![]() In this example we are using a NPN Darlington transistor. In many of these transistor circuits R1 ranges from 1000 to 2200 ohms for 5 volts. In this case it's only 100 mA limited by the light emitting diode. If Q1 has a gain of 50 and the base current through R1 is 5 mA, then the collector current will be 250 mA. In this example a digital "HIGH" on the input "sources" a current in the base/emitter of Q1 (limited by R1) which causes a larger current flow in the collector/emitter circuit and through the LED-resistor. The "HIGH" is switched to 5 volts inside the micro-controller "chip" while a "LOW" is switched to ground inside the "chip." Another digital state is known as floating that's as the name suggests is attached to nothing. Note a digital "HIGH" is 5 volts and a digital "LOW" is zero volts. We have a negative battery ground tied to digital ground. It consists of a NPN bipolar transistor controlling a high-power light emitting diode connected to a 12-volt battery. Illustrated above is the most common transistor driver circuit. All of the examples below assume a negative shared common. If we put the switch on the neutral side of the load, we say we "sink" the voltage. When a "switch" supplies a voltage (on the "hot" side) such as a household light switch, we say the switch "sources" the voltage. ![]() ![]() Also note the concept of sink/source as we go along. On this page we will look at transistor driver circuits using both bipolar transistors and power MOSFETs and will use them as electrical switches. Above illustrates a digital output driving a typical low-power light emitting diodes. Most electrical and electronic devices require voltages and currents that will destroy digital circuits, so we must rely on what I'll broadly call driver circuits. The output of most digital circuits and micro-processors is only five volts at most a few milli-amps. Bi-Polar MOSFET Transistor Driver Microcontroller Interfacing ![]()
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