[Mon Feb 6] Circuits with potentiometer

The first part of today's class focused on studying circuits in the math world, the part which I was pretty familiar with already. Oscar gave a couple analogies of resistor, current and voltage, and basically, current flows across a potential difference when flowing through a resistor. We also thought about the possibility of a circuit with no resistor, then there is still potential difference, but no power is generated. I also learned that one of the usage of resistors is to compute voltage. But of course, many of the conclusions here were based on the ideal math world and could be different in the real world. For example, the current going through a circuit could be infinite on paper; however in reality, some physical effect will keep the current from reaching infinity.

Then we moved on to talk about potentiometer - a resistor whose resistance can be controlled. We started to build our circuit again based on where we left last time; this was where I got confused again. We connected the two extremes of the potentiometer to +12V and -12V. We plugged in the power source, which gives a maximum of 12 V and tested the voltage across the potentiometer through its output end; it went from 12V to -12V. Then Oscar introduced the idea of a hysteretic comparator. There was some electric equation involved, but the basic idea (from my understanding) was if measuring the voltage at the output end, it stays at 12V until voltage at the negative input end hits below -6V. At this time, the voltage measured at the output end immediately drops to -12V and stays there until the voltage of the negative input end hits above 6V.

An important application for the hysteresis is the heater: it does not go on and off constantly when the temperature is around the set point. Instead, the hysteresis alternates the set point, so the heater won't turn off until it gets a lot warmer again. I thought that was really cool and would love to hear more applications of circuits.