A device suitable for 220V a.c. can be put on safe d.c. voltage of – ____________________
(a) 157 V (b) 189 V (c) 440 V (d) 220 V
Answers
Answer:
An incandescent bulb will do fine. The 220 V AC is the RMS value, for Root Mean Square. The sine's amplitude will be 2–√ higher than that, or 310 V. But the RMS value tells you what equivalent DC voltage you would need to get the same power, so that's exactly what you need. The bulb will use the same power and light as bright under 220 V AC as DC.
Switching on an incandescent bulb may cause a large current peak: the cold resistance is only about a tenth of what it is when the lamp is lit, and when the voltage applied is high at that time the lamp may break. You may have noticed that if a bulb breaks it always does when switching on. So at AC worst case is when you switch on at the peak of the sine, at 310 V. But there will be lots of cases when the voltage is lower when switching on, even zero if you just happen to switch on during a zero-crossing of the sine. In fact that's the best thing to do for the bulb's longevity.
At DC you don't have this; anytime you switch it on it will be 220 V. Not as bad as 310 V, but you can't use zero-crossing switching either.