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AC Rectification

It’s back to usual this week for my article, because I thought of a good topic. I’ve talked many times about AC power and how and why we use it, but many things don’t use AC power. In fact, they can’t. These are things we use every single day. Virtually all digital electronics cannot use AC power directly. It needs to be converted into DC power. Remember that AC means the current is alternating and DC means the current is flowing in one constant direction.

Any time you convert electricity from one type to another, or one voltage to another, there will be losses. Nothing is free. Every time you change something about electricity, you have to pay a tax. The goal in electronics design is to minimize those losses. AC to DC conversion is one of the simplest and most efficient conversions we do on a regular basis. DC to AC conversion is also possible (accomplished by inverters). Most of these processes are on the order of 95% efficient or greater. So how do we convert AC to DC?

First off, the process of converting AC to DC is called rectifying. Devices that accomplish this task are often called rectifiers. A very common rectifier design is called a bridge rectifier. To understand how they work, we have to recall that AC is a wave. Sometimes it’s voltage value will be positive, other times it will be negative. This doesn’t work for DC since the voltage has to always be positive or always be negative. Rectifiers take the negative voltage swing of the wave and flip it up top so that it’s positive. Now instead of a sine waveform, the output of the circuit will look like mountains. The voltage will always be positive, but it will still vary considerably, from the peak (approximately 170 volts on grid-level 120VAC) to zero and then back up again.

How is the voltage rectified? In a bridge rectifier a ring of diodes is used. Recall that a diode only allows current to flow through it in one direction. By arranging the diodes in a ring we can create a “draw off” point for the current where the voltage will always be positive. Instead of being allowed to “pull” the current backwards during the negative voltage swing, the diodes redirect it back to the output point. Of course, this only means that the negative voltages get flipped up, creating the mountain-like waveform I mentioned earlier. This is not good for DC power, so how do we fix it?

There are many ways to minimize this issue, but it can’t be fixed perfectly. One way is to use capacitors to buffer the output voltage. Instead of sinking when the mountains go back down to zero, the capacitors prop the voltage up during that period, until the next mountain peak arrives. Depending on the load, this could mean large capacitors to hold the load long enough. Remember also though that AC has a frequency on the order of 50-60Hz. This means there will be around 120 of those mountain peaks every single second, so the capacitors don’t need to hold for too long.

The other issue we have to contend with is how to reduce the voltage from anywhere between 120-240VAC to something DC electronics might expect, like 5VDC. For AC to anything, this is actually very easy; we use transformers. A transformer has two sets of wire windings, a primary winding and a secondary winding. The primary winding contains the input current, usually fluctuating AC. This fluctuating current induces a magnetic field that also fluctuates in a metal core that extends from the primary winding to the secondary winding. The changing magnetic field in the core induces a new current in the secondary winding. The induced current will necessarily be greater than the input current if the windings are configured to reduce voltage. The overall power has to remain the same. So if the input were 120VAC at 1 amp (120 watts), and the transformer was reducing the voltage by a factor of two, the output would be 60VAC at 2 amps (still 120 watts). The output of a transformer will still be AC, but this is where we would apply rectification.

This method of power conversion is extremely efficient, but as transformers are highly inductive, power companies would hate it if every single device used one. The reason for this is because inductive loads shift power around a lot without actually consuming that much. So the power company has to pay to send the power all the way from their generating station, to you, only to have your device reject almost all of the actual energy in that power. Most homes don’t have solely inductive loads though, so most of the time it’s not an issue. However, certain industrial processes rely heavily on inductive load machinery. In these cases, the power company may request that they burn off a certain amount of power in dump loads (basically huge resistors) in order to keep the grid functioning normally.

So there you have it. How we take that high voltage raw AC power from the wall and tame it to safely power all of your devices.

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