AC Voltage

248 Hacking Electronics

AC Voltage

We have not talked about AC very much in this book. AC stands for alternating current and refers to the type of electricity you get in a home wall socket’s 110V or 220V supply. Figure 11-4 shows how 110V AC household electricity voltage varies over time.

From Figure 11-4, it is apparent that the voltage actually reaches a peak of 155V and swings all the way negative to –155V. So you might be wondering why it is referred to as 110V at all.

The answer is that since a lot of the time, the voltage is quite low, at those times, it delivers very little power. So the 110V is a kind of average. It’s not the normal average voltage, because that would be (110 – 110) / 2 = 0V, and because half the time it is negative. 110V is the RMS voltage (root mean squared). This is the peak positive voltage divided by the square root of 2 (1.4). You can think of this as the DC equivalent voltage. So a light bulb running on 110V AC would appear to be the same brightness as if it were running on 110V DC.

You are unlikely to need to measure AC unless you are doing something exotic and dangerous, and you should not do that unless you are very sure about what you are doing and therefore probably already knew what I just told you.

Figure 11-4 Alternating current