In this state the transistor is in danger of overheating unless it is switching a small current. Note that the switching action of these simple circuits is not particularly good because there willīe an intermediate brightness when the transistor will be partly on (not saturated). Note that the diode is connected 'backwards' as shown High voltage produced when the load is switched off. If the load being switched on and off is a motor, relay or solenoid (or any other device with a coil) aĭiode must be connected across the load to protect the transistor from the brief Note: Where the IC and load have different supply voltages, for example 5V for the ICīut 12V for the load use the formula below for R B: R B = Use the formula below to find a suitable resistance for R B and choose the nearest standard value. The value which just saturates the transistor. The base resistor (R B) must allow sufficient current to flow to ensure the transistor isįully saturated when switched on and it is good to make the base current (I B) about five times Work out a value for the base resistor R B Times the load current Ic divided by the maximum output current from the IC.Ĥ. The transistor's minimum current gain h FE(min) must be at least 5.The transistor's maximum collector current Ic(max) must be greater than the load current:.Work out the transistor properties requiredĬhoose a transistor of the right type (NPN or PNP from step 1) to meet these requirements: Note: avoid confusing IC (integrated circuit) with Ic (collector current). Vc = IC supply voltage (usually this is Vs but it will be different if the IC and load have separate supplies).If you are unable to find this information assume a low value such as 5mA. Maximum output current from IC - check the IC datasheet.R L = load resistance (for example the coil resistance of a relay).In order to work out the transistor properties required you need to know these values: Find out the supply voltage(s) and load properties. With suitable resistors (and capacitors for AC) a transistor can amplify voltage such as an audio signalīut this is not yet covered by this website.Ģ. It is a good way to test a transistor and confirm it is working.Ī transistor amplifies current and can be used as a switch, as explained on this page. NPN transistor (BC108, BC182 or BC548 for example). You can build this circuit with two standard 5mm red LEDs and any general purpose low power When the switch is open no base current flows, so the transistor switches off This collector current is large enough to make LED C light brightly. This small current to allow a larger current to flow through from its collector (C) It is just enough to make LED B glow dimly. When the switch is closed a small current flows into the base (B) of the The small base current controls the larger collector current. The diagram shows the two current paths through a transistor.
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