Power Supplies for Beginners, Part 2
In part 1 of our series of video tutorials for beginners on power supplies, we explained how you can get set up to test, modify and use power supplies without spending a fortune on expensive equipment. Here in part 2 of our video tutorial series we look at testing and using Unregulated Power Supplies.
Watch our Part 2 video tutorial to learn more!
Following Transcript of the Video Tutorial
Time: 0:00sHello I’m Chris Richardson, and I’m an electronics engineer focused on power supplies. This is the second part in a series of web seminars for power supply enthusiasts who aren’t necessarily trained as Electronics Engineers.
In Part 1 of the series I talked about the basic supplies needed to start testing, now lets look at some power supplies that don’t actively control their outputs. This type of supply, an unregulated supply, is less and less common because its getting more and more affordable to make regulated power supplies, but we can still learn a lot from older unregulated supplies.
Unregulated Power Supply Agenda
Time: 0:31sIn this video we are going to see where the unregulated power supply can still be found, but its more and more rare to actually find them. Then we will look at the heart of most of these supplies which are based on transformers that operate at the AC (Alternating Current) line frequency, either 50Hz or 60Hz depending upon where you live.
Since transformers take an alternating current (AC), or voltage, and also up the alternating current or voltage, just about every supply needs to be rectified, meaning that the AC is turned into DC (Direct Current).
By their nature, unregulated power supplies allow their output voltage to change as their output current changes, so we will test this on some real supplies. Then we will explore the universal challenge of all power supplies, heat!
Time: 1:11sSo here are three power supplies. The first one was salvaged from a telephone, the next from another phone. The first is an unregulated line transformer based power supply and the second is a switching power supply. The first one is 6.5 volts at 500mA and the second one is 6.5 volts at 600mA. So notice the difference, but the big thing is the weight.
If you want to know the difference between a transformer or line transformer based power supply and a standard one, you just have to see how heavy it is. The first one weighs 220 grams, and a device that’s physically smaller and provides actually more power based on a switching regulator weighs only 55 grams. The last one here, is an unregulated power supply that I made myself and its even heavier at 338 grams.
Time: 2:15sI’m getting ready to test my discrete unregulated power supply here, but before I do I want to make a very important note about electrical safety. Here at the AC input we have “Earth”, those are the tabs there in brass, and whenever you test with an oscilloscope or most pieces of lab equipment, the negative connection, the silver there that we see on the oscilloscope is also earth, in fact here is a connection.
So I have my tip there connected and I am going to use the testing function here, so if I go over and touch the tabs, sure enough its electrical earth, and the reason I say this is that we can not connect the oscilloscope to either of the AC inputs, that would be basically shorting earth to line or to neutral. We would cause the differential circuit breaker to trip, or cause a lot of voltage or current across the probe which would go through the oscilloscope and likely damage something.
Time: 3:17sAnother important thing to note, if we wanted to test across both sides of the diode bridge it would not be possible with this oscilloscope and two standard non-isolated probes, because again if we were to connect one probe here and another probe on the other end of the diode bridge we would be short circuiting it.
Where to Find Unregulated Power Supplies
Time: 3:36sIf you are a conscientious environmentally inclined person then you would take your old unwanted electronics to a re-cycling centre. But you might still have a bag or a box somewhere with old electronics you have not gotten around to re-cycling yet. Look in there for the wall adaptors and find the heaviest one.
Unregulated Power Supply Voltages
Time: 3:52sHere I have my unregulated power supply setup, I have got my two multimeters. This blue one is going to measure the input voltage, the orange one is going to measure the output voltage at the output terminals of the transformer. An important note as far as safety goes, is that this multimeter will go up to 750 volts AC rms and the second one is safe up to 250 volts AC rms so we will not destroy anything.
So when I switch it “ON”, we have approximately 230 volts at the input, again AC rms, and the device says 24 volts AC, but we actually got 27 volts, but that’s normal, that’s typical. When we load it to its 12 volt-ampere rating it should be closer to 24 volts.
50/60 Hz Transformers – Volts, Amps and Volt-amps
Time: 4:49sIn general, the lower the frequency the bigger the transformer will be for a given power level. 50 or 60 Hertz transformers are therefore big and heavy because they need a lot more inductance to operate at such low frequencies.
In comparison, switching power supplies that we will talk about in parts 4 and 5 of this webinar series operate at one thousand to nearly one hundred thousand times higher so their transformers are much smaller and lighter, oh and cheaper.
Time: 5:16sThe same experiment again, but this time the oscilloscope probe is testing the actual DC output voltage. Here is the multimeter reading about 38 volts, and what I have done is to take the oscilloscope and put it into AC coupled mode at only two volts per division. So you can see here that it is very, very smooth and that’s because there is no load.
Now our unregulated power supply is loaded by this 330Ω power resistor. So from the positive output it goes to the resistor then it goes into the blue multimeter, and this one is actually measuring DC current, there we can see 93mA. The other multimeter is measuring the DC output voltage (31.6V) and from the blue multimeter the voltage returns back to the load.
Another important thing to note now that our circuit is under load, we can actually see some ripple. I did have to zoom in and now this is now 500mV per division, but we can definitely see a difference between the loaded and unloaded case.
Unregulated Power Supply Rectifiers
Time: 6:14sPretty much all modern electronics run with DC, Direct Current, so rectifiers are employed to convert the AC outputs of the line transformers into DC. In general there are three things that destroy microchips or electronics.
Time: 6:28sToo much voltage is the first and most common, a negative voltage connected where a positive voltage should be used is the second cause and that’s where the rectifier comes in. The third cause of death is heat, and we will discuss that towards the end of this webinar and at the end of all the remaining webinars as well.
Discrete diodes and diode bridges come with voltage ratings for the peak or DC voltage they can handle in reverse, and they come with current ratings for the DC or RMS current. In most cases exceeding the voltage by even a little bit destroys the diode almost immediately, whereas too much current causes too much heat. That can destroy the device, but it usually takes longer.
Here is the unregulated discrete component power supply, but now the diode bridge has been taken out of the circuit and replaced by this single rectifier diode here. At no-load we still have about 38 volts at the output and we can see with no-load the output voltage is very, very smooth.
Hold-up Capacitors – Smoothing Out the Ripples
Time: 7:25sOnce your AC is rectified into DC it still has a lot of ups and downs in terms of output voltage and it gets worse as you apply more and more load. Applying rectified AC directly to most electronics will work sporadically at best and will destroy your electronics at worst because the peaks of the output voltage can easily be too high and cause over voltage.
A large capacitor, usually hundreds of microfarads or millifards absorbs charge while the rectified AC is above the desired output voltage and then supplies that charge to the load when rectified AC is below the desired output voltage. With enough capacitance the output begins to look very smooth.
Time: 8:02sHere is the half-wave rectifier circuit again, but this time with the 330Ω power load connected in. Drawing just under 90mA, the voltage is about 30 volts, and most importantly we can see quite a big difference as there is a lot more ripple at the output voltage now.
Output Voltage Versus Output Current
Time: 8:20sAfter rectifying and smoothing there is still the drupe or loss of output voltage as more and more load current is drawn. This is why the outside of the wall adaptor says “6.5 volts at 500mA” because it has been tested to have that voltage at that current but the average output voltage will rise at lower loads and sink at higher loads. Whatever device is powered by this adaptor must either have a steady constant current draw or must be able to withstand the changes in that voltage.
Time: 8:48sHere in this experiment, what I am doing is testing the transformer at the maximum volt-amperes that it is capable of doing. So on the back of it that we can not see now it says a maximum of 12 volt-amperes (12VA). We know that when we connect it to the line we have 230 volts rms.
So 12VA divided by 230V is about 52mA (0.052A) and this is as close as I could get with the type of variable load which I have here called a constant linear current source and you can see this in more detail in part 5 of the series, so I have jumped ahead a little bit to make a point.
When loaded to its maximum we have about 25 and a half volts here, and also when load at the maximum we can see that there is more ripple at the output.
Unregulated Power Supply Ripple
This ripple can be reduced by having more output capacitance but we can see that the approximately 24 volts that was listed as the secondary voltage at maximum load is approximately correct.
Heat – How Much is Acceptable
Time: 9:43sOur basic unregulated power supply consisted of a transformer, some rectifier diodes and a whole load of capacitors and of these three components, the capacitors are the most sensitive to heat.
To get all the capacitance needed the type of capacitor used is almost always an aluminium electrolytic. These have a liquid or gel inside called the electrolyte and over time it evaporates. Once its gone the capacitor isn’t a capacitor anymore, its just a resistor. Now the hotter the air on the capacitor and the more the capacitor heats up having current pass through it, the shorter its usable lifetime.
A lot of electronics enthusiasts are familiar with the term “re-capping” and this refers to saving a piece of electronics and replacing all the dried out aluminium electrolytic capacitors.
Time: 10:25sOne of the last things I am going to do is to test how hot all of the different components in my discrete semi-regulated power supply get. So what I have done is connect it to the maximum load, in the previous test I checked to make sure we were drawing 12 volt-amperes by watching the current at the input.
Now I am watching the current at the output, about 320mA we can see that the voltage is following approximately the nominal voltage, 25.5 volts in this case, and if you are wondering what was the use of that ATX power supply I turned into a lab supply I am now using it to run this DC fan.
Time: 11:01sNow I am not blowing the air onto the actual power supply, I am blowing the air onto the load. The load again is basically a 10-turn precision potentiometer connected to a linear regulator makes me a current source but the watts that I am dissipating here are a lot higher here than what it can normally take, so the air is keeping this part cool.
Here is my thermocouple and the tip is resting in free air right now and you can see it is a pretty hot day here inside my house its almost 30 degrees Celsius. So what I am going to do is grab the tip here and I am going to touch it to the tops of different components. So for example, the actual transformer top is maybe 33.5 degrees or so, so at the maximum load it is not getting very hot.
Unregulated Power Supply Temperature
However what is even more interesting is to look at the aluminium electrolytic capacitor and that device is about 32oC. So overall this power supply is not heating up very much at all. The two critical components are only maybe 2 to 3 degrees, maybe 4 degrees higher than the ambient air temperature and they are also below the 40oC maximum listed on the transformer.
Time: 12:23sThere is one other component in this power supply whose temperature I want to measure and that’s the diode bridge I am looking at here. Again, its about 30oC ambient temperature.
When I do the temperature test here I need to be very careful as the blue and the black wires we see here are connected to the 230 volts AC. So I definitely do not want to short that with my hand. So very carefully put the tip on the component and we can see that we get to almost 39oC, almost 40oC, so this is the hottest component in the circuit.
Time: 13:01sThat concludes part two of power supplies for non-EE’s. Stay tuned for part three were we will look at regulated linear power supplies.
On behalf of myself and Electronics-Tutorials.ws thanks for watching and we hope to see you again for part 3.
End of video tutorial transcription.
You can find more information and a great tutorial about unregulated power supplies by following this link: Unregulated Power Supply.
In part 3 of our video tutorial about power supplies for beginners, we will look at using Linear Power Supplies and see how series and shunt linear regulators are better at controlling their output.