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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