In my first tutorial about power supplies, I mentioned I picked up this adapter by accident. This adapter has an AC output which is pretty useless to me, unless I build a rectifier for it. That’s what this second tutorial is going to be about.
I was taught about rectifiers by this guy who just threw up a whiteboard of equations every lecture. The labs didn’t help because they were boring (ie. Get the circuit working, get a signature, go home, wonder what I’m doing with my life). While I sort of understood what was going on, I definitely needed some time to register it all, even if that time is like two years later. Looking up information online to refresh my memory was difficult because it was the same formulas being thrown back at me.
In this tutorial, I’m going to show you how a full-wave rectifier works. It changes an AC signal into a DC voltage. My goal with this tutorial is to explain how it works with little to no formulas, enough so that you could practically use everything here if you happened to be stuck with an AC output adapter like me.
Whenever you’re analyzing a schematic, the best thing to do is group components into blocks and figure out what they do, and then see how they work together. This is what the first piece of the rectifier looks like. When the sinusoidal input is above the x axis (positive values), the current takes the path of the red line starting from the top of the source (the circle). For now, think of the two dots as the places where you’d put your oscilloscope probes to see what’s happening.
When the source voltage is below the x axis, you can think of the source as flipped upside down so that current is now coming out of the bottom of the source and following the blue lines. Notice how the flow of the current is going through your top oscilloscope probe and exiting the bottom again as it did in the previous diagram.
This is what the waveform now looks like with the AC source going through that diode configuration. The negative values of the sine wave is flipped over the x axis so all of the values of the signal are positive.When you add in a capacitor, the capacitor charges once the input signal is applied and then stays at a certain voltage (the voltage level will be discussed later). Since there is no load, it doesn’t discharge so the voltage remains a nice DC output. Obviously, this is useless to us, but it shows the reason for the capacitor.Once a load is added (the resistor in the diagram), you start to see ripples in the voltage. The peak-to-peak value of the ripple is known as the ripple voltage. This ripple happens because the capacitor discharges as the input signal heads toward zero. The capacitor charges again as the signal heads back toward its peak.
The reason I did the simulation is because I don’t have an oscilloscope. I still did it on a breadboard with my multimeter in hand, where I did a couple simple calculations. Let’s go through it.
The source measurement is giving us 14Vac…… at 60Hz.
The 14Vac that we’re seeing is an RMS, or root mean square, value. Any value taken by a multimeter is an RMS value. All you need to know is that if you are given an RMS value, the peak is actually the RMS value multiplied by √2. To go the other way if you are given the peak value, the RMS value is the peak voltage divided by √2. If you’re unsure what to do with √2, just remember that the RMS value is always less than the peak.
So, with our 14 volts read by the multimeter, the peak is actually 19.8v. If we account for the two diodes that are conducting for each half of the cycle, that’s a 1.4v drop (each diode is a 0.7v drop). That leaves us with an 18.4v output. How satisfying. Remember that we have to use the peak value because the capacitor charges up the peak, not the RMS level (which is 70.7% of the peak).And obviously, with a DC output, the signal is 0Hz.
18.4 volts is still pretty high for the applications I have for my supplies. The LM7805 voltage regulator can handle input voltages between 7-35 volts to produce a 5 volt output. You can use other voltage regulators to have other DC levels for your projects.For an example of a practical use, here’s the rectifier and 5v regulator driving one of my new blue LEDs.
So that’s it! Hopefully you understood everything in this tutorial. While I probably wouldn’t use this in a project (for space issues, primarily), it’s good to understand this concept as it is basically what’s going on in some DC output adapters.