I was doing a question on transformers and found this really confusing question:
A 100% efficient transformer converts a 240V input voltage to a 12V
output voltage. The output power of the transformer can be a maximum
of 20W. The output is connected to two 0.30A bulbs in parallel. One of
the bulbs fails. How does the current in the primary coil change?
What I did:
Since I have been given power and voltage I thought it would be helpful to figure out the current, so I did that which was fairly easy
20W/12V = 1.67 A
the output is connected to 2 0.3A bulbs so if one fails that's only 1 0.3A bulb
What I am stuck on
Now, this is where I kinda fall apart since I don't know where to go, I thought I would need to figure out some sort of ratio due to the transformer rule of the ratio of coil turns is equal to the ratio of voltages, but I don't really see where I would get information of the number of coils turns, however, my intuition is telling me that the current would increase…
Am I missing something that I haven't calculated from the question?
Best Answer
The 20 W is a maximum rating for the transformer and is of no concern unless it is exceeded. At 100% efficiency the power in equals the power out. In this case the power out (7.2 W) gets cut in half, so the current in (0.03 A) will also be cut in half.