Having a bench power supply in your workplace is an essential for an electronics engineer or anyone doing DIY electronics projects. You can use it in powering up basic circuits, microcontrollers, etc. It is most of the time assigned as a project for students taking up electronic engineering and alike.
Here is a basic tutorial of how to make a +/- 5V power supply from scratch.
Step 1: Materials
Here is an approximation of how much will you spend for this project:
Voltage regulator 7805 and 7905 = P30
Case = P50
Center tapped Transformer (6V-0-6V 750mA) = P150
AC Cord = P20
x4 Diodes 1N4001 = P5
Electrolytic Capacitors (the black/blue ones) 470uF x2 (any value more than 10V) 0.1 uF x2 (any value more than 10V) = P25-30
Jumper wires = P20
Breadboard (for trial) = P70
Switch 50V SPST = P30
Binding posts = P50
If you are required or you want to place it on a PCB:
Soldering Lead (buy those that are in tubes) = P25
Soldering Gun (even a disposable one) = P100
Ferric Chloride = P30
Presensitized PCB = P70
P455+P225 = P680
Step 2: Component Analysis:
This component acts as an indicator once the DC supply is opened. Don’t forget to put a resistor next to its cathode so that the filament won’t burn up easily. It acts like a diode but it differently has a light output.
There are not only one kind of switch in electronics. Some of the switches used are the SPST or single pole single throw and SPDT single pole double throw. For this tutorial, you shall use the SPST since it acts as a simple switch for ON and OFF function.
7805 and 7905 voltage regulators
The VRs are used for converting input voltages into a certain voltage output. The 7805 and 7905 converts an input voltage of 6-12V into an output of +5V and -5V respectively.
This component is used for rectifiying the signal. This only means that they cut your AC signal into two parts, the positive and negative. This is by means of a wheatstone bridge.
This component is for removing noise and making the input signal smoother. You will see how important these capacitors on the next steps. A note is, an electrolytic capacitor is entirely different from other capacitors. Since electrolytic capacitors have polarities unlike the others.
Step 3: Simulation
From the photo above, you will be able to see the first part which is the 220V AC signal to the transformer. In this step, you are actually converting a 220V AC voltage into an 6V AC Signal. This is done by the transformer depending on its number of turns. Most transformers have labels so you don’t have to worry that much.
The next part is the full wave rectifier using a wheatstone bridge. This configuration recitifies or cuts the signal into two parts, the positive and negative. This is now done by the diodes forming a bridge-like circuit.
Capacitors C1 - C4 acts as signal cleaners. What do I mean? They make the signal smoother so that once it reaches the regulators, they would be able to convert it easier.
Lastly, C5-C6 acts as noise removers as they remove noise from the output voltages. Also, the LED acts as an indicator.
Taking the voltages at the positive and ground node, you shall receive +5V and -5V for negative and ground. Once you take the voltages at the positive and negative node, you shall read more like +- 10V depending on how your probes are used.
NOTE: This simulation is done using Multisim. You can download it for free by searching for it on google. It is mostly used for students for simulations as a precaution for not harming any electronic component before an experiment.
Step 4: Trial Circuit
The photo above actually explains everything on how to use a breadboard. As you can see, the rail (as we shall now call it), at the left and right part of the breadboard are all connected. And the center holes are connected horizontally.
Now, let’s go to our power supply. I want you to connect your diodes like this and refer to the schematic I showed you above:
This is your wheatstone bridge. Next, you must connect the capacitors. One end of capacitor is always connected to the ground and the other one is connected to the polarity it is used. Again, refer to the schematic since you must also learn on how to read a schematic on your own. Continue until you arrive to a circuit that looks like this:
Now, solder the two wires of your AC cord to your transformer and add wires on the +6V, 0 and -6V of your transformer so you can use it to connect to the breadboard. The red jumper wires refer to the + signal and black for -. The grounds are indicated by the white and gray jumper wires. All the rails are used for the ground.
Once you are finished, your set-up should now look like the photo above.
The image above does not include the soldered wires of the transformer for tutorial purposes. Now, if you are confident enough, plug in your power supply to a 220V AC socket and see what happens.
If at this point a capacitor blew up, replace it and make sure you replace it by putting it in with the right polarity.
Try touching your VRs. If the VRs are getting hot, that means that you placed it incorrectly. Check out the pin configurations again and put it back once you get it.
Now, once you have finished checking out the problems, try measuring the output voltages using a multimeter. The read voltages should be like these:
+ and GND:
+ and - :
- and GND:
Once you have your desired voltage readings, you are done with your power supply circuit! Let’s proceed with preparing a case for it.
Step 5: Soldering (optional)
This is optional since soldering is a bit tough for first timers. If you wanted to learn proceed with this step since it will make your circuit strong enough to withstand damages.
The first time I did this power supply, I immediately placed the breadboard inside the casing but I made sure the jumper wires were not loose so that it will still be working no matter what happens. Then again, this is plan A.
And if you still have time, you can solder your circuit like this:
To remind everyone, this is optional and if you have not enough time to do this, skip this step and proceed with the casing. But as you can see, I used different components but to tell you the truth, the output of this circuit is not as accurate as what I showed you on the previous steps.
Step 6: Casing
This part is up to you and I have done my part in showing you how to connect the circuit. You can also include a switch on your casing if you want.
The only thing I’m going to remind you about this part is the use of the binding posts. The binding posts will enable the user of your power supply to connect it through banana plugs or alligator clips.
Solder the positive node to the red binding post. Then the GND and negative node are soldered separately on the black binding posts.
Your power supply should somehow look like this.
Step 7: DONE
If you have questions, feel free to ask me especially if you wanted to solder the circuit.
Keep on tinkering!
Please visit my blog for a better version of this tutorial: