Resistors and Ohms Law - Voltage-Current Characteristics

In Class: 
We did a circuit exercise, analyzing potential difference.

We also calculated the hot resistance of a Tungsten filament using voltage and power ratings. Cold resistance is less than cold resistance.

We talked about branches, loops and nodes of circuits and how to describe independent loops in a circuit.
# Branches  = # nodes + # loops - 1
(b = n + l - 1)

Kirchoff's Law of Charge: Total Charge in and out is Zero, ie. Sum of Currents is Zero. We finished with a second lab where we measured the current out of a MOSFET while varying the voltage supplied to its gate.


In Lab:
We began by measuring the resistance across our resistor, which was supposedly 100 Ohms.  It was not 100 Ohms! We measured 67 Ohms.

We went to find another resistor closer to the desired 100 Ohms.  Our second resistor was 130 Ohms! We gave up and went with it.

Our power supply indicated the voltage supplied, but we measured the voltage across to be certain.  We found typically about 1% error in voltage values.  We then measured the current across the circuit by setting our DMM to current, and putting the DMM in series.

3. We found a direct relation between voltage and current:











2. We also played with a MOSFET circuit lab and the Waveforms software. We varied the voltage on the gate pin with waveforms software, and measured the current allowed to pass through the MOSFET. The graph on the right clearly points the MOSFET threshhold voltage of 1.2V. 
4. The MOSFET behaves like a voltage dependent current source. 
5. Estimated value: g = 10.5 Amps/Volt on gate beyond 1.2V.




















In conclusion:
We did a circuit analysis! We learned the difference between hot and cold resistance.
We plotted voltage against current in a simple series circuit with a 130 ohm resistor.
Using Waveform software, we observed how a MOSFET transistor passes current with different voltages on the gate.