V cc is the supply voltage, V ce is the voltage drop across the collector to emitter at saturation. If this is not known - or we need to limit this current to protect the device then the resistance can be worked out using ohms law. Depending upon the type of device it may be possible to take this from the datasheet based on the current required to activate or operate the device. 10 times the minimum saturation base input current).įirst we need to determine the current flowing through the resistor R L. The input is normally driven by a much higher current to ensure that it is well into this saturation region (eg. To work out the appropriate level of resistor you need to calculate the appropriate input current to saturate the transistor.
Whilst the aim of this is to keep the maths to a minimum we need to use some simple formula to determine the appropriate value for the base resistor R b. If the load being switched is an inductive device, such as a motor, solenoid or relay then a diode should be connected in the reverse direction across the load to prevent any back EMF from damaging the transistor. When switched on in saturation the transistor acts as though it is a closed switch allowing current through the load.
#TRANSISTOR SWITCH HOW TO#
Details of how to determine the size of the resistor are explained below.įor a transistor to act as a switch it needs to be activated as the saturation region. This needs to be large enough to prevent damage to the transistor, but should still allow sufficient current to ensure the transistor switches on. The resistor at the base R b is a resistor used to prevent damage at the base of the transistor. A resistor may be needed if the device being switched does not have sufficient resistance of it's own (eg. This could be a lamp or relay or some other device that needs a larger current than the input is able to drive directly. The resistor R L is not necessarily a resistor, but represents the resistive value of a device that is being switched. It comprises of one NPN transistor, and depicts two resistors. The circuit below shows a common simple configuration for a transistor switch circuit. As a switch the transistor is often used to take a signal from a digital circuit and use it to switch larger loads than the integrated circuit (IC) can provide. It can be used as a switch by only using the transistor in an off state or an on state using the transistor's saturated region. The transistor is an amplifier, which can increase the amount of current flowing through a circuit. Electronic circuit design - transistor switch
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