| 36 | | There are many different types of transistors, but most are three-terminal devices where the current or voltage at one terminal controls the flow of electricity through the other two terminals. The most common transistors used today are either bipolar-junction transistors (BJTs) and field-effect transistors (FETs). BJTs are an older technology, but are very inexpensive and are still in common use. You can learn about BJTs [https://www.youtube.com/watch?v=sRVvUkK0U80 here]. You can use a BJT as a switch for motor control, but metal-oxide-semiconductors (MOSFETs) offer many advantages. A primary advantage of the MOSFET is that it has a very low resistance when on (when the control voltage on the gate terminal allows current to flow between the other two terminals (source and drain). Because there is very little resistance, the transistor can pass large amounts of current without getting hot. If you recall Ohms law, E=IR, if the resistance is very low (a few tenths or hundredths of an ohm), a lot of current can pass without the voltage drop getting large. This is important because if you passed 10Amps (a lot of current) through a transistor that offered 10 ohms of resistance, it would drop 100 volts and the power dissipated (as heat) would be E*I = 1000 Watts! |
| | 36 | There are many different types of transistors, but most are three-terminal devices where the current or voltage at one terminal controls the flow of electricity through the other two terminals. The most common transistors used today are either bipolar-junction transistors (BJTs) and field-effect transistors (FETs). BJTs are an older technology, but are very inexpensive and are still in common use. You can use a BJT as a switch for motor control, but metal-oxide-semiconductors (MOSFETs) offer many advantages. You can learn about BJTs for motor control in this [https://www.youtube.com/watch?v=sRVvUkK0U80 brief video tutorial]; you should watch this before moving on. A primary advantage of the MOSFET is that it has a very low resistance when on (when the control voltage on the gate terminal allows current to flow between the other two terminals (source and drain). Because there is very little resistance, the transistor can pass large amounts of current without getting hot. If you recall Ohms law, E=IR, if the resistance is very low (a few tenths or hundredths of an ohm), a lot of current can pass without the voltage drop getting large. This is important because if you passed 10Amps (a lot of current) through a transistor that offered 10 ohms of resistance, it would drop 100 volts and the power dissipated (as heat) would be E*I = 1000 Watts! |
