ELI5: What is exactly happening when AC power is being converted to DC energy?

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There are electronic components called diodes. These are sort of like one-way streets for electricity. If you try to pass AC through one of these, only half the wave will get through, say the positive half. This is called a *half-wave rectifier*. Now, if you do two of these, one in each direction, you can have one stream that gets the positive halves of the waves and a second that takes the negative half. You can then combine these together using some more diodes so that you basically get the full wave, but with all the negative bumps flipped to positive. This is a *full-wave rectifier*. Sometimes, your circuit uses 3-phase power. There are 3 wires running AC, but ⅓ of a cycle out of step with each other. If you fully rectify this, you get a signal that’s just a slightly bumpy version of DC. Regardless of single or 3 phase, the next step is to smooth things out as best you can. This can be done using a *low-pass filter*. LP filters let through signals that don’t vary much, and are resistant to quickly changing signals. If you’ve got a sound system with a woofer and a tweeter (low and high speakers), there’s an LP filter making sure the woofer only gets the bass. A simple LP filter is just a coil of wire, known as an inductor. When you pass a current through it, it generates a magnetic field. This field is resistant to changes in the current. It store energy, and when the supply drops, it gives off some of the stored energy to stabilise it. When the current tries to increase, the inductor takes in some of that energy to ‘charge up’ the magnet. This means the current coming out the inductor will be smoother than what went in.


Imagine the energy as some water. Now imagine a big glass of water – something like a water tank full of water with a hole in the bottom with a continuous output flow. This is your D.C. current. Now, imagine that you bury this water tank in the sand on a beach and that each wave that arrives fill it a little bit (and you still have a the water going out of this tank). Well, the waves are you A.C. current entering in pulses in your tank. When an A.C. to D.C convertet is designed, its basically a calculation on how much you can bury your tank in the and the size of your tank (voltage and capacitance value for the capacitors) in order to have a more or less stable output flow out of the tank – also depending on the frequency of the waves arriving.


AC is electricity that goes from positive voltage to negative voltage over and over again. Each time it does this, we call that a cycle. The number of cycles per second are called hertz. DC is electricity that is always a positive voltage. There are 2 components you need to know about to converting AC to DC. The first is called a diode. A diode only allows electricity to flow one direction through it. Since AC voltage goes positive to negative, the current flows backward and forward. We want to stop that. The second component is a capacitor. More on that later. So, imagine a sine wave of electricity, that is AC. The diode chops off the bottom half of the wave, or the negative part. What you have now is like hills of positive voltage with 0 volts between them. This is called pulsing DC. We want to make the pulses smooth, so that the voltage is constant. This is where the capacitor comes in. The capacitor charges and then slowly releases that charge. So it charges up from the pulse of DC and the releases a longer slowly decreasing voltage that bridges the gap between the hills. Its still not perfectly constant, but its much closer to a constant voltage and most DC items don’t really care about the noise. Extra credit: there are circuits you can build called full-bridge rectifiers that flip the negative part of the wave to positive and add it to the positive wave so that there is no time when the voltage is at zero and the hills are closer together. This is both more efficient and has less noise. These are made with 4 diodes in a clever arrangement.


You use some diodes and a capacitor to “rectify” the signal. They are arranged in such a way that when the voltage is positive, current flows, but current is blocked by the diode when your AC voltage is negative. The capacitor then starts discharging some stored energy it got when the signal was positive, and that keeps your DC voltage constant.


There’s a few steps Usually the first step is an AC/AC transformer which reduces the voltage closer to the final voltage. Then some one-way valves (diodes or rectifiers) are wired in such a way as to change the electrons from moving both ways to moving one way only. However it’s still pulsing on and off. Then there is usually a filter which smooths out the pulsing. This usually consists of one or more capacitors, which are kind of like batteries, or an electron-balloon. Finally there is usually a regulator which acts like an automatically adjusting variable resistor designed to maintain a constant output voltage. This is much more important for electronics than, say, a fan. Some of the fancier regulators (switching regulators) actually convert the DC back to AC and then back to DC again; this is more efficient, because instead of acting like a variable resistor, it’s merely turning on and off very fast, which has the same effect, but without heating up the resistor. (although it can heat up the switching transistors)