[Chikis (OP)]

I was given an assignment to draw a diagram to explain the working principle of a transformer which can produce 24 V from a 240 V supply. My question now is what type of transformer are my suppose to draw - step-up transformer or step-down transformer and why?

[evan_au]

It is our policy not to do people's homework for them - so have a go, and show your working.

They are asking for a transformer that steps down from 240V (the input, available from your wall plug in the UK or Australia, connected to the Primary of the transformer) to 24V AC (the Secondary output that may power a motor or low-voltage lighting, for example).

However, to a first approximation, a step-up transformer can also be used as a step-down transformer. If you were studying electrical engineering, there are considerations like the amount of insulation you might use, or slight allowances you might make for magnetisation current or internal resistance. But for a physics question like this appears to be, you can ignore any subtle design differences between step-down and step-up transformers.

[teragram]

Draw a thick vertical line, maybe 20 mm long, to represent the iron core of the transformer. On the left hand side of this draw the primary coil, coils are often represented as a curly line, spring like. Next to this write something like "Primary, 2400turns". Draw another coil on the right hand side of the core line and write next to this "Secondary, 240 turns".
The ratio of the transformer needed to reduce 240Volts to 24Volts is ten to one ie 10:1. The numbers I have chosen for the turns are arbitrary but in your case must have a ratio of ten (primary) to one (secondary). With 240Volts applied to the primary 24Volts will appear on the secondary.

[Chikis (OP)]

It is our policy not to do people's homework for them - so have a go, and show your working.

They are asking for a transformer that steps down from 240V (the input, available from your wall plug in the UK or Australia, connected to the Primary of the transformer) to 24V AC (the Secondary output that may power a motor or low-voltage lighting, for example).

However, to a first approximation, a step-up transformer can also be used as a step-down transformer. If you were studying electrical engineering, there are considerations like the amount of insulation you might use, or slight allowances you might make for magnetisation current or internal resistance. But for a physics question like this appears to be, you can ignore any subtle design differences between step-down and step-up transformers.

The original turns ratio from the problem is 1/10. They showed it.

I drew a step-down transformer for the problem. When I checked the answer provided for the question, I saw that a step-up transformer have been drawn for the problem. A small note was added " in the diagram, there should be more turns in the primary than in the secondary coil ( the diagram should be change to a step-down transformer) and the turns ratio should be 10 if that is done".

If the question is asked the other way round, that is if the 24 V becomes the primary voltage and 240 V becomes the secondary voltage, should I draw step-down transformer and add a small note telling them that there should be more turns in the secondary coil than there is in the primary coil to make it a step-up transformer and the turns ratio will become just as they have done?

[syhprum]

There is no real difference between a step up and a step down transformer the same device can be regarded as either depending on which way it is used.
If you have a transformer with two winding that have a turns ratio of 10 to 1 you can apply 220 v to the winding with the greater number of turns a obtain 22 v from the winding with the smaller number then you would regard it as a step down transformer or you can apply 22 v to the winding with the smaller number and obtain 220 v from the winding with the greater number and regard it as a step up transformer.
You must bear in mind there is a limit as to how high a voltage you can apply to either winding depending on the number of turns and the area of the core, if you have a transformer designed to step down 220 v to 22 v you could not use it in the reverse manner to step up 220 v to 2.2 kv as the core would saturate magnetically.

[Pumblechook]

Transformers are actual quite complex to analyse as there are copper and iron losses as well as magnetic effects like hysteresis and saturation.  If you want 24 V output from a 240 V supply you will need something like an 8 to 1 ratio as the output voltage will drop when the transformer is loaded.   Modern power supplies these days (Switch-mode)  (TVs, computers, all sorts of thing) are often far more complicated than simple transformers.  They usually chop up the incoming mains at a high frequency which allows a much smaller more efficient transformer to be used.  The output voltage  is adjusted by changing the on-off period of the switching and a feedback system can stabilise the output voltage so it doesn't vary (much) with loading.   A simple transformer either with AC out or rectified to DC is poor as regards the voltage dropping as the load is increased.   They can be made stable (DC out) by using a 'linear regulator'. 

[alancalverd]

I drew a step-down transformer for the problem. When I checked the answer provided for the question, I saw that a step-up transformer have been drawn for the problem. A small note was added " in the diagram, there should be more turns in the primary than in the secondary coil ( the diagram should be change to a step-down transformer) and the turns ratio should be 10 if that is done".

If the question is asked the other way round, that is if the 24 V becomes the primary voltage and 240 V becomes the secondary voltage, should I draw step-down transformer and add a small note telling them that there should be more turns in the secondary coil than there is in the primary coil to make it a step-up transformer and the turns ratio will become just as they have done?

Textbooks are illustrated by fallible humans. But that doesn't excuse them being edited by lazy idiots. The rule, when communicating scientific matters, is to make the drawing show what you want, using the established conventions and standards. The joy of this is that thanks to the conventions of chemistry, physics, maths and engineering, we techie-types can communicate explicitly with each other across boundaries of language and culture, and thus build things like space stations and particle colliders. If you want a step-down transformer, draw one! 

And just to make it easy, you don't need to draw the exact number of turns. If it is more than about 3, draw a few turns or even a solid block, and write the number next to it. 

[Pmb]

Draw a thick vertical line, maybe 20 mm long, to represent the iron core of the transformer. On the left hand side of this draw the primary coil, coils are often represented as a curly line, spring like. Next to this write something like "Primary, 2400turns". Draw another coil on the right hand side of the core line and write next to this "Secondary, 240 turns".
The ratio of the transformer needed to reduce 240Volts to 24Volts is ten to one ie 10:1. The numbers I have chosen for the turns are arbitrary but in your case must have a ratio of ten (primary) to one (secondary). With 240Volts applied to the primary 24Volts will appear on the secondary.

It was just stated that we don't answer homework questions. There's a very good reason for that. So why did you just give him the answer?