The Phototransistor has a base pin, but it is left unconnected in circuits since base bias is through light. That is, in dark, it remains off and in bright light, it turns on fully. The current flow depends on the intensity of light. When light falls on the base, transistor conducts. In some makes the glass window is a lens to focus light (L14F1). It is a kind of transistor with the base exposed through a glass widow. High power transistors are used in Power amplifier, Inverter circuits, UPS etc. If the load is large and current requirement is high, Medium power transistors like BEL 100 N (SL 100 / CL 100), BD 139 (all NPN), BEL 100 P (SK 100 / CK100), BD 140(all PNP) etc are used. In common circuits, low power, general purpose transistors like BC 547, BC 548, BC 549 (all NPN), BC 557, 558 (all PNP) are used. These are Low power, Medium power and High power transistors. Transistors are classified into three types based on the power output. Higher voltages can damage the base so that current limiting resistor is used at the base. The transistor turns On and drains current only if its base gets more than 0.7 volts. The flow of current in NPN is from Collector to Emitter while in PNP, it is from Emitter to Collector. NPN transistor conducts only when its base is positive and PNP transistor conducts only when its base becomes negative.Ģ. There are two basic differences between these types.ġ. Silicon transistors are now widely used in electronic circuits. The semiconductor material used in the transistor may be Silicon or Germanium. Transistors are semiconductor devices act as switch, voltage amplifier etc. This section describes the features of Transistors, their working and Pin identification methods.