4/3/2023 0 Comments Npn transistor![]() The most commonly used devices are the vertical npn and the lateral pnp transistor. Read more Navigate DownĪ typical bipolar process contains npn and pnp transistors which can be orientated vertically or laterally. In the case of equipment operating at mains voltage there is no need for the transformer. Circuits of this type are in common use for controlling the brightness of room lights and the speed of motors on equipment such as electric drills. In this way the average current flowing to the external circuit can vary between the maximum (equivalent to half-wave d.c.) and zero. By altering the timing of the trigger pulse we can arrange for conduction to commence earlier or later in the cycle. As a result, current flows for only one third of a cycle. It does not conduct again until it is triggered again during next positive half-cycle. Immediately, the thyristor begins to conduct and the waveform is a sine wave until the a.c. The triggering pulse is timed to occur about one third of the way through the positive half-cycle. The action can be seen in the diagram opposite. Conduction stops and does not start again until the thyristor is triggered during the first half of the next cycle. voltage falls near to zero, the current dies away and eventually falls below the holding current. When triggered, the thyristor behaves like the diode of a half-wave rectifier ( p. A positive pulse to this switches off the pnp transistor, cutting off the current even though the anode remains positive to the cathode.Ī triggering circuit produces a short positive pulse at the same stage during each positive half-cycle of the supply. Some thyristors are supplied with a fourth terminal, the anode gate. But, unlike a diode, it does not turn on again when the p.d. In this way, the device acts as a diode, allowing only one-way current, as its symbol suggests. The device is also turned off by reversing the anode-cathode p.d., making the cathode positive with respect to the anode. This reduces the current through the device, but it remains on until the current reaches a minimum value, known as the holding current. ![]() The device may also be turned off by reducing the anode-cathode p.d. The only way to turn them on is to reconnect the supply and then apply another positive pulse to the gate. They cannot be turned on again by reconnecting the supply to the anode. If there is a break in the circuit that prevents current flowing to the anode, both transistors are switched off. But, if the gate potential drops 0.6 V or more below the positive potential, a small current flows out of the gate, causing a much larger current to flow through the transistor and out of the collector. ![]() No current flows as long as the gate is held at, or close to, the same positive potential. This is usually connected to the positive rail of the circuit. In the diagram of a pnp transistor, the arrowed line is the emitter. ![]()
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