Transistor Summary:
Here is a summary of general transistor characteristics and nomenclature. The following are the symbols for npn (left) and pnp (right) transistors. The characteristics described in this note will apply to npn transistors and you will need to reverse them to get the equivalent characteristics of pnp’s
Physically these devices look like the following:
Transistors Rules:
1) iC=0 unless VC > VE . This means that the transistor won’t turn on unless C is held at a higher potential than E.
2) BE and BC look like diodes to an Ohm meter. When a transistor has been toasted it will often show by destroying one of these diodes.
3) iC, iB, and VCE have maxima that can’t be exceeded without toasting the transistor.
4) When conditions 1-3 are in effect then iC ≈ b iB where b ≈ 100. This is the big feature of transistors but it is important to realize that b is not a reliable or fixed number.
5) When in operation there is a single diode drop (.6-.7 V) between the base and the emitter.
FET Transistor Exploration:
The particular type of transistor that we are working with in the PWM Motor controller is a special type called a FET (Field Effect Transistor). These are generally used for higher current applications in power supplies like we are building. Here is a link to a description of and FET and a host of useful ideas and details about it (go read!).
Complete your motor controller circuit from pins 14 and 11 to the FET and the fan motor (use a 1 k resistor if I don't have a fan motor for you). Use a 1 or 2 Ohm resistor to connect the source leg of the FET to ground. You may find this DC Fan Motor site useful in determining how to hook up the fan motor. Disconnect the gate wire from the FET. Then verify that the output of the PWM chip is still the same as you had in last weeks lab. Set the duty cycle to roughly 80-90%. Then turn off the project board and reconnect the gate wire. Continue to monitor the output of the PWM chip and monitor the voltage drop across the 1 or 2 Ohm resistor grounding resistor with the other channel of your oscilloscope.
Measurements:
1) Sketch the dual trace plot of the gate voltage on the FET and the voltage drop across your grounding resistor in your notebook with correct scales.
2) Calculate (from your oscilloscope traces) the current flowing through the grounding resistor when the FET is "on". How is this related to the current flowing through the motor? If you adjust the duty cycle of the gate voltage does the fan speed up or slow down?
3) Set the control resistors on your circuit to provide the maximum possible duty cycle to the output of your PWM chip. Now turn down the 12 V supply until your PWM chip output disappears or your fan stops - which ever comes first. For some of the fans this behavior is frequency dependent so explore the possibilities and make useful notes in your notebook. Then turn it up just enough to get the fan/chip running. Now as you adjust the duty cycle the fan should turn at slower speeds.
4) As an open ended question - can you determine whether the frequency of the gate signal has a significant effect on your ability to control the speed of the fan?
