Resistor Transistor Switch

- October 29, 2018

Usually an npn transistor in a common emitter configuration used as a switch drives the relay. Transistor as a switch for relay transistors are capable of driving small relays without any problems.

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Resistortransistor logic rtl sometimes also transistorresistor logic trl is a class of digital circuits built using resistors as the input network and bipolar junction transistors bjts as switching devices.

Resistor transistor switch. In an ideal switch the transistor should be in only one of two states. Leds on or off by using a transistor in its saturated or cut off state when used as an ac signal amplifier the transistors base biasing voltage is applied in such a way that it always operates within its active region that is the linear part of the output characteristics curves are used. The switch is on when the base is saturated so that collector current can flow without restriction.

The circuit is designed by using a transistor as a switch to light the bulb in a bright environment and to turn it off in the dark and a light dependent resistor ldr in the potential divider. Typically these use an npn bjt or an n channel mosfet. Resistors are placed to limit the currents through the base and led.

Then the transistor is switched off. When the environment dark ldrs resistance become high. Here is a simple circuit that uses a bipolar junction transistor bjt to drive a small 5 v relay coil.

Transistor switches can be used to switch a low voltage dc device eg. The base resistor is chosen small enough so that the base current drives the transistor into saturation. Since the transistor is switching the path to ground or is sitting on the low side of the load it is called a low side switch.

In this example the mechanical switch is used to produce the base current to close the transistor switch to show the principles. When the transistor is connected to ground that means the load is between v and the transistor. The transistor is off when theres no bias voltage or when the bias voltage is less than 07 v.

When the switch is closed base current starts flowing through the transistor and then drives into saturation results to led become on. Biasing a transistor to operate as an ideal switch involves understanding transistor characteristics graphsthis article focuses exclusively on biasing and if you have gaps in your knowledge then i recommend reading the earlier pages of this multi page article.