Power Supplies and How to Make One

Nicholas Schleicher - Engineering Intern

Stay in Charge: Chapter 2

Power Supplies and How to Make One

Do you ever wonder why there is a box you have to plug into the wall when you need to charge your phone? Notice how you always have to buy a specific one for each type of device? It can be a real pain (and I'm not talking just in the wallet). Those boxes are called DC Power Supplies. These clever devices are what make the energy coming into your home usable for the electronics that you want to charge. Let's take a look further into this and see if we can make one ourselves by understanding how it works. Why we need it and the math behind it.

 

DC Power Supplies are pretty straightforward: the basic premise of a power supply is to take the incoming AC current, then change it to a DC current, and regulate it so the device you want to charge can use the electricity properly. This is done by using a few electrical components to force the current to change it to the way we want it. The first thing we have to do is change the current from AC to DC: this way, we can focus on regulating it after we get the type of charge we want. Since almost every portable device in the world runs on DC, that will be our starting point. To do this, we need to make something called a rectifier: this is composed of four diodes put in a configuration that limits the AC ground drop and changes it so it can only focus in one direction (meaning we are going to flip the ground and loop it back). The picture below should make this clearer: the first graph shows the incoming AC current. Notice how the wave goes all the way through the middle line (also known as 0 volts)? Now, look at the one below it: this is what we need to change it to. The wave now does not go below the 0-volt line. This means it was changed to DC.

incoming AC current DC current

The component we will need is a transformer: this is used to change the incoming 120v AC to 12v AC by creating a magnetic flux, thus, decreasing the voltage and increasing the amperage. Next, are the diodes: these will change the AC to DC by looping the ground (also called a rectifier) configuration. After that we need a regulator: this is what changes the 12v DC to 5v DC. Last, we will need a few resistors—depending on what we want to add, like an LED, for example—to show that we know it is on and working correctly.

Now that we know what the electrical current looks like, let's build the first part, the rectifier. As I said earlier, this is just four diodes in a configuration. Below is a picture of what this looks like: each one of the triangles with a line on it is a diode. Two of them are facing towards each other, while the other two are facing away from each other. Look at the ground: it is the one with the wavy line. See how it loops back on itself? This changes the direction of the ground by not letting it go down but forcing it to loop back up. This is what makes AC to DC; it goes in from the left and goes out through the right. The positive side just goes through because it is not as important as the ground. The energy really comes from the ground. Think of the ground as the pressure in a tube, and the positive as the water that it is pushing.

AC to DC

The regulator second part of the power supply is the regulator: this is going to change the 12v DC to 5v DC. To the right is a picture showing what the most common ones look like. What we do is take the positive output coming from the rectifier configuration, and put it on the first pin to our left: this is called the input. Next, we will ground the second pin, which is the middle one. Last, we will use the third pin as our output to be connected to our device. Side note, if you look at the top of the regulator see how there is a metal plate sticking out on top with a hole in it. This is just a heat sink.

Most portable devices will have this same exact setup. This is because almost all devices run at 5v by 2 amps. As long as you can make the power supply have the right voltage and the minimum amperage, it will charge.

 

Let's have a recap of what we just did: we were able to break down what a power supply does and how the basic electrical current works and how to manipulate it. We showed what components are required and what order they fit—transformer then rectifier—regulator then appropriate resistors. Below is a completed picture of what the power supply should look like. Then we showed why they fit in this way. I think you are now ready to try to make one yourself. This can be a little tricky at first (especially if you are not used to tinkering with electronics in general), but before you know it, you will be a pro!

completed power supply



Tuesday June 5th, 2018#phone repair