Today we start off the class with a demonstration, a hot dog connected to two prongs hooked up to the 120VAC wall outlet. We were asked what would happen to the hot dog once it was connected and draw a current versus time graph.
We assumed that since the hot dog has water in it, current will flow though it and slowly cook it as the water evaporates. Once the hot dog was powered, we saw it slowly steam and then start smoking as the electricity burned the hot dog at the point of contact. Then, LEDs were inserted on the hot dog, and we were asked what the LEDs would do. We said the the LEDs that had legs parallel to the hot dog would light, with more spread out legs lighting more, and the ones perpendicular to the hot dog would not light at all.
The hotdog with LEDs stuck in it.
The hot dog being powered and we see that our predictions are correct.
There a a potential drop across the hot dog, and with the LEDs being powered at two different spots, we see a potential difference and current flows through the LED, obviously the further the spread, the more potential and the parallel ones receive no current as there is no potential drop.
Next we're given a simple KCL problem to be introduced to dependent sources. We're told that the current source is dependent on the current flowing though the middle branch. We do a simple KCL equation and find the unknown current flowing though the branch, then we found the potential drop across the resistor.
We're given a KCL problem and told to find i_0 and v_0, we solve and find is equal to 4A and v_0 is equal to 8V.
Professor Mason shows us that there a much simpler method to solve the problems and avoid so many currents, we can redraw the circuit to cluster the top wire into a single connection and eliminate many of the currents we were dealing with.
Next up, we were given the problem of the varying voltage source. It's a simple circuit with a power source v_s, a voltage out v_out, and two resistors with resistance R1 and R2.
We solve for the current though the whole loop to and multiply it by R1, and since we know that is the potential drop the second resistor has to have, we can conclude that the potential between the resistors is that value.
Next, we are given a design problem involving an an LED that has specific requirements to run off of a 12VDC supply. We set up the initial loop and solve for R1 and R2 using the formula derived from the previous step. We first found total resistance and then found values of resistance that would give us our maximum operating voltage from the 12VDC source.
Professor Mason then tells us that resistors do not come in those values we have just solved for, and to obtain such a resistor, it is extremely expensive. So,with a table of E-12 grade resistor values, we found two that would be proportionally close to our values we solved for. We calculated the new voltage and current and found the voltage is nearly identical and the current is roughly halved.
Another problem, we have a LED that runs off of a specified voltage and we need to find a resistor that would drop the potential down. So we find the maximum drop possible and the maximum current possible and solve for resistance.
There is also a lovely comment from our neighboring group.
Next, another resistor problem that we found the equivalence resistance to be just a simple answer. We were quite disgruntled when we solved it.
Lastly, for the lab, we were introduced to a light sensor that has a variable resistance based on the amount of light received. We're also introduced to a new circuit element called a Bipolar Junction Transistor, or BJT for short. The BJT works similar to the MOSFET from last time except that rather than using voltage as a activation value, it uses current. The light sensor varies between a very high resistance and a lower resistance when in the dark, with the circuit, we cam make a LED light turn on an off dependent on the light level. We solve for the current that would flow and the potential at each value.
Here we have the circuit set up on the breadboard with the Analog Discovery as a power source. We also have a multimeter set up to measure the potential across various elements.
Here the sensor is uncovered so the potential across the bulb is not enough to turn on the LED.
Here we cover the sensor and we notice the potential across the bulb increases to turn on the LED.
Here we have the measured values of the light sensor's resistance and calculated our expected voltage across the LED, and then compared it to the actual voltage. We found that the voltage when it was off was close, but the voltage when it was on was fairly off.
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