Power Supplies for Beginners, Part 1
A Video Tutorial for Beginners – Getting set up to test, modify and use power supplies without spending a fortune.
Watch our Part 1 video tutorial to learn more!
Following Transcript of the Video Tutorial
Time: 0:00s Hello I’m Chris Richardson, and I’m an electronics engineer focused on power supplies. This is the first of a series of videos for viewers who aren’t necessarily Electronics Engineers but, want to learn more about test and used power supplies.
If you are a student, hobbyist or someone who needs to modify power supply for pretty much any reason, electronics-tutorials.ws and I hope that these videos will get you started.
Power Supplies Agenda Slide
Time: 0:25sOne important goal of this first video is to show you some basic items that will help to test a power supply, but to do so without spending thousands of dollars or euros or the equivalent wherever you are watching this from. I have put together a list with some of the approximate costs here in Spain where I live and work.
Time: 0:43sHere I’ve gathered some of the basic supplies needed to work with and to test power supplies, so, wire strippers, some clippers, some thin tweezers for grabbing small components. Here two silver box power supplies that I salvaged from an old PC, two different old PC’s. This one is a very old one, it actually has a 20-pin connector, and here we can see after modification. I will get into the details of this one later.
Time: 1:13sIf you focus in closely, a silver box power supply will tell you how much power it can provide overall and also how much of the different voltages it gets. Also salvaged from some old PC’s were two DC fans, these run off 12 volts plus they come with a convenient connector also. Seems like something basic, but for the basic plugs here you can turn-on and turn-off at the switch, very nice.
Time: 1:53sA soldering iron, one that has a fairly thin tip that will allow us to solder some small components. Some fairly thin solder and of course safety goggles.
As far as electrical tools go, I like to have two multimeters, they come with these kind of tips here. Two multimeters are good for measuring two voltages, but also for measuring either a current and a voltage, and at least one wire that has a banana plug on one end, an alligator or so called grabber clip on the other end.
Time: 2:27sThe last tool here looks a lot like a multimeter but this is actually a thermocouple, I’m going to turn it on. What is does is actually measure temperature, measures the temperature out of the tip here. I am using a plastic workbench here, so this is the kind of thing you can find anywhere, nothing special.
Cheap Oscilloscopes – The Pros and Cons
Time: 2:46sIn planning for this series of videos, I debated very seriously the topic of using or not using an oscilloscope. Do a quick search and you’ll find plenty of devices like the one on the screen that attach to your PC and make it into an oscilloscope. In the end I decided that this was better than nothing because actually seeing some power supply voltage waveforms really helps to understand them.
But be aware that 20MHz, even though it sounds high, isn’t enough to see many of the so called transient effects on power supplies. That means things that happen very quickly so, during these videos we will stick to things that happen mostly in steady state.
Time: 3:21sHere we have an oscilloscope that is not the 60 euro model I talked about, its a slightly fancier one. But what I am going to do to make my waveforms that I show in these presentations more realistic and closer to what you would see in the cheaper model you can get off of the internet is to do two things:
One, these aren’t the probes that came with my fancier oscilloscope these are some lower quality probes. A lower quality probe has lower output resistance or impedance and has higher output capacitance. Those are the things that will distort the waveforms.
The other thing I will do is, focusing in here you will see the BW written there stands for Bandwidth. That means that the oscilloscope is bandwidth limited to 20MHz. That’s the same limit that the cheaper oscilloscope has. So that will make the measurements I show closer to what you see if you had the less expensive device.
Turning an ATX PSU into Your Lab Supply
Time: 4:19sJust about everyone will have and old PC gathering dust in their basement or attic. The floppy disk drive may be useless, but that power supply, the so called silver box may still be good. As the web based tutorial on Electronics Tutorials shows, an ATX Power Supply provides a whole host of different voltages and quite a bit of power.
Also take a moment to remove any fans you find in the case of your old PC. Those will be great later for blowing air and keeping your power supplies and other electronics cool.
Time: 4:48sHere is the pin of an ATX Power Supply. This actually has the 20-pins of the older ones and the four extra pins too attached. Its coming of a power supply that was donated to the cause. Of course there are lots of extra wires connected. One thing to keep in mind is that they are colour coded.
Pin outs of the 20-pin ATX PSU connector
Reference: ATX Power Supply Tutorial
| Pin | Name | Colour | Description | |
| 1 | 3.3V | Orange | +3.3 VDC | |
| 2 | 3.3V | Orange | +3.3 VDC | |
| 3 | COMMON | Black | Ground | |
| 4 | 5V | Red | +5 VDC | |
| 5 | COMMON | Black | Ground | |
| 6 | 5V | Red | +5 VDC | |
| 7 | COMMON | Black | Ground | |
| 8 | Pwr_Ok | Grey | Power Ok (+5 VDC when power is Ok) | |
| 9 | +5VSB | Purple | +5 VDC Standby Voltage | |
| 10 | 12V | Yellow | +12 VDC | |
| 11 | 3.3V | Orange | +3.3 VDC | |
| 12 | -12V | Blue | -12 VDC | |
| 13 | COMMON | Black | Ground | |
| 14 | Pwr_ON | Green | Power Supply On (active low) | |
| 15 | COMMON | Black | Ground | |
| 16 | COMMON | Black | Ground | |
| 17 | COMMON | Black | Ground | |
| 18 | -5V | White | -5 VDC | |
| 19 | 5V | Red | +5 VDC | |
| 20 | 5V | Red | +5 VDC | |
Time: 5:08sEvery single wire that is Yellow delivers positive 12 volts (+12V). Every wire that is Black is ground or the reference (0V). Every Red wire is 5 volts (+5V). My suggestion is to follow what the tutorials says as the main connector also has some negative voltages so this is the one we will actually cut-off.
Here is the other silver box power supply after I cut-off the main connector and converted into this breakout pcb. You can see here that I have these spring loaded clamps that allow me to connect different wires.
ATX Bench Power Supply
I have soldered a lot of the wires parallel here to give me more power. This particular ATX power supply doesn’t have a switch on the back, so when I want to turn it on I am going to use one of my little independent switches here.
When I do, we don’t hear anything. The fan is not running and that’s because it actually has an on-off switch, that’s the Blue wire here. Switch it “on” and now it makes lots of noise. It’s definitely running and I have switched out the negative lead so that I can test the different voltages with the multimeter.
Negative 12 volts (-12V). Standby power, this is always on even if I flick off the switch. Minus 5 volts (-5V). The power good signal is a logic level signal that actually tells us whether on not the power supply is operating. Also notice the positive 5 volts (+5V), the positive 12 volts (+12V) and the positive 3.3 (+3.3V) don’t have a particularly great tolerance, and that’s because there’s not much load.
Meaning to say that when they are delivering much current and in this case they are delivering almost no current they are not particularly precise. That will improve once they start to deliver some power.
Time: 7:20sI used some so called perf-board (perforated board) to make the back side of my connector for the ATX power supply here. It lets me put lots of wires in parallel. In this case I used a type which has a 2.54mm or 100 mil pitch and the rows are all connected together in parallel. Here is another kind of perf-board which is good for other types of experiments, also with a pitch of 2.54mm or 100 mil but with each little square separated from its neighbours.
Stay Earthed and Safe
Time: 7:46sEarth in the context refer to the potential of so called Safety Earth, or Protective Earth. That’s the third connection in your wall power outlet. In the European Union (EU) there are little tabs in each electrical socket is the only connection that your finger can touch easily because it is perfectly safe to do so.
In fact if your work space is a plastic or wooden table like the one that I’ll be using, then you want to earth yourself by touching an earth connector regularly especially before handling any semiconductor microchip or anything else that is sensitive to ESD. That’s Electro Static Discharge.
Time: 8:18sSince I am using a plastic workbench here, it could build up electro static discharge or ESD. So, what I want to do is earth myself fairly regularly, especially before I touch any semiconductor chips. I am using the continuity tester, the beeping function of my multimeter here and I am connected to the earthing clip which I can touch with my finger.
The actual power supply is connected through the cable and in theory is a device which the case should be connected to earth. So I can take the other end of my multimeter and test. If I press hard enough to go through the coating I can see it does, but what I want to do is to touch the screws as they are connected to the frame.
So when I go ahead and do any actual testing, regularly I will reach a finger over here and actually touch to discharge any ESD built up on my body before I transfer it to anything sensitive like a semiconductor chip.
By Careful with Charged Capacitors
Time: 9:18sAs the next video segment shows, a charged capacitor with nothing to drain the voltage out of it can stay charged for a long time. A typical practical joke among Electronics and Electrical Engineers is to charge up a capacitor and then hand it to someone who is not expecting it.
I do not recommend you try that at home, and the charged capacitor phenomenon is why a lot of electronics still recommend that when you need to reset them, you turn them off, wait a while, and then turn them on a again. That’s to allow all the internal capacitors to discharge to zero to be sure that anything digital inside the device is actually turned-off.
Time: 9:53sTo demonstrate how a capacitor that is not loaded or not connected to anything can hold charge for a long time, I am going to use a laptop here and its charger.
This laptop battery is almost dead, so wants some power and when I turn-on the charger, they included a little white LED here that turns on to let us know that it is charging.
Power Supply Capacitor Discharging Time
Time: 10:13sThe laptop itself is drawing a lot of current, so when I turn it off, the LED begins to fade. When the LED has faded all the way we know that the output capacitor, and there is a lot of output capacitance in a power supply in this laptop adaptor is completely drained.
Now if I disconnect it, and perform the same test, the LED turns on and when I disconnect it, nothing seems to happen. That’s because the LED is barely drawing any current at all and there is a huge amount of capacitance. Milli-farads, (mF) that’s to say thousands of micro-farads, (uF) of capacitance here. It’s going to take possibly a minute or so for this output capacitance to completely discharge.
Note: 100 seconds of the LED’s intensity reducing intentionally removed from video to save time.
We can see the LED getting dimmer here, but the moral of the story is, whenever a capacitance is charged up and there is no load on it, it may still be charged minutes later so you need to be careful especially if its charged up to a voltage higher than say 30 to 40 volts. That’s enough to give you a nasty shock.
Time: 11:25sThat concludes part one of power supplies for non electrical engineers and hopefully you are now set up to begin testing an actual power supply. Stay tuned for part two were we’ll look at unregulated or semi-regulated power supplies, just to be clear, the ATX we have converted is a regulated power supply.
On behalf of myself and Electronics-Tutorials.ws, thanks and see you next time.
End of video tutorial transcription.
You can find more information and a great tutorial about converting an old computer ATX power supply into a bench power supply by following this link: ATX to Bench PSU.
In the next video tutorial about power supplies for beginners, we will look at using Unregulated Power Supplies and see how an unregulated power supply has difficulty controlling its output.
