This is why it's important to use the data sheet figure for the guaranteed minimum h FE at the correct collector current. The relationship is usually shown in a typical characteristics graph. So choose one that's comfortably over-rated (at least 50% higher than I C(max)) but not unnecessarily over-rated.Īlso it's important to know that h FE decreases as I C increases, for any given device. Larger transistors normally have lower current gains. A higher h FE makes the transistor easier to drive - i.e. When you're choosing Q1, also consider the minimum current gain, h FE(min), at that collector current. This current, and the V1 voltage, enable you to select a suitable transistor for Q1. If it's a motor, use the starting or stall current (whichever is higher) from the data sheet. If it's a buzzer or a relay coil, the resistance and/or current are given in the data sheet. If the load is an LED with a series resistor, you probably already know the current. If the load is resistive, you can calculate the maximum collector current I C(max) using Ohm's Law, from the supply voltage V1 and the load's resistance. Start with the maximum collector current I C The load is energised: an LED lights up, or a relay activates and closes its contacts.Ī common question is: "How do I calculate the value of RB?" Current flows from V1, through the load, into Q1's collector, out Q1's emitter, and back to 0V. When the controlling device's output is high, current flows through RB into Q1's base, biasing Q1 into conduction. If the load is a relay coil (with a reverse diode across it for back EMF suppression), it does not energise. If the load is an LED (with a series resistor), it does not illuminate. Therefore, no current flows through the load in the collector circuit. When the controlling device's output is low, no current flows through RB, and Q1 is turned OFF.
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