The cooling system in many houses is controlled by three electrical switches. These are:

Question 11

The cooling system in many houses is controlled by three electrical switches. These are:

• a thermostat switch that closes when the temperature rises to a given value,

• a clock switch that closes at times when cooling may be required,

• an override switch that closes to turn on the system when exceptional temperature

rises occur.

Which diagram shows the switches correctly connected between the power supply and the cooling system?

Reference: Past Exam Paper – November 2015 Paper 13 Q35

Solution:

Answer: A.

We can think of the cooling system as having 2 major purposes.

(1) it operates when it is hot (temperature exceeds a certain value) for a certain amount of time until the temperature decreases to normal again – this is the NORMAL operating state.

(2) it turns on when the temperature is too hot (e.g. in case of a fire) – this would be the ALERT state. Here, we want to cooling system to remain on until the emergency state is over. In this situation, the system is usually turned off manually.

Current flows from the power supply, through the switch connections and finally to the cooling system.

To allow operations for the 2 situations mentioned above, we need to arrange the switches correctly.

When connected in series, current would flow through both switches.

When connected in parallel, current can flow through either of the branches available.

The cooling system normally needs to be ON when the temperature increases to a given value (i.e. it becomes hot). A thermostat is used for this purpose. The thermostat switch closes.

If the thermistor has not risen to that given value (i.e. it is still cold), we do not want the cooling system to operate. So, the thermostat switch would not close.

So, CHOICE B is incorrect as current could flow through all the switches. Even when the house is still cold (the thermostat switch would be opened), the cooling system would still operate as current flows through the clock switch.

The clock and thermostat switches must be in series for normal operation of the system. → When the temperature reaches a given value, the thermostat switch closes. That is, cooling is required. And as cooling is required, the clock switch closes. [CHOICE A is correct]

The override switch is used for the ALERT case. When the temperature is too hot, the override switch closes and operates the cooling system.

In choice C, current needs to flow through all the switches for the cooling system to operate. But the override switch only closes when exceptional temperature rises occur, so the cooling system will only operate in the ALERT state and not in the NORMAL state. [CHOICE C is incorrect]

Similarly, in choice D, the override switch will always need to be closed for the cooling system to operate. That is, the cooling system will only operate when exceptional temperature rises occur and not under the NORMAL mode. Additionally, the parallel combination of the thermostat and clock switch is irrelevant. [CHOICE D is incorrect]