![]() ![]() When saturated, increasing the base current has no effect on the collector current. The potential difference across the load is approximately equal to the power supply voltage.įor efficient switching, the transistor should be saturated. The potential difference across the load is zero. V CE is equal to the power supply voltage. When the transistor is cut off (fully turned off), Vce = Vs. ![]() When the transistor is saturated (fully turned on), the collector current depends only on the load resistance and the power supply voltage. This could be enough to destroy the transistor.īy adding the diode, the current through the inductor can die away gradually (by flowing through the diode). When the current through an inductor is turned off suddenly, a large Back EMF is produced. Inductive loads are electromagnetic devices such as motors, relays and some buzzers. This could be a bulb, an LED, a relay, a small motor, a buzzer and so on.ĭ1 is only needed if the load is inductive. For example a BC182L with a current gain of about 300 R1 limits the base current to a few milliamps To calculateR 1 R 1 <= R L x h FE R 1 should be less than or equal to the current gain times the load resistance. This is also known as a common emitter switch.Ī small base (b) current controls a much larger collector (c) current supplying the transducer.Ī high logic level in the input will switch on the bipolar junction transistor. Here is a switch or transducer driver based on an NPN bipolar junction transistor. Mechanical switches and relays can switch high voltages and currents and also alternating current.Ī small base current is used to switch a much larger collector current.īipolar Junction Transistor Switches - OK for Low Power Mechanical switches are less reliable due to the moving parts.Įlectromagnetic relays are less reliable due to the moving parts. Logic GATE switches are fast and cheap but not very sensitive to small input voltage changes. ![]() OP-Amp comparators are slow, more expensive, but much more sensitive to input voltage changes. A plus point is the minute current needed to operate the switch - nearly zero. MOSFETS are even faster but they cost more. A minus point is the current needed to operate the switch. BJT switches are fast and the transistors are not expensive. ![]()
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