Lab 8 - GilesVolmir/Skylar-Scott--308L-Junior-Lab GitHub Wiki

A. First up is to measure the voltage of a full bridge with an oscilloscope which has the problem of needing a floating ground. First the circuit is rebuilt. After troubleshooting i found it is important to have both probes on the same voltage scale. I am measuring with a floating ground by using the CH1-CH2 mode on the oscilloscope and connecting the inputs of the channels across my resistor. I can see the usual small sawtooth wave on top of AC.

B. Transistors

I. Transistors as Diodes Found an appropriate transistor (2N3904) and began performing tests suggested. The resistance meter is not giving expected range of response: BE is around 5Mohm and BC is around 4Mohm. The resistance is lower across BC as expected. Also the Fluke 111 multimeter's diode checking setting checks out BE and BC as diodes in the same configuration as the figure in the electronics manual. BE is 0.688 volts and BC is 0.675 volts on this "diode" setting. Since the positive (red) lead had to be on B as I was measuring, I assume current will only flow from B-->C or B-->E when used as a diode. This implies that it is an NPN transistor since electrons flow from N to P, so formal current (imagined positive current) flows from P to N. Now I will construct a half wave rectifier using the BC and BE pairs as diodes (separately). I used a 1kohm resistor, a 430ohm resistor, and a wavetek model 180 function generator on "lo" output. The EB pair gives the easily recognizable half way rectification response. The CB pair also gives a half way rectification response, though rather more attenuated.

II. Transistors as Switches I made the circuit in figure 4-1 of "lab 4" from the manual provided. When the switch is engaged: *the voltage across the 270ohm in series with the LED is 2.83 volts (current: 2.83V/270ohm=10.48mA) because it they are connected in series this is also the current through the LED. *the voltage across the 8.2kohm at the base of the transistor is 4.22 volts. (current: 4.22/8200ohm=0.51mA)

The manual suggests connecting a 10kohm resistor to ground between the switch and the 8.2kohm resistor to the base of the transistor. I think this would be for a faster turn off of current if there is 'stray capacitance' nearby. After trying it cannot tell any difference. To test I put a 100 microfarad catalytic capacitor (that huge so i could see the delay with my eyes) from ground to the switch (in parallel with the 10k resistor) when i take out the resistor i can tell the LED takes a lot longer to turn off.