Introduction: Dual Polarity Power Supply

By The right place to be
About: When I started my course in electronic, I dreamed of building audio gadget and synthesizer. Unfortunately, my course was a 'how to repair audio/visual equipment' type and not 'design cool stuff' type. Neverthe…

When I first started playing with electronics and circuits, I bought myself a cool variable power supply... A HQ 0-15V/2A simple power supply. I still use it most of the time but when I want to play with OP Amp and audio stuff, it just doesn't do it.

This is why I decided to fight my fear of high voltage and tackle the design of my own Power Supply.

This power supply is all made from scratch and output +15V and -15V fixed at around 1A.

Supplies

  • Transformer with center tap (more details in the appropriate section)
  • Full bridge rectifier
  • Fuses
  • Wires
  • Power Switch
  • 120V Light (or 230V)
  • LM7815 regulator
  • LM7915 regulator
  • LED (2x)
  • 4,7K ohm resistors (2x)
  • Lots of Capacitors (more details in the appropriate section)

Step 1: Introduction, Yeah There Is 2 Intros.

Hi there! You must be a complete electronic fan geek to read a tutorial like this one... or maybe you're just curious and telling yourself : 'Hey could I really be designing my own Dual Polarity Power Supply from scratch without risking electrocution or death?!'

Well, if you follow the advice in this tutorial, there is much chance that you build your first Dual Power Supply successfully! I hope you learn along the way following this step-by-step design guide!

First of all, what is a dual polarity power supply and why would you need one?

Let's start really simple with the explanation of a Wall-Wart... A Wall Wart is a device that you plug into your wall and will convert the AC live power to a DC Stable Voltage. We can say that this is the simplest PSU (Power Supply Unit). Often, these wall adapter produce 9V, 12V or even higher voltage but what is the big trend right now is 5V with a USB port to charge cellphone and small devices. I think everyone know now what I'm talking about.

Another kind of PSU is the one you find in your computer! These power supply are really strong and are made to handle a load of Power! Let's say it can handle 100 Watts, that's a lot of power but not that much compared to some top PSU that can go up to 600 Watts or more!

The output of these power supply is often 12V (yellow wires), 5V (red wires) and Ground (black wires). Sometimes you will find the green wire which is a control wire to turn the supply on.

So right now we've identified 2 type of supply, one that would gives us a single Voltage and one that produces 2 different voltage. The Dual Polarity Supply is a special PSU that would provide a Positive Voltage, a ground (common) and also a Negative Voltage. Wow! You say?

The Negative Voltage provided is a less positive voltage that the ZERO, the common or AKA the Ground. What?! Yes, if you look at this on an Electronic scale, let's say it would look like that :

+9V
+5V
0 (ground, common)
-5V
-9V

Okay I think you figured this out... but why using dual polarity?

Dual Supply is really useful in some particular application like the following:

In the Audio World, we are working with Audio Signal and this mean a waveform or AC signal. The audio Signal is AC (alternative Current) which means sometimes it's over the Zero Level and sometime under the Zero Level. You could see this as the Amplitude of the Sound.

Working with a Positive and Negative Voltage in this case will allows us to work with a bigger scale of audio (bigger range of amplitude, which means more sound level)! If you have +15V and -15V supplying an Operational Amplifier (OP amp), the output will have a much fuller kick than a single 9V battery providing 9V and a Zero. The range we have between a +15V and -15V dual supply is 30V differential. When using a single supply of 9V and 0V we only have 9V differential.

But you see, there's something interesting with all this, in most of the Guitar Pedal I've used so far, the supply is Single Polarity or just a single 9V battery... These are not made to amplifier or Drive a huge speaker, they are made to modify, boost or transform the audio signal.

So how do they make OP amp works properly? Let me tell you, there is this weird but useful technique to create a reference point or a fake Zero (ground).

Let's say you have a single 9V battery supplying power to a circuit and you really need to create a more positive and a less positive edge. Take that 9V and split it in two by using a simple voltage divider... you can now use the 9V as the positive, the 4,5V as the ground and the 0V as the negative. This will never be as efficient as a Dual Polarity Supply but will enable you to get 4,5V on both negative and positive side. This is the way it's done with single supply, you just created a reference point at half of the full Voltage which is 4,5V.

So, do you need a Dual Polarity Power Supply? If you're an experimenter, a scientist or an electronician I will answer you right away : ‘Yes you always need a Dual Bench Supply in case you have to test some OP amp or just to produce a Negative Voltage. We never know how much this tool can be useful!’


Safety Rules

Okay now that you know a little bit what you're about to deal with, let's explain some safety rules that none shall break!

  • Never plug anything in the wall if you don't know what to expect
  • Double check everything
  • Apply protection such as fuse and Isolation to your wires
  • No short circuit allowed...
  • If you don't know what you're doing, don't do it.
  • Call an electrician...
  • Ask questions before playing with live POWER.

**Risk of electrocution, damage and even death**

The last part was intended to scare you... because yes it's dangerous to work with the AC power. But if you really want to make this project, you will be careful, you will learn and finally you will succeed!

Step 2: Protection and Voltage Reduction

If you want to skip the reading, you may watch this video:

First when working with AC live power, you must know what comes out of your wall, is it 120V or 240V ? My tutorial is specially made for North American who works with 120V AC but don't worry, if you have 240V you may also create this circuit but the parts you will need will be somehow different. (make sure all parts are specs for 240V if it’s your case)


I think that everyone know what an IEC cable is but maybe this term sounds strange to your ears. An IEC cable is the regular cable that you plug into your wall and than into most of your machine. Some have only the Live and Neutral and some have the third pin which is the Ground (green wire). Right now we will work with the ground one to isolate our power supply. If you don't have a ground in your house call an Electrician.

What you want to know first before plugging the Power Cable in the wall and start measuring or striping any wires is the following :

  1. White Wire is Neutral, you won't take any shock if you hold it.
  2. Black Wire is Live, touch it and it may get hot or zappy! Be careful so you don't touch it bare hand and make sure it doesn't short with the ground or the Neutral.
  3. Green Wire is Ground, this get back to the Earth and it's there to isolate the circuit from shock or short circuit.

Strip this cable and identify all your wires, **DON'T PLUG ANYTHING YET WE'RE JUST DESIGNING**. The Black wire(live) will go connected directly on one end of the fuse.

Fuses

Fuse is what will prevent you from putting your house in fire or just turning off your breaker uselessly.


Fuses are electronic component with a specified capacity to handle Current.

A Fuse with a capacity of 1 Amp will ‘blow’ after the current of the circuit goes over 1 Amp. The delicate wire inside the tubing will just burn and create an Open Circuit. That is exactly why you want to put the Fuse in series just after the Live Power before any other component is placed. So let's say you have a short at the input of the circuit, the fuse will blow and protect every component in your circuit and preventing the breaker to trip.

In our case, we are going to work with +15V 1A and -15V 1A so it makes about 2 Amp of Power at the output of the PSU. We don't want the circuit to draw more than 2 Amp of current and the fuse will protect the circuit from overheating and even burning.

**In this design, the output of our circuit is Short-Circuit Protected but always apply a Fuse in anything you design that will be plugged in the Wall. First, because it's the law and secondly, it may just save your life.**

When choosing a fuse, you may get the Fast Blow or the Slow Blow. These make some differences and I won't get into the details. Since we're having an Experimenting Bench Supply, go with a Fast Blow. As soon as there will be a problem or an overdrive in the current, the fuse will blow. This way, it will protect you and even your part such as the OP amp you will be testing.

Fuse : 250V 2A Fast Blow

Switch

''Since we're building a bench power supply, we want to be able to turn it on and off.''


That's what I said in the video part 1. And this is true, you don't want your PSU on your bench to be always ON and ready to zap anyone who pass by and accidentally touch two bare wires. What we want here is a Rocker Switch. Rocker Switch can be found anywhere, from Canadian Tire to any Electronic Local Store. There is a wide variety that you couldn't even imagine! What we want here is something that can handle Voltage over 250V and current as well! I know there will never be up to 2 AMP passing in that switch but please choose something way over! You want your fuse to blow and protect your circuit but never a switch to melt before anything else. Choose 5A or 10A it doesn't matter as long as you like the Switch and it's fun to press!

Transformer

Now comes the time to choose a Transformer. It may be a stressful moment but don't worry I'll try to guide you in this path.


Like I told in the video part 1, a transformer is just 2 coils of wire isolated by metal or air gap. (it’s much more complex than that but huh... we’ll skip some explanations)

My preference for Transformer is Hammond so we will navigate through their site and try to choose the perfect transformer in their list. So I will explain briefly why we need a transfo while this is all explained in the video already. The Transformer here will be used as a step-down for the voltage. We will take our 120V live AC and step it down to something more around 18-24V AC. This voltage will be harmless for our components and will enable us to create our desired output voltage without any problem.

Really important like I said, we absolutely need a Transfo with a Center Tap. (3 wire output Secondary)

Here's the Hammond website : http://www.hammondmfg.com/5cindex.htm

Go into the Power Transformer section and then this section: Low Voltage/Filament -Economical Single Primary which should lead you to this link bellow.

http://www.hammondmfg.com/5cindex.htm

Here, choose something between 24V and 18V AC C.T. (Center Tap)

You need enough voltage to produce a +15V and -15V but not too much to blow the input of the regulator. I won't get too much into the details with transformer because it could take 100 years to explain... so once you have choose your transformer, your are good to go and make sure you identify the primary and the secondary. Primary is where you plug your Live AC. Secondary is the Output which should have 3 wires because of the center tap.

**Follow the schematic at the end of this guide before you connect anything**

Step 3: Rectifying and Filtering

Guys we're 1/4th of the way done! Which is pretty good because all the rest will go smoothly since I prepared this guide to be as simple as possible to understand and build.

By now, 'theoretically' you should have 2x 24V AC at the secondary of your Transformer and a Center tap to form a Ground.

What do I mean by 2x 24V AC? Well, if you measure your AC voltage between one of the wire and the center tap, you should have 24V AC, for sure if you take both opposite wire you will measure 48V... but the point is to have 2 wires of 24V AC and a common (center tap).

Bridge Rectifier

In school we built our first Bridge Rectifier with an arrangement of 4 diodes... some of us were real dummies and some of us were OKAY/smart/clever teenagers... so there was nothing much to expect from us. Coming to the point that around 20% of the class, which means 3-4 students, had configured the bridge of diode correctly on the first try. Which comes to the conclusion that 80% of the students in this course end up putting one or more diodes in fire... Yeah, it was the course on Live Power so we could make mistake but we would learn from these mistakes.

Even if I'm going to propose these all ready Bridge Rectifier, I suggest you take a look at how a bridge is configured with 4 diodes. (see first picture of this step)

This is what a well made bridge rectifier should look like. Since I'm not giving a course to student, I have choose to skip this step and make use of these ultra safe Bridge Rectifier. (please see the second picture)

  • Long Leg is Positive Output
  • AC is your 2x AC input (check for specs but 24V AC shouldn't be a problem at all)
  • Minus is the Negative Output **DON'T PLUG THIS TO GROUND**

Important note, don't plug the negative output to ground... it won't explode or anything but we don't want a single polarity supply right?

Hum, these Bridge Rectifier are really common and comes in various of model, size and shape. What really matters is that it will fit into your casing and Most Important of all is 2 specifications that you MUST watch:

  1. Maximum repetitive peak reverse voltage if this is above 300V you will never get any kind of problem.
  2. Average rectified forward current as long as it's over 5A you're over-safe!

We won't loose any much time on the bridge rectifier since it's really a plug and play device but to conclude, I can tell you what it will do to your AC Signal.

The internal configuration of Diode will allow two thing :

  1. Allow only the positive edge of the AC to pass through the Positive Output
  2. Allow only the negative edge of the AC to pass through the Negative Output

So we will end up with some Positive and Negative Pulse at the output of our Bridge Rectifier.

How to measure or see that waveform on the scope? Plug your probe on the negative or the positive output (not both at the same time please) and the ground lug to the GROUND and there I mean the center tap! You should see something like pulses if your scope is well set.

Filtering Capacitor

The filtering Capacitor is one of the nicest part of this circuit. Take the biggest caps you can find. Why? Not only it will look like a badass power supply but it will also filter a lot of the ripples and also allow stability to the voltage Inputted at your regulator. Yes, cause next step is the regulator already and we want to prepare the signal to be the cleanest possible so that regulator as only it's job to do, output a steady voltage.

Here you will need 2 capacitors, here are the specifications:

10,000uF / 50Volt / 85 or 105 Celsius (105 is better but don't break your wallet for it)

So is this the result of a long and scientific research? No this is just what I've tried first and the result of experiences over time. These value will work perfectly and don't be shy to try bigger value like 47,000uF but never go under 50V... Why? This is the maximum tolerance your capacitor can endure before blowing up.

Remember we are building a Bench Test Power Supply? You don't ever ever want that supply to be in fault or cause you any trouble, that's a bit why we're going with these values. If you want to use higher voltage specification than only 50V, go on, this is perfectly fine.

So make sure you follow the Electronic Schematic to plug your capacitor correctly, I mean the polarity of these 2 capacitors will be extremely important and respect the schematic, please always!

Step 4: Regulating and Filtering Again

Yeah, the next stage shall be the REGULATION. We are already into a big process to achieve regulation but the step we’re going to take is the Regulation via ‘all ready made’ Regulators.

I present to you the family of the LM78XX and LM79XX :

The LM78XX are made to regulate voltage and output a Positive DC Voltage.
The LM79XX are made to regulate voltage and output a Negative DC Voltage.

The XX are there to inform about the output voltage :

  • 7805 = 5V 7905 = -5V
  • 7809 = 9V 7909 = -9V
  • 7812 = 12V 7912 = -12V
  • 7815 = 15V 7915 = -15V

These are the most common one. These regulator are usually housed into a TO-220 which is the exact same packaging as a standard power transistor. They have 3 pins and you may consult the pin out in the pictures.

**The 7915 will look very similar but watch out! The pin-out is not the same and don’t make that mistake or you’re sure to fry the regulator.**

So pay close attention to the pin-out of these component and everything should be fine. Input something between 17 and 30V into these regulator and they should give you the right voltage at the output. Another excellent reason why we are using LM regulators in this circuit is because these IC are short circuit protected. So they won’t blow up if you make an error on your breadboard or two alligator clip would comes in contact by accident.

**I don’t recommend shorting your Power Supply for no reason, just don’t treat this like the regular bench power supply that will cost more than 100$. This circuit is not flawless**

Filtering Capacitor (stage 2)

We already used Filtering Capacitor at the input of these Regulator and now we’re taking the filtering to another level. At the output of our regulator, in parallel with the ground (zero), we’re gonna put another but smaller capacitor. 100uF at 50V will be just fine.

Then we have the stage 3 which is optional but I recommend you use it. 104 ceramic capacitor. Or 0,1uF capacitor to filter all spike and variation on the output of your regulators.

We’re about done now, at the output of your LM you should read 15V and -15V with a 5% tolerance. 14,75 is something acceptable but make sure you don’t have more than 0,20V difference between your positive and negative output. This will create a more balanced power supply when powering OP amp.

The 2 LED indicator are mostly optional but will add cool effect to your circuit. Put the LED at the output of your regulators and if your LED is a regular LED make sure you have a 1K to 10K ohm resistor in series. It will not only create a cool effect, it will also tell you if your output is working and will indicate if you have a short somewhere or a fault in the circuit you are powering.

Step 5: Assembling and Housing Everything

There is not much to say here except that this is a crucial moment, make sure that no live wires are exposed, that your fuse is well installed. Please, isolate your circuit, specially if the case you choose is in metal.

Be careful when drilling holes for your component and most of all, take your time.