變壓後電流計算

12v 3w 燈泡消耗電流係 0.25Amp。
而總消耗電流約為 3w + 20% ( 火牛損耗 ) = 3.6W。
初級平均電流為 220V / 0.0163Amp，等於 220V / 16mA。
需然火牛有 50w，接上 3w 燈泡後，消耗也就只算燈泡功率就可。
input AC220V / 16 mA，output DC12V / 250 mA。

Current through the light bulb = 3/2 A = 1.5 A
Assume the transformer is ideal, i.e. with 100% efficiency. By conservation of energy,
power input = power output
220.I = 3
i.e. I = 3/220 A = 0.0136 A = 13.6 mA
The actual current would be larger than 13.6 mA due to various energy losses in the transformer.

雖然火牛功率為 50w, 不過負載只有 3w, 所以係計 3w 唔係計50w, 不過火牛總共負載唔可以超過 50w。