16-10-2017, 10:54 AM
19-10-2017, 09:01 AM
One of the most common uses for transistors in an electronic circuit is as simple switches. In summary, a transistor conducts the current through the collector-emitter path only when a voltage is applied to the base. When there is no base voltage present, the switch is off. When there is base voltage present, the switch is on.
In an ideal switch, the transistor must be in only one of two states: off or on. The transistor is off when there is no polarization voltage or when the bias voltage is less than 0.7 V. The switch is on when the base is saturated, so that the collector current can flow without restriction. This is a schematic diagram of a circuit that uses an NPN transistor as a switch that turns an LED on or off.
![[Image: 308965.image0.jpg]](http://d2r5da613aq50s.cloudfront.net/wp-content/uploads/308965.image0.jpg)
Look at this circuit component per component:
• LED: This is a standard red 5mm LED. This type of LED has a voltage drop of 1.8 V and is rated at a maximum current of 20 mA.
• R1: this resistance of 330 Ω limits the current through the LED to prevent the LED from being consumed. You can use Ohm's law to calculate the amount of current the resistor will allow it to flow. Because the supply voltage is +6 V, and the LED falls 1.8 V, the voltage at R1 will be 4.2 V (6 - 1.8). By dividing the voltage between the resistance, it obtains the current in amperes, approximately 0.0127 A. Multiply by 1,000 to obtain the current in mA: 12.7 mA, well below the 20 mA limit.
• Q1: this is a common NPN transistor. Here used a 2N2222A transistor, but almost any NPN transistor will work. R1 and LED are connected to the collector, and the transmitter is grounded. When the transistor is turned on, the current flows through the collector and the emitter, which illuminates the LED. When the transistor is off, the transistor acts as an isolator and the LED does not turn on.
• R2: This resistance of 1 kΩ limits the current flowing to the base of the transistor. You can use Ohm's law to calculate the current at the base. Because the base-emitter junction drops by about 0.7 V (same as a diode), the voltage across R2 is 5.3 V. The division 5.3 per 1,000 gives the current to 0.0053 A, or 5.3 mA. Therefore, the 12.7 mA (ICE) collector current is controlled by a base current of 5.3 mA (IBE).
• SW1: This switch controls whether current can flow to the base. Closing this switch turns the transistor on, which causes current to flow through the LED. Therefore, when closing this switch, the LED lights up even if the switch is not placed directly inside the LED circuit.
It can be understood in the following video: