Week 4

Blog sheet week 4

IMPORTANT:

1.     (Table and graph) Use the transistor by itself. The goal is to create the graph for I_C (y axis) versus V_BE (x axis). Connect base and collector. Use 10K potentiometer to generate the voltage. Use 5 V but DO NOT EXCEED 1 V for V_BE. Make sure you have the required voltage value set before applying it to the base. Transistor might get really hot. Do not TOUCH THE TRANSISTOR! Make sure to get enough data points to graph. (Suggestion: measure for V_BE = 0V, 0.5V, and 1V and fill the gaps if necessary by taking extra measurements). The circuit should look like below:

Table 1. Ic(mA) v.s Vbe(mA)

Figure 1. The graph of Ic v.s Vbe

Figure 1 shows the relationship between Ic and Vbe of an NPN transistor: I_C relates exponentially to V_BE.

2.     (Table and graph) Create the graph for I_C (y axis) versus V_CE (x axis). Vary VCE from 0 V to 5 V. Do this measurement for 3 different VBE values: 0V, 0.7V, and 0.8V. The circuit should look like below:

Table 2. Ic v.s Vce with Vbe = 0V, 0.72V, and 0.8V

Figure 2. The graph of Ic v.s Vce

Figure 2 shows the relationship between Ic and Vce (with different Vbe) of an NPN transistor.

      As you can see when the voltage for the collector increases, the current increases fast at first. But when Vce is above two volts  the current levels off.

3.     (Table) Apply the following bias voltages and fill out the table. How is I_C and I_B related? Does your data support your theory?

Table 3. The relationship between Ic and Ib with different voltages

As shown in the table 3, the ratio (Ic/Ib) are all close to 10, which support our theory that Ic = a*Ib (a means a number).

4.     (Table) Explain photocell outputs with different light settings. Create a table for the light conditions and photocell resistance.
     
     

Table 4. Photocell resistances with different light conditions

       As the photocell sees more light the resistance decreases. So when there is no light present the resistance if the photocell is very high, this makes the total resistance of the circuit very high also. When the total resistance is very high in the circuit the current in the circuit is very low.

5.     (Table) Apply voltage (0 to 5 V with 1 V steps) to DC motor directly and measure the current using the DMM.

Table 5. Current of DC Motor with different voltages

      Table 5 shows the current of DC motor by applying voltage from 0 to 5V with 1V step.

6.     Apply 2 V to the DC motor and measure the current. Repeat this by increasing the load on the DC motor. Slightly pinching the shaft would do the trick.
     
      Under normal load with no tension applied on the shaft the current was 32 mA. When we put tension on the shaft, increasing the load the current increased.

7.     (Video) Create the circuit below (same circuit from week 1). Explain the operation in detail.

      When the photocell is exposed to light the resistance of the photocell decreases. Since some of the current that passes through the photocell goes to the base of the transistor, it determines the current for the collector of the transistor. When the photocell sees more light more current goes to the base of the transistor. When more current if applied to the base, there is more current allowed to flow through the collector. The amount of light that the photocell sees determines how much current the motor gets. The more light the photocell gets, the motor spins faster.

Video 1. The operation of photocell

8.     Explain R₄’s role by changing its value to a smaller and bigger resistors and observing the voltage and the current at the collector of the transistor. 

        
        The role of R₄ is to limit the current of the collector circuit. Too much resistance for R₄ would result in to little current to power the motor. Without R₄ the current could damage the transistor. As you can see from the table below when the resistance is low, the current is higher. When the resistance is high the current is lower.
    

Table 6. Voltage current of collector with different R4

9.     (Video) Create your own Rube Goldberg setup.

Video 2. The set up for Rube Goldberg

        In our Rube Goldberg set up when light it applied to the photocell the motor will spin, causing the golf tee to hit the ping pong ball.