Voltage Dividers

Voltage Dividers

In this lecture, we talk about finding the equivalent resistance and in the lab, we used a photocell resistance to power the LED light. 

The Picture above shows how equivalent resistance is calculated. First, the two resistance from the right are parallel. Thus, the two resistance are added with this equation: (1/R1+1/R2)^-1. 

Then, afterwards, the combined Resistor is added to the resistor on top due to the fact that it's connected in series, and then it is added parallel to the 3R resistor. Thus the Req = R. 

This picture above shows how to find the Voltage of the Photocell. From picture for the photocell, the resistance can be controlled from 5kΩ to 20kΩ. Thus, we used the equation of the photocell resistance plus the total resistance and multiplied that by the current to find the Voltage of the Photocell. 

Speaking of the Photocell, in this lab, we make a circuit to power the LED using the photocell. This lab would show why resistance is important. 

From this video, we can see that by preventing the light from entering the photocell. The LED lights up. This would be because decreasing the voltage from the light will also decrease the resistance. This causes the LED to light up because by having too much current in the circuit, the LED is "fried". 

Measuring the Photocell Resistance:

When it's dark, the resistance of the photocell is: 110 kΩ

When light reaches the photocell, its resistance is 1.3 kΩ

The above two pictures represent the voltage from the diode Vd measured when light is prevented from entering the photocell to when light contacts the photocell.

The above two pictures is the same as the next two above except it's measuring the voltage from the base to the ground Vb. 

The calculated values of Vb for photocell resistance of 5kΩ and 20kΩ is 5/3 and 10/3 respectively. 

Summary
In lecture, we calculate the equivalent resistance by combining the resistors that are connected in either parallel or series. Then in the alb, we find the resistance of the photocell. Here, it is shown why resistance or resistors are needed as too much current onto the LED will "Fry" it. This causes the LED not to light up meaning that if the photocell was blocked of light, the LED will light up because resistance will significantly increase.