Electronics lessons: Mains Power -single phase and three phase

Whilst writing this blog I've stayed away from things that I would consider dangerous.

That is to say I've completed some projects that I've felt were either too big to document, (like when I put up my new storage shed in the garden and put electrics and lights etc in.) -at the time my thoughts were like this. it's OK to tell someone to stick bits of wood together to make a speaker, it's OK to say to people use this step down transformer and deal with the safe voltage that comes out the other side. I even thought it fine to tell people to stick electrodes into water to generate hydrogen/oxygen gases!

But big scary voltages have been something that I've stayed away from, the thing about mains utilities is that, you can smell gas, and know to shut off the supply and get out. you can see water and hopefully have the intelligence to get out before drowning. Electricity is completely invisible, it's not a substance, it's a force.

Anyway, that might give you an insight as to how I feel about mains electricity. it's dangerous to the point of deadly, and too much confidence can lead to mistakes that ultimately can kill you.

As a point of reference I live in the UK, this means that for me, voltages come in 120v for site tools, (where the 120v is considered safer due to lower voltages) this voltage is obtained by using those little yellow transformer boxes.

240v comes into every homes, (at least every connected home, I suppose off grid homes might make their own standards).

440V is considered industrial and is what most three phase power outlets are going to be using.

Voltages are measures with respect to neutral, live - neutral.

Single phase

Single phase power is an alternating waveform going from the positive extreme of the voltage swing to the negative extreme of the voltage swing in one wave form.

Single phase is what I've mostly dealt with, it's easy to visualise, easy to look at and easy to understand. remember that the positive is measured with respect to neutral, not with respect to ground.

Sometimes a single phase supply is actually a 2 phase supply, where instead of a virtual earth, or earthed neutral, there are two hot phases where the phases are 180 degrees apart.
the sum of the phases is zero, (e.g at 0 degrees phase 1 is 0v, phase two is at 180 degrees and is also 0v, at 90degrees, phase 1 is +120v and phase two is 270 degrees and -120, (+120 + -120 = 0v), the total potential difference between the two phases is 240 volts, (120 - -120 = 240)

Three phase

Here's where it starts to get a little more complicated.
Now there are three phases, each phase is 120 degrees behind the next, they are still sine waves, and they are still measured with respect to neutral, but there is no neutral in the transmission line, -when power enters your premises in a single phase set-up there is a live and neutral line. when you get three phase there is only 3 live wires, no neutral. (you can make a neutral by connecting all the phases together, because the sum of the phases is zero) -but I'll cover that in a later lesson.

You can see at 90 degrees now phase 1 (blue) is at at the top of it's rising cycle 240v, and phase 2 (green) is just coming up from the bottom (so during it's rising cycle) and is -120v, whilst phase 3 (red) is getting towards the bottom of it's falling part of the cycle, and is -120V
240 + -120 + -120 = 0

Colour Codes

Colour codes for wires are important, in that they will let you know what wire does what. everyone knows that green/yellow stripe is earth, and it's safe to touch. but what if I wired a plug so that the earth wire was used as a live conductor.
it's just a bit of copper, fundamentally there is nothing to stop me from doing that, provided I wire the other end the same way, where's the problem?

The problem is what happens when I'm out, and whatever appliance I've wired badly breaks, what if my wife/girlfriend/boyfriend/husband/friend/son/daughter/mother/father/grandparent etc decides that they just want to check the fuse on the plug, then they see the wires connected wrongly, so think that might be the problem. or something see's an earth wire, and decides that they can splice that wire to earth something else, -except it's not earth, it's live, and they are probably now dead.

Colour codes are set by the IEE.

for single phase systems.
Brown = Live, (the old colour was red)
Blue = Neutral, (the old colour was black)
Green and yellow stripe = Earth, (the old colour might be plain green)

for three phase systems,
Brown = Phase 1 (old colour was red)
Black = Phase 2 (old colour was yellow)
Grey = Phase 3 (old colour was Blue)
Blue = Neutral (old colour was black)

This means that if you are adding new wiring to an old system, don't just connect the new blue wire (N) to the old blue wire (P3), or the new black wire, (P2) to the old black wire (N)

you may also find three phase systems where the conductor colours are Brown, brown brown for P1, P2 and P3, with a blue for earth.

Earth (as always) is green yellow stripe.

earth may also be the bare wire in fixed installations, (where solid core wire is used rather than stranded wire).

Electronics lessons: Improving the power supply.

So a few lessons ago we took a simple circuit for a power supply and mocked up a rough circuit for a power supply.

To make the lesson a little easier we made a few assumptions, one of those were that diodes were perfect conductors:
So lets re-visit with realism, and see what the circuit is really doing.

We had a step down transformer at 20:1 with a supply voltage of 240 volts.


therefore a secondary coil output of 12 volts.

then there was the diode bridge, the forward conduction voltage for a diode is 0.7v this leaves up 11.3volts.
the load of the circuit was 1200Ohms.
and the smoothing capacitor in the circuit was 1 Milifarad

we ignored the diode voltage drop and said that the ripple current was 0.2volts saying that this means that the voltage would ripple between 11.8 and 12 volts.

well, actually it ripples between 11.1 and 11.3 volts because there is a voltage drop in the diodes.

Lets fact it, what good is 11.3 volts. have you ever seen this as the supply needed in a circuit?
What is we really wanted 9 volts.

Well, where do we want the ripple to be, between 8.8 and 9, or 8.9 and 9.1, where am I going to find a transformer that outputs 9.7 or 9.8 volts, so that I can satisfy the voltage drop across the diode and the ripple current and keep enough energy for my circuit? to put it simply, nowhere.

What if I were using germanium diodes, now I need a 9.3v output.

What we use in this situation where we want the power supply to be at a specific set voltage is a zener diode.

The zener diode sits across the positive and negative power supply rails.
in this case we'll stick with out 12v transformer, and use a 9v zener diode.

 What this means now is that whenever the voltage is above 9v the zener diode will conduct, meaning that the supply voltage will fall, but if the voltage were below nine volts then the zener diode would not conduct and the supply voltage can rise.

An even better alternative.
An even better alternative to using zener diodes is to use a voltage regulator.

These components have three legs.
the first leg is the input, this connects to the positive rail of the power supply circuit.

the middle leg is the ground leg, this connects to the ground of both the power supply circuit and the zero volts rail of the circuit that you are powering.

The final leg 3 is the Vout leg, this connects to the positive power rail of the circuit that you are powering.

Common voltage regulators are the 78 and 79 series.

the 78 series are positive, you apply positive voltage to the input leg and a positive voltage is returned at the output leg.

The part numbers relate to the output voltages:
7805 = 5v
7809 = 9v
7815 = 15v

the 79 series are negative voltage regulators, you apply a negative voltage to these and you get a regulated negative voltage out.

7905 = -5v
7909 = -9c
7915 = -15v

Converted ATX Power supply

Recently you could be forgiven for thinking that this had changed from the idiots guide to puttin' shit together, to the opinionated guide to how you should have done your projects, or the idiots musings and bad teachings.

I'm definitely going to continue the tutorials because it's actually quite fun to write them, and I'm finding that as I go over the ground again, I'm re-remembering some stuff that I'd completely forgotten about. But this post is right back to the core of why I even started this blog, and that's to share something that I've made.

Firstly,

There's your warning, but if you can't figure out for yourself that there is an element in danger in modifying anything that plugs directly into the wall then you should probably stop reading now and go elsewhere.

Theory
The theory for this is so simple, I have a power supply (a computer power supply), and I want to turn it into a bench power supply, so I pretty much just need to cut off the wires that aren't needed, and attach the appropriate plugs to the wires that are need!

How it works
The computer power supply maker has done all the hard design parts for me it's a switch mode power supply, mains supply and as it's switching faster, the transmission of the energy inside the ferrite core, basically that means that inside the power supply is switching on and off at a very high frequency, this is much faster than your 50Hz s of the transformers inside is more efficient, more efficient means that you're not generating as much heat, and not as much energy is wasted as heat.
A traditional power supply, would require an absolutely huge iron core surrounded by pounds of wire to be able to get the same kind of output power that a switch mode power supply (SMPS) gets.

Why it's important
The high frequency is important as that's what allows the supply to be so efficient. The fact that someone else made it is important because it means that you can make this supply inside a single afternoon.

If I was designing my own SMPS, I'd probably need to research for at least days, if not weeks, I'd need to source components. Create circuits to sense the output voltage and adjust the input if necessary, it'd be a lot of work, yet I can turn an old supply from a broken PC into a perfectly reasonable bench supply in a matter of hours.

Simple experiment
Before even breaking out the screw drivers, or plug in the soldering iron. I took a look at the computer power supply unit.

There is a big plug with either 20, or 24 pins on it. inside all these pins there will be a single green wire, with a black wire either side of it.

Using a small bit of wire I pushed it into the pin connected to the green pin, and one of the black pins from either side. The fans spun into life, and voltage could be measured at other pins.

You can just use the supply like this with no further modification, inserting your bit of wire to turn it on, removing your bit of wire to turn it off.

On the side of the case there was a table that telling me what colour wire carries what voltage, and what the limits of the current capabilities are for each voltage. this made it a whole load easier that searching for a pin out on-line for the ATX plug to know what pin was what.

Converting a power supply

Parts and tools required
>an old computer power supply
>a screw driver
>some terminal posts
>Wire cutters
>A switch (push to make locking switch)
>A LED
>A 10mm drill bit
>A drill
>A 6mmDrill bit
>Insulated screw terminals -these are not important, you may chose to leave these out
>A 470Ohm resistor
>A soldering Iron
>Solder (flux core)

Taking it all apart
The first thing that I did was take off the lid of the box, The fan is connected directly to the board with no plug to remove it, so I removed the fan from the case lid.
Modifying the part
Lets deal with the lid first,
I knew that in the lid i'd need to put a switch (basically in place of the wire that I had before),
You might want (I did) a nice LED to tell you that the supply is on.
I also wanted to put plugs that to connect the wires to in the lid.

I chose to mount my plugs in two columns, (2 x 12v, 2x 5v, 2x3.3v and 2x0v) as these are the voltages that I'll use the most often, then I have one plug for -5 and -12, as I'll use these less often.
As it happens, banana plugs are usually stackable, so you won't really loose an functionality just using a single row of connectors.

When I looked on the inside, of the power supply, I saw that there were basically two sides, one side has big heat sinks in it, and the mains plugs next to it, the other side is fairly low profile leaving the top of the case empty. I decided that I'd like to keep my plugs away from the side with exposed mains connectors that the bottom of the terminals might hit, and away from the heat sinks for the same reason

I marked out the lid and drill the holes for all the part to go into.
Then installed the plugs, LED and switch (I used a simple black collar for the LED it's called a panel mount and costs about a penny. -you could leave the LED out completely).

I just used a marker pen to write the voltages on the top of my supply, you might use nothing relying on the colour of the plugs to tell you the supply voltage, perhaps you might like to try etching or engraving, or using a label printer, I don't really care, pen works for me, and that's good enough.

Next as I have two plugs for one voltage, I connected these plugs, using a piece of wire running between them.

I also want to add a 470Ohm resistor to one leg of the LED (this is to limit the current that goes into the resistor).

(That'll connect to the 5v rail 5/470 = 0.01A or 10mA, enough to make the LED glow, you might like a bigger or smaller resistor depending on the needs of your LED, it's also 1/20th of a watt, so a simple small 1/8th watt resistor will be fine.)

I soldered leads to everything ready to connect to the terminal block connector.

Finally I was able to stop working on the lid and move onto modifying the actual PSU.

One thing that you'll notice is that the wires are colour coded depending on their job. And there are a LOT of wires that are the same colour, in the supply I used there were about 10 black wires, all 0v, all from the same pad on the PCB!
I needed 3 black wires, could have got away with 1, but three was good (one for the power button, 1 for the LED and 1 for the output plug).
I really only needed 1 -12 wire, only 1 -5 wire, only 1 +12 wire. and two +5v wires (one for the plug, and one for the LED.

You do only need 1 3.3v wires, but in addition to the thicker wires you must keep the very thin orange wire, this is the sense wire that determines what voltage is being generated, and regulates the supply, without this wire your supply can float.

Anyway, I cut away the wires that I didn't need (that I could get to)

I couldn't get to cut the red or yellow wires, so I'm going to tie these off later in some spare terminals on the connector.

Putting is all together
So I'm now in a position where I've thinned out all the wires that I don't need, and I've trimmed all the wires that I do need to about four or five inches long so that I have plenty to work with.

Now I striped back the wires, and attached them to the terminal block.

It doesn't matter what order you make the connections in, just that black wires connect to ground, terminals, and the negative side of your LED, also that there is a black wire attached to one terminal of your switch.

You want the green wire to attach to the other leg of your switch, and a red wire to attach to the LED for your "on indicator".
Yellow wires are alwyas 12v, Orange is always 3.3v, Red is always 5v, minus five and minus 12 however do not appear to have a set colour scheme, and as such may vary.

The colours were written on the outside of the supply I used, you may have to poke around with a multimeter. or look for an ATX pinout and just select the correct wire from the plug.

Once all the wires were connected I attached the fan to the top of the case again, and put the top of the case onto the bottom. as I said earlier, make sure that the bottom of your sockets do not touch the metal bits inside. Also make sure that no wires are touching the heat sink where the insulation could melt, and that no wires or the large terminal block is getting in the way of the fan.

Now I measured The output voltages using a multimeter.

Since I'm always seeming to run out of power outlets, I chose a power supply that lets me get mains power out as well so I can daisy chain equipment together.

Costs
This is not the cheapest project in the world, but it is cheaper than buying a power supply with the same amount of usable outputs, regardless of whether those outputs are of a fixed voltage, or five variable voltages. I had everything either in my parts box/junk bin so this really cost me nothing.

There are 10 4mm sockets, these are 20 pence each from Rapid Electronics
http://www.rapidonline.com/Cables-Connectors/Connectors-Single-Pole/4mm-Connectors/4mm-Sockets/63893
(total cost £2)
The Square Single pole single throw switch cost 36 pence.
http://www.rapidonline.com/Electronic-Components/Switches/Push-Button-Switches/Square-push-switches-1A/30275
(toal cost £2.36)
LEDs cost around 9 pence (or less) (when you buy them in bulk).
here's one for 6 pence http://www.rapidonline.com/Electronic-Components/Optoelectronics/5mm-LEDs/Low-current-5mm-LEDs/29336
(total cost £2.45)
Resistors cost even less than this when bought in quantity.
http://www.rapidonline.com/Electronic-Components/Resistors-Potentiometer/Carbon-Film-Resistors/CR12-0.125W-Carbon-film-resistors/65192

65 pence for 100, seriesly people stop shopping at Maplin/Radioshack

Rapid cost 65 pence for 100, (or ~ half a penny per resistor).
Maplin cost 25pence per single resistor. (50 times more expensive).
http://www.maplin.co.uk/components/resistors/metal-film
If your project requires 3 resistors, it's cheaper to order on-line and have 97 spare/left over!

(total cost £3.10)

Now comes the expensive part, if you don't have a spare power supply laying about, (and don't have the resourcefulness to find one for free), then you'll have to buy one. for the princely sum of £7.86!
http://www.ebuyer.com/product/20083

Total cost £10.96! Much cheaper than any other power supply that I know of. in fact we're really knocking on the cost of a single voltage wallwart cost here, which has a lower power capacity and only 1 voltage output (which may or may not be adjustable!)

Alternatives, -and Why I didn't use them
This is something I started when I wrote up the red light torch, there are so many ways to crack an egg. So my way is not the only way, just the way that I chose on that day.

Spring terminals:
Spring terminals are useful if you're doing a lot of breadboard work, without a breadboard that has power terminals, but practically everything I've got is set-up for 4mm banana plugs, my multimeter, the power terminals on my breadboards, crocodile clips connect to 4mm plugs. Spring terminals may well have been cheaper, and indeed wouldn't require me to buy 4mm plugs to go with them, but since I'm set-up for using 4mm plugs it makes sense to use them

Binding posts:
Yes, these are just like 4mm sockets, except that you can also attach bare wires and screw them down, making them a whole heap more versatile. The only reason that I didn't use these is that I didn't have any in my parts box, and I did this project one evening after the shops were closed.

Directly soldering the leads:
I could have missed out the terminal block connector, and indeed the first time I made this supply I did. without that chunky connector however there are two things.

The wires that you can't cut off at the board end up loose in the case, you can wrap them in tape, but it's a bit messier than being able to put them in a spare space on the terminal block.

The first time I built this supply I soldered straight to the plugs, I broke that supply, and it was quite a PITA to remove all the old connections to put new connections on it. if I ever need to change the actual board again I can just take the case lid as I did this time and bolt it to a new supply just cutting and stripping the few wires that I need to use.

Sandbar resistor
Instead of just a locking switch on the green "power on" line, you should use a resistor on the 5v line to apply a load to the circuit that'd sense that it's on. That's the "proper" way of doing it.
I didn't do it like this because...
1> The sandbar resistor has to be a high power resistor, tied to the case for cooling, this seemed like it's take a lot of space inside the already cramped case, and is a part that's going to increase the idle power consumption of the power supply.
2> My supply works without it, so I'm not too worried really.

External plug-set with ATX/Molex connections
In the theory section you saw that it's possible to turn the supply on with nothing more than a piece of wire, I could have kept the supply as is, put molex connections on my projects, of I could have just made a board with my 4mm sockets arranged just as they are where the power supply just plugged straight into it.

The good side of this is that I wouldn't have had to open the case at all. So if you have a skittish partner or parents then you may wish to go this route. (If they aren't happy for you to plug things in after you're taken them apart and soldered bits to them!)

On the other hand, I think that you'll end up with something a lot messier.