flow and acts as a switch or gate for electronic signals. The transistor was invented by William Bradford Shockley, John Bardeen, and Walter Houser Brattain at the Bell Laboratories in 1947.

     

Fig. 1 Transistors

Transistors is divided into several types that are bipolar-junction transistors, field-effect transistors, unijunction transistors, Darlington transistors, insulated-gate bipolar transistors, phototransistors, and transistor oscillators.

Unijunction Transistors (UJT)

Unijunction transistor (UJT) is an electronic semiconductor device that has only one junction (P-N). Unlike a true transistor, a UJT does not amplify but it works more like a voltage-controlled switch.

Fig. 2 Unijunction Transistor

The unijunction transistor contains one emitter and two bases. The resistance between base 1 and base 2, when the emitter is open-circuit is called interbase resistance. The UJT operates like a base 1-to-emitter switch that switches from OFF to ON once the thresh-old voltage level is reached. When the variable emitter supply voltage is set to zero, a small current flows from base 1 to base 2. No current flows from base 1 to the emitter. When the positive emitter voltage reaches a certain threshold value (several volts), the UJT switches ON, and a high current flows from base 1 to the emitter.

There are two types of unijunction transistor:

• The original unijunction transistor, or UJT, is a simple device that is essentially a bar of N type semiconductor material into which P type material has been diffused somewhere along its length, defining the device parameter η. The 2N2646 is the most commonly used version of the UJT.
• The programmable unijunction transistor, or PUT. Like the thyristor, it consists of four P-N layers and has an anode and a cathode connected to the first and the last layer, and a gate connected to one of the inner layers. They are not directly interchangeable with conventional UJTs but perform a similar function. In a proper circuit configuration with two “programming” resistors for setting the parameter η, they behave like a conventional UJT. The 2N6027 is an example of such a device.

Fig. 3 Programmable Unijunction Transistors

References :

• Petruzella, Frank D.,1996, Industrial Electronics 2nd edition,Singapore : McGraw-Hill Book Co.
• http://whatis.techtarget.com/definition/transistor
• http://www.allaboutcircuits.com/vol_3/chpt_7/8.html
• http://www.circuitstoday.com/ujt-uni-junction-transistors

 

 

 

 

Thyristors

Fig. 1 Thyristor

Thyristor is an electronic component that can be used to create a latching circuit. It is also known as a bistable. Similar to FET (Field-Effect Transistors), they are digital switches that respond to an input voltage. However, unlike FETs the thyristor will remain on even if the input voltage is removed, allowing electricity to flow until the thyristor is reset. This means that they are very useful in alarm circuits.

Thyristors have three leads:

• cathode
• anode
• gate

When the gate leg receives a momentary signal of at least 2V it switches on fully, allowing electricity to flow between the cathode and the anode. The current continues to flow until the thyristor is reset by interrupting the current flow through it.

(Source : http://www.bbc.co.uk/schools/gcsebitesize/design/electronics/switchesrev4.shtml)