Explanation

Where resistors are wired in series (one after another) it is relatively simple to find out what single resistor could be fitted to replace them.

This simulator has 4 sliders which allows you to alter the resistance of 3 resistors to values varying from 1 to 100Ω and voltage from between 0 to 240V.

Sliders can be dragged to approx positions and then fine tuned by using left and right arrows.

On changing these sliders the total resistance RT, the current flowing in the circuit and voltage drops across the 3 resistors adjust automatically. The effect of these changes can be hidden or displayed by ticking/unticking various checkboxes.

Tasks

In a previous module you looked at resistors in series (one after another) and saw how an equivalent resistance could be calculated by taking the total of those resistors. From this equivalent resistance Ohms Law could be used to find the current that would flow in that circuit, and then using this current Ohms Law could be used to find the voltdrop across these resistors.

This exercise will extend this to look at volt drop in circuits and how the choice of a circuits cable size and length affect the volt drop, how well the intended load will work and power may be lost in the cable supplying the load.

The interactive tool represents a radial like circuit with a single point of load such as a shower, cooker or heater with 3 resistances wired in series. These 3 resistances represent

• R1 - the resistance of the phase conductor from board to load.
• R2 - the resistance of the load in the circuit which should be the main consumer of power.
• R3 - the resistance of the neutral conductor from board to load.

It starts with R1 and R3 having a resistance of 1Ω each and R2 (the load) having a resistance of 100Ω which results in 2.25A being drawn and approx 500W being consumed by the load R2 when connected to a supply of 230V.

Imagine that R2 represents a lighting load. At this level the voltdrop in the circuit is 4.5V (2.25 in the phase and neutral conductors) which is within the allowable limits for a lighting circuit volt drop.

The following steps should link series resistors and voltage drop together giving a greater understanding of both. Your findings and explanations should be recorded in the task sheet downloadable from the bottom of this page then be submitted as an attachment to me at Robert.Eyre@Hull-College.ac.uk

1. With the voltage set to 230V and R1 and R3 both at 1Ω each reduce the R2 value using the slider so that the circuit current reaches 3A, 5A and 6A. For each of these note the value of R2 where this is achieved along with voltages dropped and power lost across the 3 resistors.
2. Comment on the voltages dropped for each in the supply cables (R1+R3).
3. With the voltage set to 230V and R1 and R3 both at 1Ω each reduce the R2 value using the slider so that the circuit current reaches around 13A (roughly what a 3kW Immersion heater would draw). Note the value of R2 where this is achieved along with voltages dropped and power lost across the 3 resistors.
4. Comment on the voltages dropped for each in the supply cables (R1+R3).
5. Without changing anything else from the previous question reduce R1 and R3 down to 0.4Ω each. Note the value of R2 where this is achieved along with voltages dropped and power lost across the 3 resistors.
6. Comment on the voltage dropped in the supply cables (R1+R3).
7. With the voltage set to 230V and R1 and R3 both set at 0.4Ω each reduce the R2 value using the slider to 5Ω. Note the voltages dropped and power lost across the 3 resistors.
8. Comment on the voltage dropped in the supply cables (R1+R3).
9. With the voltage still set to 230V and R2 still at 5Ω reduce R1 and R3 both to 0.25Ω. Note the voltages dropped and power lost across the 3 resistors.
10. Comment on the voltage dropped in the supply cables (R1+R3).
11. With the voltage still set to 230V and R2 still at 5Ω reduce R1 and R3 both to 0.1Ω. Note the voltages dropped and power lost across the 3 resistors.
12. Comment on the voltage dropped in the supply cables (R1+R3).
13. With the voltage still set to 230V and R2 still at 5Ω increase R1 and R3 both up to the 1Ω in the initial lighting circuit question. Note the voltages dropped and power lost across the 3 resistors.
14. Comment on the voltage dropped in the supply cables (R1+R3).
15. Based on what you have discovered in this exercise explain the importance of choosing appropriatly sized conductors for each circuit covering such areas as complying with voltage drop requirements, preventing cables overheating and allowing devices to work as expected.ted and checked.