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Lab Work (continued)

4. Part D: Serial and parallel concept

What are the functions or applications of electrical circuits?

For virtually all of them, there are three major applications:

• power distribution,
• signal/information processing (communication/computing), and
• control (related to signal processing).

Consider the simplest power distribution application that we all are familiar with:

  

The household circuit is designed to distribute the power from the street or electrical pole to appliances in the house. The appliances should be independent from each other, hence, each is directly wired to the power source, resulting in parallel configuration. An essence of the parallel configuration power distribution is that it must be a voltage source, which supplies variable current to the demands, up to the maximum current capacity.  A circuit short can take up the entire current capacity, and all other appliances will fail.

An analogy of parallel circuit is the water distribution system to the home:

 

Every household has the same pressure, but the water flow rate can vary depending on usage.

The opposite of parallel configuration is the serial configuration. Consider another analogy:

Assume for simplicity that a river volume is conserved over its course (neglecting minor water adding and subtraction from small tributary streams and rivulets), then the water pressure or potential energy can vary over different segments as illustrated above, but the flow rate is the same (water mass conservation). If each dam produces hydroelectricity with identical turbine system for all dams using the same water flow rate, the power output would be different depending on the pressure differential (water potential energy) at each dam.

In the old time, we sometimes see serial circuits such as the one below:

The advantage of this configuration is that a small current (low risk of fire) can be used with high voltage to supply the power. The disadvantage is well known: if a light is broken (open) or removed, it breaks the current and the entire circuit is off. But if a light is shorted, the circuit is still working.

For Part D, we will study two very simple examples below:

Fig. 4.1	All LED's should be of the same color. If you are short of LEDs of the same color, you can mix LEDs of very close colors, such as mixed red/orange, or mixed yellow/green. Blue cannot be mixed with any other color (unless you have purple). The circuit is scalable, the number of LED's can range from 4 - 8 depending on how many you have.
Fig. 4.2	Same as above with regard to LED color.

To implement circuits in Fig. 4.1 and 4.2 onto a breadboard, consider Fig. 4.3 below.

Fig. 4.3

Figure 4.3 above shows 5 wiring configurations of group of LED's, labeled A, B, C, D, E. The last block, configuration E, is the obvious diagram of 8 LEDs in parallel. It is shown only for illustration. For the Lab work of Part D:

- determine which of A, B, C, D LED blocks represents the circuit in Fig. 4.2
- build the circuit, apply the voltage while monitoring the current until the current is 15±1 mA. (if your LEDs are too dim, raise the current to the level you feel sufficient to see).
- measure the voltage and obtain the total power (P= V x I).  Divide the total power by the number of LEDs to determine the average power per LED.

- determine which of A, B, C, D LED blocks represents the circuit in Fig. 4.1
- build the circuit, apply the voltage while monitoring the current until the current is n*15±1 mA where n is the number of LEDs.  For example, if n=7, the current should be 7*15 mA=105±5 mA.
 - measure the voltage and obtain the total power (P= V x I).  Divide the total power by the number of LEDs to determine the average power per LED.

- compare the per-LED power of circuit 4.2 and circuit 4.1. Discuss.

- discuss what's wrong with the other circuits that are not selected for measurement.

5. Part E: Parallel current distribution

Fig.5.1

Consider the circuit in Fig. 5.1. The 4 LEDs should be red, yellow, green, blue. The resistors are of your choice such that the current in each LED should be approximately 15 ± 5 mA. (depending on your LED type, you can go higher or lower, as long as it is safely below the damage threshold of the LEDs - check with manufacturer's specs). Use the app in Section 3 (Part C) to get an estimate for the resistance value for each LED. Use resistors in parallel if necessary to obtain the value you need. Work to be done and reported: - draw the entire circuit schematic with measured resistance values (after selecting appropriate resistors) - measure the current of each LED circuit, which should be 15 ± 5 mA as indicated above - measure the voltage of each LED - measure the net current supplied by the voltage source (nothing else should be connected to the voltage source besides this circuit). Verify if the measured net current is equal to the sum of all LED currents.

Hint: see the figure below. The calculation uses a fixed value of R1 for all LED colors. Hence, it is up to you to find and match the resistance for each LED color such that the current is as close to 15 mA as possible.

    

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