Basic Transistor Circuits

Posted on Feb 4, 2014

This circuit will drive a relay coil from a low power output, usually from an IC like 555 or a TTL/CMOS. It is used to switch high loads or loads that needs AC current to operate. The relay will be actuated when the input of the circuit goes high. The protection diode Dp is used to protect the transistor from the reverse current generated from the

Basic Transistor Circuits
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coil of the relay during the switch off time. The values for Rb and Qs vary accordingly. The way to calculate them is: Then we calculate the transistor hFE. It must be at least 5 times the load current IL divided by the maximum output current from the Input to the base of the transistor Then you calculate the base resistor RB, If the input is taken from a component (possible an IC) that uses the same power supply as the transistor (that is Vs), then the form is: The output from a 74LS series TTL IC is required to operate a relay with a 160 Ohm coil. The supply voltage is 12V for the transistor and 5V for the IC. The IC can supply a maximum current of 2mA. A transistor may be connected in a way that will switch on and off a load (RL) according to a sensor. In our example, the sensor is an LDR. The LDR is a resistor that will change it`s resistance according to the light falling on the sensor. The left circuit will switch on the load when the LDR is in dark, and the right will switch on the load when light fall on the LDR. The potentiometer RS will control the sensitivity of the automation. You should consider using a potentiometer 10Kohms. Also note the resistor on the left circuit labeled RP. This resistor is used to protect the transistor when the potentiometer is switched to low values. This RP should be from 1Kohm to 22Kohms. Select according to your LDR. You should consider using low current loads using this circuit....

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