Power Supplies 101 - Part 2
In this set of articles, I'm going to be looking at mains powered linear power supplies (PSU’s) but before we start, a word of caution.
You should already be aware that the domestic mains voltage, which ranges anywhere from around 110v to 250v AC depending on your country, is lethal and will kill you if you don’t show it proper respect. Please, if you’re uncomfortable with the idea of working with mains voltage don’t take the chance.
Just remember, there’s no substitute for common sense and being careful.
For safety help and information, please read part 1.
You should already be aware that the domestic mains voltage, which ranges anywhere from around 110v to 250v AC depending on your country, is lethal and will kill you if you don’t show it proper respect. Please, if you’re uncomfortable with the idea of working with mains voltage don’t take the chance.
Just remember, there’s no substitute for common sense and being careful.
For safety help and information, please read part 1.
The oscilloscope display and interpreting the data.
Figure 1 - Pure 50 Hz sine wave
Figure 1 shows a 50 Hz sine wave. The image was produced by an oscilloscope and it will be useful to explain exactly what is being shown on the display as there are going to be a lot of these types of images to look at.
The bottom line of text on the display shows the major scope configuration settings, and you need this as a point of reference for interpreting the display. You can see from the image that there is a grid of dark red squares overlaid on the display and these are 1cm square, and using these squares and the information from the bottom status line makes it possible to interpret the waveform that’s displayed.
CH1 –This tells us the scope’s channel that is capturing the data.
6.00 – Vertical resolution. This tells us that the height of each square represents 6 volts.
V~ - This indicates that the channel is set to measure AC voltage. (V= would mean DC)
MTB 5ms – Indicates that the Master Time Base is set to 5ms. Each square is 5ms wide.
At the top of the display is a value; 20ms in this case. It is used to calculate the frequency between the two vertical lines on the display. The vertical lines in this image are four squares apart, and as the MTB is set to 5ms that equates to 20ms over four squares.
In the UK and many other countries, the mains voltage is around 230 volts, AC at a frequency 50Hz.
If you look at the left hand side of figure 1 and half way up, you will see a little mark, “1-“. This is the Zero point for channel 1 and is important to know when interpreting the data from any scope display and It's common for the Zero point to be vertically centred on the display.
Following the trace display starting at the far left, you will see that it curves down below the level of the Zero point, then swings back up, crosses the Zero point and rises to a peek and then curves back down to repeat the cycle.
This indicates that the voltage first goes negative; in reference to the zero point, and then positive in reference to the zero point; The signal is “alternating” between negative and positive hence it’s called AC; Alternating Current. You can see that from the zero point to the upper peek of the signal is about one and a half squares; this equates to around 9 volts.
It’s also possible to use the display to calculate the frequency of the waveform that’s being displayed.
The display tells us that the waveform “period”; a complete cycle of the waveform which is from any point along the trace to the next identical point on the trace, is 20ms
To convert the period to frequency is simple with the following formula:
1 / 20ms = 0.05 = 50 Hz
The bottom line of text on the display shows the major scope configuration settings, and you need this as a point of reference for interpreting the display. You can see from the image that there is a grid of dark red squares overlaid on the display and these are 1cm square, and using these squares and the information from the bottom status line makes it possible to interpret the waveform that’s displayed.
CH1 –This tells us the scope’s channel that is capturing the data.
6.00 – Vertical resolution. This tells us that the height of each square represents 6 volts.
V~ - This indicates that the channel is set to measure AC voltage. (V= would mean DC)
MTB 5ms – Indicates that the Master Time Base is set to 5ms. Each square is 5ms wide.
At the top of the display is a value; 20ms in this case. It is used to calculate the frequency between the two vertical lines on the display. The vertical lines in this image are four squares apart, and as the MTB is set to 5ms that equates to 20ms over four squares.
In the UK and many other countries, the mains voltage is around 230 volts, AC at a frequency 50Hz.
If you look at the left hand side of figure 1 and half way up, you will see a little mark, “1-“. This is the Zero point for channel 1 and is important to know when interpreting the data from any scope display and It's common for the Zero point to be vertically centred on the display.
Following the trace display starting at the far left, you will see that it curves down below the level of the Zero point, then swings back up, crosses the Zero point and rises to a peek and then curves back down to repeat the cycle.
This indicates that the voltage first goes negative; in reference to the zero point, and then positive in reference to the zero point; The signal is “alternating” between negative and positive hence it’s called AC; Alternating Current. You can see that from the zero point to the upper peek of the signal is about one and a half squares; this equates to around 9 volts.
It’s also possible to use the display to calculate the frequency of the waveform that’s being displayed.
The display tells us that the waveform “period”; a complete cycle of the waveform which is from any point along the trace to the next identical point on the trace, is 20ms
To convert the period to frequency is simple with the following formula:
1 / 20ms = 0.05 = 50 Hz
The secondary winding
Figure 2 - Display from the secondary winding of a 6 volt transformer
The secondary winding of a transformer is where we get our useable power from and transformers can have one or many secondary windings providing different voltages outputs and current ratings.
The trace in figure 2 was taken directly off the secondary winding of a 6v transformer; the transformer isn’t connected to anything other than the mains supply and the scope’s input.
Also, if the scope is set to display 6 volts per square, and it's a 6v transformer, why is the signal peeking at around one and a half squares above and below the centre line; that's around 9v? Come to think about it, isn't AC supposed to be a pure sine wave like the one being displayed in figure 1, as whilst the trace in figure 2 looks like a sine wave, there is a slight flattening at the top and bottom of the peeks; the sine wave is clipped.
The trace in figure 2 was taken directly off the secondary winding of a 6v transformer; the transformer isn’t connected to anything other than the mains supply and the scope’s input.
Also, if the scope is set to display 6 volts per square, and it's a 6v transformer, why is the signal peeking at around one and a half squares above and below the centre line; that's around 9v? Come to think about it, isn't AC supposed to be a pure sine wave like the one being displayed in figure 1, as whilst the trace in figure 2 looks like a sine wave, there is a slight flattening at the top and bottom of the peeks; the sine wave is clipped.
Clipped sine wave output
Firstly, the AC supply isn’t perfect; it has a Thevanin equivalent series resistance greater than zero ohms.
Then there are long power cable runs, distribution transformers and substations along the way, each introducing additional resistance and imperfections. Finally there is your transformer which isn’t perfect either and all these factors add to distort the sine wave.
Then there are long power cable runs, distribution transformers and substations along the way, each introducing additional resistance and imperfections. Finally there is your transformer which isn’t perfect either and all these factors add to distort the sine wave.
Transformer output voltage
The transformer in question does have a 6 volt secondary and its details are even printed on the side, but the scope display clearly shows that the output voltage is much higher than that specified.
Remember that the secondary winding (which is a huge long piece of wire) has resistance so that as the current draw increases, the voltage will start to drop and because of this, manufacturers design transformers to output a nominal voltage under load. Because the transformer being used to generate the trace for figure 2 is not being loaded, the secondary voltage is higher than expected and you need to sometimes be mindful of this.
You must also consider that the transformer output voltage is directly related to the input voltage on the transformers primary side, and any input fluctuations will have knock on affects on the output.
Lets assume that our mains supply voltage is 240v, and using the transformer above, the secondary output voltage is 6v.
240v / 6v = 40 so for every 40 volts on the primary winding, we get 1 volt on the secondary; the transformer is said to have a 40:1 ratio.
Now we know the ratio, we can calculate what happens if the mains voltage should drop to say 225 volts.
225v / 40 = 5.625 volts.
Or, what happens if the mains voltage should rise to say 250v
250v / 40 = 6.25 volts.
So a slight variation of mains voltage on the transformer primary affects the output voltage on the secondary.
Remember that the secondary winding (which is a huge long piece of wire) has resistance so that as the current draw increases, the voltage will start to drop and because of this, manufacturers design transformers to output a nominal voltage under load. Because the transformer being used to generate the trace for figure 2 is not being loaded, the secondary voltage is higher than expected and you need to sometimes be mindful of this.
You must also consider that the transformer output voltage is directly related to the input voltage on the transformers primary side, and any input fluctuations will have knock on affects on the output.
Lets assume that our mains supply voltage is 240v, and using the transformer above, the secondary output voltage is 6v.
240v / 6v = 40 so for every 40 volts on the primary winding, we get 1 volt on the secondary; the transformer is said to have a 40:1 ratio.
Now we know the ratio, we can calculate what happens if the mains voltage should drop to say 225 volts.
225v / 40 = 5.625 volts.
Or, what happens if the mains voltage should rise to say 250v
250v / 40 = 6.25 volts.
So a slight variation of mains voltage on the transformer primary affects the output voltage on the secondary.
Transformer VA ratings... again
In part 1 I briefly mentioned how to use the VA rating of the transformer to calculate the maximum current that it can supply from its secondary.
To recap:
Transformer VA rating / secondary voltage = available current
I also stated that transformers can have multiple secondary windings and that these windings don’t have to be of the same voltage; and they don’t.
Back in the days when TV’s had CRT’s and were full of valves (tubes), many different voltages were needed to run everything. Typically a low voltage; perhaps 6v was required for the value heaters, then a higher voltage for the electronics and then a really high voltage to generate the HT for the CRT.
But a VA rating won’t really help you much if you’ve got multiple secondary’s all of different voltages.
I’ve tried to think back to the last time I remember using a transformer with multiple secondary voltages and the last one I can remember was in an oscilloscope that I built when I was a lad, and a quick look through my favourite electronics suppliers catalogue confirmed what I’d started to suspect; nobody seems to be generally supplying transformers with multiple secondary windings of different voltages. Multiple secondary windings are very common indeed but they are usually for the same voltage and current rating.
What you need to remember is that the available current result from the VA calculation, is split across all secondary windings. A 50 VA transformer has a total of 50 VA, it’s not 50 VA per secondary winding.
To recap:
Transformer VA rating / secondary voltage = available current
I also stated that transformers can have multiple secondary windings and that these windings don’t have to be of the same voltage; and they don’t.
Back in the days when TV’s had CRT’s and were full of valves (tubes), many different voltages were needed to run everything. Typically a low voltage; perhaps 6v was required for the value heaters, then a higher voltage for the electronics and then a really high voltage to generate the HT for the CRT.
But a VA rating won’t really help you much if you’ve got multiple secondary’s all of different voltages.
I’ve tried to think back to the last time I remember using a transformer with multiple secondary voltages and the last one I can remember was in an oscilloscope that I built when I was a lad, and a quick look through my favourite electronics suppliers catalogue confirmed what I’d started to suspect; nobody seems to be generally supplying transformers with multiple secondary windings of different voltages. Multiple secondary windings are very common indeed but they are usually for the same voltage and current rating.
What you need to remember is that the available current result from the VA calculation, is split across all secondary windings. A 50 VA transformer has a total of 50 VA, it’s not 50 VA per secondary winding.
Basic transformer configurations
Next, we will look at the three basic options of secondary windings that are available. This doesn’t cater for any specialist transformers; just the more common ones.
Single Secondary winding
Figure 3 - Transformer has a single secondary winding
This is the simplest transformer you will find.
It has a single secondary winding and so all available voltage and current is available from the one secondary winding via connections S1 & S2.
It has a single secondary winding and so all available voltage and current is available from the one secondary winding via connections S1 & S2.
Single Secondary winding with centre-tap
 Figure 4 - Single secondary winding that is centre tapped |
Connection options Figure 5 - centre-tap connection options
You have several options when using a centre-tap transformer in a design. The transformer shown has a single winding but has a tap from the centre of the winding and this makes the transformer very useful in some applications.
Figure 5 shows possible connection options. |
Whilst the term "centre-tap" is common enough, the "tap" dosn't actually have to be in the centre and this was common in older transformers. For example, in the above transformer the centre tap could actually be moved 25% of the way down the winding instead of 50%, this would allow for different output voltage combinations.
These days, it's much more common for the tap to be in the centre of the winding as this gives symmetrical voltage outputs (+6 and -6 in out example) and these are very usefull when creating power supply units for driving operational or audio power amplifiers.
These days, it's much more common for the tap to be in the centre of the winding as this gives symmetrical voltage outputs (+6 and -6 in out example) and these are very usefull when creating power supply units for driving operational or audio power amplifiers.
Dual Secondary windings
Figure 6 - Transformer with dual secondary windings
As common as the centre tapped transformer configuration above (and probably much more flexible) is the transformer with dual secondary windings. These transformers seem to becoming the norm these days and probably one of the reasons is that for no real additional labour or material costs, you get some additional benefits over other designs.
Figure 6 configurations for T1, T2 & T3 are identical to the T1, T2 & T3 configurations for a transformer with a centre-tapped secondary from figure 5.
The big difference is the configuration used for T4.
Here the two windings are connected in parallel. Whilst this keeps the voltage output the same, it doubles the available current; this is something that’s not really possible with a centre-tapped secondary.
A word of caution. You MUST make sure that when connecting secondary windings together to combine their current capabilities, they are of the same voltage, and connected in phase (see how the 0 pins are connected together and then the 6v pins are connected together) for T4 in figure 6. If you get the phase wrong the transformer will burn itself out, quite spectacularly sometimes and remarkably quickly.
Figure 6 configurations for T1, T2 & T3 are identical to the T1, T2 & T3 configurations for a transformer with a centre-tapped secondary from figure 5.
The big difference is the configuration used for T4.
Here the two windings are connected in parallel. Whilst this keeps the voltage output the same, it doubles the available current; this is something that’s not really possible with a centre-tapped secondary.
A word of caution. You MUST make sure that when connecting secondary windings together to combine their current capabilities, they are of the same voltage, and connected in phase (see how the 0 pins are connected together and then the 6v pins are connected together) for T4 in figure 6. If you get the phase wrong the transformer will burn itself out, quite spectacularly sometimes and remarkably quickly.
Next time...
AC is great, but for most electronics applications what we really need is a stable DC voltage and we will look at this in the next part.