Oakley Sound PSU2 User manual

Oakley Sound Systems
PSU2
PCB Issue 1
Project Builder's Guide
V1.3
Tony Allgood
Oakley Sound Systems
CARLISLE
United Kingdom
1

Introduction
This is the Pro ect Builder's Guide for issue 1 of the power supply unit, PSU2, circuit board
from Oakley Sound. This document hopefully contains everything you need to know to build
and install the Oakley power supply unit.
The PSU2 allows for various options in the installation. You can use the unit either in full
wave rectification mode for connection to tapped linelumps or twin transformer secondaries,
or in half wave rectification for single phase AC output wallwarts and linelumps. If all this
sounds very confusing at the moment, do not worry, in this manual I will try to make it clearer
so that you make the right decision about what power source you will need.
It is designed to be mounted onto a metal panel which is used as a heatsink for the two power
devices used on the board. Mounting your power supply to a metal panel on the outside of
your case helps keep your modular cool.
For general information regarding where to get parts and suggested part numbers please see
our useful Parts Guide at http://www.oakleysound.com/parts.pdf.
For general information on how to build our modules, including circuit board population,
mounting front panel components and making up board interconnects please see our
Construction Guide at http://www.oakleysound.com/construct.pdf.
2

Safety Warning
The PSU2 has been designed to work with isolated low voltage AC inputs. Connection to any
other supply, such as an internally mounted mains transformer, is done at your own risk. Low
voltage is classified as being less than 25V with respect to the ground potential. Voltages
above this level can, and often are, lethal to living creatures.
Oakley Sound Systems will not advise on building or modifying this board to allow for direct
connection to the mains, or other high voltage sources, further to what is provided in this
document. Please do not ask me for any additional information pertaining to direct mains
connections or using internally mounted transformers as I will not give it.
or safety and legal reasons I cannot recommend powering this board from any other
supply than low voltage AC output mains adapters.
Oakley Sound Systems are not liable for any damages caused by the misuse of this product. It
is your responsibility to use this product safely. If you have any doubt about installing a safe
power supply, then please do not attempt to do so.
3

The Oakley Power Supply Board
The issue 1 PSU2 board from Oakley Sound Systems.
The power supply board will allow the conversion of a suitable low voltage alternating current
(AC) to be rectified, smoothed and regulated for operation with the Oakley Modular. The
module is designed to be fitted to a suitably large metal panel which functions as a heatsink for
the regulator ICs. This metal panel should have adequate airflow around it. The recommended
method is to use a 4U or 5U high 19” rack blank panel. These are particularly suitable if you
are mounting your modular synth in a 19” rack. The large surface area on a 19” blank panel
allows you to mount not only the power supply module but also any Dizzy or MU-Dizzy
PCBs used for power distribution. And the large surface area also allows the power supply's
regulators to dissipate heat safely.
The voltage output of the power supply module is a split rail or bipolar 15V supply. This
means it generates both +15V and -15V. That is, two power rails, one of a positive voltage,
the other a negative one. It's sometimes written as +/-15V. These voltages are measured with
respect to a common 0V (typically called 'ground') which is normally connected, via your
house's wiring, to the earth that you stand on. The voltage across both rails is 30V, with the
common ground sitting exactly in the middle of this at 0V. Both output voltages can be finely
ad usted with their own trimmers.
The output current capability is the maximum current you can draw out of the power supply.
The current taken from the supply is, for the most part, determined by the amount and type of
modules you are connecting to the power supply. However, the actual patch also has an effect
4

on the current draw – the more outputs that are connected to inputs increases the current
draw slightly. It may also increase when any LEDs light up.
An M4 ring terminal crim ed to 24/0.2 insulated wire. This sort of connector is suitable for connecting to
the out ut terminals of the PSU2.
I recommend that you use a Yamaha PA-20 or PA-30 power supply. These are neat tidy
external power supplies that keep the dangerous mains voltage away from your modular. The
PA-20 will allow a maximum current of 520mA (0.52A) to be drawn from each rail. While the
PA-30 will allow up to 780mA (0.78A). Both the PA-20 and PA-30 are centre tapped AC
supplies with three wires within their output cable and use a three pole connector.
Other power supplies are available and they come in lots of different variations. Other than AC
output voltage the two key specifications are output current (please don't call it 'ampage'), and
whether the output is centre tapped or single phase. In almost all cases the outputs of standard
AC output power supplies are single phase. You can tell because they only have a cable with
two wires inside which terminates in a two pole connector.
A single phase AC output supply will allow only slightly more than a quarter of its rated
current output. For example if you are using a 500mA (or 0.5A) AC wallwart* then the most
current you can take from this power supply module is around 125mA from each rail. That is,
take no more than 125mA from either the +15V supply and 125mA from the -15V supply.
Various companies make linelumps** with a greater capacity than 500mA. If you can get a
single phase 1A output one than this will be able to drive up to 250mA per rail.
The Oakley PSU2 can be used with full wave or half wave rectification. The former allows it
to utilise split AC outputs. With full rectification and using a centre tapped power supply the
amount of current taken from each 15V rail can be up to ust over half the rated current
output of the power supply. The Yamaha PA-20 supply is rated to give an output voltage of
35Vac (with a centre tap) at a load of 0.94A. Once rectified and smoothed this means that a
maximum current of 0.52A can be drawn from each rail.
5

Two sets of screw terminal blocks are provided for connecting the low voltage AC power
source to the board and the optional power switch. If you are using a single phase wallwart to
power the PSU module than you need only to use two terminals per terminal block.
The PSU2 board uses chunky individual screw terminals for the outputs. These allow good
solid connections to suitable ring terminals which are crimped onto your connecting wires.
Four output terminals are provided, +15V, 0VA, 0VB, and -15V. 0VA and 0VB are
connected together on the PSU2 board. I recommend that the metal panel the PSU module is
mounted onto be securely connected to 0V. This can be done either via the PSU2's 0VA or
0VB terminal, or using a suitable point on your distribution system.
The power supply has two integral fuse holders in case of a problem with the power supply
circuitry itself. Two fuses are needed if you are using full wave rectification, but only one, F1,
is required for ordinary half wave rectification. The fuse type should be a slow blow or anti-
surge type. The size is 20mm. It should be rated at between one and two times the maximum
current of your wallwart. Thus a 500mA AC output mains adapter should have a fuse that is
rated between 500mA and 1A, ideally 750mA. A 1A linelump should have a fuse that is
between 1A and 2A, ideally 1.25A.
Two on-board LEDs, a red or orange one for +15V, and a green one for -15V, provide a
quick visual reference that all is well. These two LEDs could be fitted externally to the board
and be mounted on a front panel. A third LED is supported, normally a red one, which lights
when any AC power is applied to the unit. This would normally be mounted on the outside of
the unit alongside the AC power switch and as such the PSU2 makes space for a 0.1”
(2.54mm) two way header to connect to this LED.
The board has four mounting holes for stable placement onto your modular case. Care should
be taken so that the board's various board mounted components do not come into contact with
any part of your modular's enclosure. Use of 8mm hex spacers between the board's bottom
surface and the panel is the preferred option.
The PSU2's output terminals must be connected to the modular's power distribution system
with the shortest and thickest wires possible. I normally use 24/0.2 wire but the both the 1/4”
Faston blades and the screw terminals will support larger wires with the correct crimps.
Multiple Dizzy and MU-Dizzy boards should never be connected in daisy chain fashion. That
is, each Dizzy board should always go back to the PSU2 separately and with the shortest and
thickest wire you can use.
* A wallwart is the vernacular term for a low voltage mains adapter that plugs directly into the
wall. These take the form of a black plastic block that is shaped like an oversized mains plug.
It is called a wart simply because its appearance is somewhat uglier than a normal slimline
plug.
** A linelump does the same ob as a wallwart but it generally can handle greater currents.
Because of its increased size it cannot be made so that it will safely fit into a plug socket
directly. Thus the adapter sits in a black plastic box and connects to the wall via a cable and
traditional mains plug. It is therefore a black plastic lump connected to a line. The Yamaha
PA-20 and PA-30 are such linelumps.
6

Our Recommended Power Supply
The safest available option is to use a ready made 'wallwart' or ‘line lump’ supply. As already
mentioned one can use any 15V or 18V AC output wallwart of linelump you can source. The
current capability of the mains adapter will be the chief limiting factor in determining the
maximum current draw of your PSU. For a variety of reasons I recommend the Yamaha PA-
20 and PA-30 supplies.
Yamaha PA-20
The Euro ean version of the PA-20. Other country's units are similar but will have the local mains
connector fitted.
This is a linelump supply and features a fixed 17.5-0-17.5 volt AC output at 0.94A maximum.
This means it gives us two AC outputs with a centre tap or mid point reference voltage. So
unlike the single phase AC adapter output with two leads, this one has three. This means you
need to use the Oakley PSU in full wave rectification mode.
The PA-20 is made for Yamaha products and they are available from Yamaha spares
departments as well as many music shops, eg. Thomann. These are CE approved and connect
to the mains via your local mains connector. They will be different types depending on the
country you need them for. It comes with a handy three way plug at the low voltage end that
you can use with an appropriate socket. If you wish you can ditch their connector and use
your own. Oakley Sound sell a suitable three way connector to fit the Yamaha one perfectly.
In the UK the line lump’s part number is V9812300 and the total cost is around £30 including
VAT and postage.
7

Once rectified, smoothed and regulated the Yamaha PA-20 can deliver up to 0.52A
continuously into both 15V rails.
You should fit both fuses and both should be anti-surge types and rated at either 1A or 1.25A.
Yamaha PA-30
This is essentially a bigger version of the PA-20 as detailed above which supplies 18V-0-18V
at 1.4A maximum. Once rectified, smoothed and regulated it can supply up to 0.78A
continuously. You should again fit both fuses and both should be 2A anti-surge types.
If you have successfully used the Oakley PSU with any other types of power pack please do
let people know via the Oakley Sound forum at www.muffwiggler.com
8

Parts List
For general information regarding where to get parts and suggested part numbers please see
our useful Parts Guide at the pro ect webpage or http://www.oakleysound.com/parts.pdf.
The components are grouped into values, the order of the component names is of no particular
consequence.
A quick note on European part descriptions:
For resistors: R is shorthand for ohm. K is shorthand for kilo-ohm. M is shorthand for mega-
ohm
For capacitors: 1uF = 1,000nF = 1,000,000pF. Sometimes the F is not included on the circuit
diagram to indicate a capacitor's value, ie. 100n = 100nF.
To prevent loss of the small ‘.’as the decimal point, a convention of inserting the unit in its
place is used. eg. 4R7 is a 4.7 ohm, 4K7 is a 4700 ohm resistor, 4n7 is a 4.7 nF capacitor.
Resistors
1% 0.25W metal film are recommended
240R R2, R3
2K4 R1, R4
4K7 R5, R7
6K8 R6
Capacitors
100nF, 63V polyester film C3, C4, C10, C11
10uF, 35V electrolytic C5, C6, C9, C12
22uF, 35V electrolytic C13
1800uF, 35V or 50V electrolytic C1, C2, C7, C8
Note: C1, C2, C7, and C8 should be 105 degree Celsius radial types and have standard wire
ended leads. The lead spacing is 7.5mm. For example, Panasonic type EEUFC1V182 or
EPCOS B41888-C6188-M, but any decent 105 degree part that will fit on the board will do.
50V parts will have a longer lifetime than 35V parts but will be physically slightly larger.
9

Integrated Circuits
LM317T 1A variable regulator U1
LM337T 1A variable regulator U2
Ensure that both devices are TO-220 types and not any surface mounting or TO-3 packages. I
much prefer the devices that are made by National Semiconductor. They are available from
other manufacturers but National's devices have a thicker and more rigid heatsink tab.
Do not fit solder these into the board ust yet. They are only to be soldered once the board is
fitted to the panel. See the section on mounting the PSU2 board later in this document.
Discrete Semiconductors
1N4004 rectifier diode D4, D5, D6, D9, D11
1N5819 Schottky diode D3, D10
1N5401 rectifier diode D1, D2, D7, D8
For 1N4004 you can use any other 1N400X part such as 1N4001, 1N4002, etc.
D2 and D8 do not need to be fitted if you are using a single phase wall wart or line lump.
5mm green LED -15V
5mm red or orange LED +15V
LED cathodes are denoted by the square pad on the PCB. LEDs will not light up if fitted the
wrong way.
The component marked as LED is the optional front panel power on or standby indicator.
This LED connected to the board via wires and is not fitted to the board.
5mm red LED LED
Trimmer
500R multiturn (eg. Bourns 3296W) ADJ+, ADJ-
Miscellaneous
20mm fuseholder PC mount F1, F2
Antisurge 20mm fuse F1, F2 For current rating see page 5
4-way screw terminal 5mm POWER, SWITCH
TO-220 insulator pad. U1, U2
Suitable power switch
10

Suitable power inlet socket
Keystone 8191 PCB terminal & screw +15V, 0VA, 0VB, -15V
4mm 'banana' socket Optional – fitted on panel. *
4mm ring terminals (crimp/solder) For connection to outputs
2-way 0.1” Molex KK header LED (optional)
You will also need thick wire to connect between the power inlet, switch and any connected
distribution boards. I recommend 24/0.2 (0.75 sq. mm) insulated wire.
If the standby LED is to be fitted then you will also need standard hook up wire – I use 7/0.2
for all my low current connections. The thicker 24/0.2 wire would be too thick for this.
For internal mains transformer (or any installation not requiring a standby switch) then you do
not need to fit the screw terminal SWITCH.
F2 and its associating fuse does not need to be fitted if you are using a single phase wallwart
or linelump.
* See sections on 'Fitting a Grounding Point' and 'Using an Internal Mains Transformer'.
Mounting hardware
M3 6mm pan head screws 4 off For PCB
M3 20mm pan head screws 2 off For power devices
M3 hex threaded male-female 8 mm spacers 4 off For PCB
M3 shakeproof washers 10 off For PCB and power devices
M3 plain washers 6 off For PCB and power devices
M3 hex nuts 6 off For PCB and power devices
M4 16mm pan head screw 1 off For 0V/Earth bond point
M4 washer 2 off For 0V/Earth bond point
M4 star washer 2 off For 0V/Earth bond point
And any mounting hardware for the Dizzy boards if needed.
11

Mounting the PSU2 Board to the Panel
The LM337, U2, fitted into a 5U 19” rack anel. Note the grey TO-220 insulator ad underneath the
regulator IC.
The PSU2 PCB needs to be fitted to your chosen panel. Use the PCB as a template for
marking the panel and then drilling the four 3.5mm holes needed for the mounting pillars. The
board should be spaced high enough off the panel with suitable mounting pillars so as to not
short out any of the components' leads should the board be flexed downward, and also not be
too high so that the leads from the regulators can't reach through the board to be soldered. I
find an 8mm spacer is best.
For the four PCB mounting holes, insert a 6mm screw through each of the four holes in the
panel. Fit a star washer over the exposed thread on the inside of the panel. Now fit a hex male-
female 8mm spacer over the washer and tighten. Check that the four holes in the PCB line up
with the threaded tops of the hex spacers by temporarily fitting the PCB onto the spacers.
Loosen and re-align any spacer if it does not fit properly.
Now you need to prepare the leads of the two regulator ICs. The three legs need to be bent
upwards so that the PCB can be fitted over them. Note that the top surface of the device is
marked with the name of the component and it is the flat side on the bottom of the device that
will be in contact with the panel. You should be able to see that the leads have a thicker
section close to the body of the device. Make a ninety degree bend upwards at the point where
the lead thickness changes. Do this for all three legs of the device.
Remove the board from the panel and fit the regulators to the board by poking their legs up
through the bottom of the board. Do not solder them and fit the board back into place on the
panel. Use the hole in each regulator's metal tab to mark out where you need to drill the
12

mounting hole for the two devices. Now remove the board and regulators. Carefully drill a 3.8
or 4mm hole in the panel for each of the regulators. Clear off any swarf and, twisting with
your hand only, use an 8mm drill bit to lightly deburr the edges of the holes on both sides.
There should be no bumps around the holes.
The linear regulators are TO-220 power devices. They need to be fitted to the panel
mechanically and thermally, but not electrically. This means that the metal tab on each device
should not be in direct 'metal to metal' contact with the panel. To achieve both thermal transfer
and electrical insulation we use a special insulator. These can be made of a 'soft' flexible
material in the form of an insulating pad, or a rigid thin glass like plate made from mica. If
using mica you will also need to use a small amount of heat transfer paste that needs to be
spread very thinly across each side of the mica.
Since the paste is somewhat messy I recommend you use the insulating pads. However, mica
and paste does offer better performance in terms of keeping the power device cool so if you
are planning to draw over an amp from your power supply it may be better to use mica and
paste. Mica also has the advantage of being reusable should the devices need to be taken off
the heatsink in the future. The flexible pads are probably OK being reused but they do get a
little deformed when the nuts are tightened so it is probably a good idea to replace them each
time the devices are removed from the heatsink.
Both types of insulation are normally available in 'mounting kits'. The kit also contains a
mounting bush. This top hat shaped piece of stiff plastic prevents the mounting screw from
touching the regulator's metal tab.
To fit the device to the panel first place the mounting bush into the hole of the power device,
with the flange of the bush lying on the top side of the device. Normally, but not always, the
plastic bush fits tightly enough so that it tends to stay in place after it has been pushed through
the metal tab. Now take one of the insulating pads and place it against the rear of the
regulator. Match up the hole in the pad with the bush that is sticking out from the underside of
the tab. If you have used a flexible pad you may find that it will happily stay put held in place
by the mounting bush.
Now place the power device, bush and pad flat against the rear of the panel so that the bush
fits into the panel. Make sure the pad does not slip out of place when you do this. Insert a
10mm or 12mm M3 screw into the hole from the reverse side of the panel, and fit a flat
washer, a shakeproof washer and nut onto the screw but don't tighten it up ust yet. Do the
same for the other regulator making sure, of course, that each one is in the correct hole.
Now if you have done all this correctly, you should find that the when the power supply PCB
is presented back onto the four threaded spacers, you can coax the power devices’ legs
through the respective solder pads on the board. Because the power devices have not been
fully tightened you will still be able to move them about a bit on the panel to ensure a good fit.
Make sure also that the insulating pads are sitting square under the devices and haven't slipped
out of position.
Fit the washers and nuts onto the four PCB mounting screws. Tighten to secure the board in
place. Do not over tighten the nuts as this will damage the board. Now gently tighten the nuts
on the power devices. Do not tighten these too much as this will distort the mounting tab and
13

squash, or even tear, the insulating pad. All the nuts need to do is hold the power devices up
against the panel.
With both the board and power devices secured to the panel with their mounting hardware
you can now solder the regulators' leads from the top side of the board. Snip off any excess
lead lengths above the solder oints.
Assuming you haven't wired up the PSU2 board to any earth bond or distribution system yet,
it is worth checking the insulation under the two regulators is working correctly. Check with
your continuity tester, or using the resistance setting on your multimeter, that there is a very
high resistance between the tab of each regulator and the metal panel. That is, there should be
no continuity between the tab and the screw that runs through it.
Power distribution comes in many sha es and forms. The Oakley Dizzy and MU-Dizzy modules make it
easy to distribute ower to multi le modules, however, if you have just a few modules then one of these
Faston blocks can be used for a chea but very good solution. This articular set u allows just four
modules, actually in this case it's two Oakley ASVs, to be connected via 1/4” blade terminals.
14

Linelumps and Wallwarts: Wiring Diagrams
Input wiring will depend on the type of wallwart or linelump you will be using.
Standard AC output wallwart
Single phase, two wire, wallwarts or linelumps need to use half wave rectification so the
Oakley PSU can generate both positive and negative supplies simultaneously. They only need
the terminal's AC1 and 0V1 wired to the power socket. AC2 and 0V2 are left unused.
Wallwart with single hase AC out ut.
The front panel switch is a single pole single throw (SPST) switch which simply connects S1R
and S1S together when switched on. You can replace the switch with a simple wire link, but I
do recommend that a switch be fitted so the socket doesn't have to take the full surge current
when you insert it with the wallwart powered up.
I also recommend fitting the AC indicator LED too. This is so you know the wallwart or
linelump is on. The AC indicator is designed to indicate the status of incoming power and is
not determined by the position of the standby switch.
The standby switch should not be used to turn the unit off for any long length of time. This
should be done by either switching the adapter off at the mains socket, or by pulling the
adapter's plug out of the mains socket.
An optional 0V or grounding connection can be made via the 0VB terminal. See later for
more details.
15

Recommended Option: Centre tapped wallwarts and linelumps
Centre tapped linelumps like the Yamaha PA-20 will have three wires coming from their
connector. It will have two AC outputs and one 0V. Take one of the AC outputs to terminal
AC1 and the other AC output to terminal AC2. It should not matter which AC output goes to
AC1 or AC2. The 0V should go to the 0V1 terminal. The 0V2 terminal is left unused.
Linelum wiring with centre ta ed out ut, eg. Yamaha PA-20
The front panel switch is a double pole single throw (DPST) switch which connects S2R and
S2S together, and S1R and S1S together, when switched on. You can replace the switch with
two wire links, but I do recommend that a switch be fitted so the socket doesn't have to take
the full surge current when you insert it if the linelump is powered up.
I also recommend fitting the AC indicator LED too. This is so you know the linelump is on.
The AC indicator is designed to indicate the status of incoming power and is not determined
by the position of the standby switch.
The standby switch should not be used to turn the unit off for any long length of time. This
should be done by either switching the adapter off at the mains socket, or by pulling the
adapter's plug out of the mains socket.
An optional 0V or grounding connection can be made via the 0VB terminal. See later for
more details.
16

Fitting a Grounding Point and Grounding the Panel
Using double insulated wallwarts and linelumps mean that you do not have to have a mains
safety earth fitted to your modular. However, if your modular is to talk to the rest of the
studio you need to make sure that the modular's 0V is tied to earth somewhere in your system.
The most usual way of doing this is via the connecting cable's shield or screen connection.
Your mixing desk or monitoring equipment will be earthed and simply connecting a cable to
any module within your modular will tie the modular's 0V to the other equipment's earth. This
seems pretty straightforward and it is so long as you have a small system and only have one or
two interconnecting cables in use.
However, a larger more complex system will have perhaps more than one modular, more than
one mixing desk and perhaps a heap of other outboard equipment. This is when it makes sense
to look at grounding your modular cases together.
The additional 4mm socket to the right of the ower inlet rovides a way of connecting the 0V lines
between cases
Let us consider a more simpler scenario for the moment. Say we have built ourselves two
modular cases and we would like to connect the modules in them together to form an
awesome monster patch. Each case has its own PSU and each one is powered by a Yamaha
PA-20. It is useful in this situation to ensure that both PSUs are grounded together. In other
words, the two 0V lines from each power supply are electrically connected together. Although
this will be done the moment that one patch lead goes from one case to the other it is
beneficial to do this with a dedicated thick bonding wire. The thicker the wire the lower the
17

resistance which reduces any unwanted voltage drops as return currents travel through the
wiring.
This can be done in a variety of ways but one useful and simple method involves having a
4mm banana socket mounted near each power supply. The banana socket is then connected to
0V, using either the 0VA or 0VB screw terminal on the PSU2 board. Use at least 24/0.2 wire
to make this connection.
If both the power supplies have a banana socket then it is a simple matter of patching the two
modular cases together with a banana patch lead. The great thing about bananas is that they
are stackable so it's easy even if you have more than two cases to connect up.
I recommend that you use thick multistrand cable to make your grounding leads and that you
use good quality 4mm banana sockets and plugs like the type sold for speaker connections and
not those used for cheap test equipment.
A single M4 crim ed ring terminal secured to a anel bonding oint. A solder tag with a 4mm hole could
also be used but for thicker wires, like this 24/0.2 wire, crim ing offers better erformance and is sim ler.
To reduce electrical noise it is wise to ground the panel on which the power supply is
mounted. To make a 0V bonding point is straightforward. Simply drill a 4mm hole in your
panel. Scrape back around the hole any finish on the inside surface, such as paint or anodising,
to reveal the shiny metal underneath. Fit an M4 screw from the front and pop on a toothed
shakeproof washer and flat washer on the rear. Fit your ring terminal (or terminals) onto the
screw, place another flat washer and shakeproof washer on top of that. Secure tightly with an
M4 nut. This should ensure that the panel is robustly connected to 0V.
18

A 4mm banana socket is taken to anel bond oint which then is connected to the PSU2's 0VB terminal.
Personal Note...
One has to a little careful when using the word 'ground'. I sometimes talk about local ground
and 0V as being the same thing. This is technically incorrect but it is used a lot. I worked at
Marconi in the 80s and Soundcraft in the early 90s, and ground and 0V were used
interchangeably even by seasoned engineers. We'd talk about chassis ground, dirty ground,
signal ground and clean ground. They'd all be connected to 0V somewhere in the system but
the term ground was in common usage.
Ground, when used in this way, is then a local common reference connection tied to the 0V of
the unit's power supply. It is not the same as mains earth. Indeed, it may not even be tied to
mains earth in the unit in question.
Strictly speaking, electrical ground is mains earth and historically it was solely referred to as
that, but usage, incorrect or not, has meant a shift in the meaning. Ideally, we should call our
common reference connection within our unit as 0V and not use the term ground.
19

Using an Internal Mains transformer
A 50VA toroidal transformer owering an Oakley PSU2 designed to deliver u to 750mA er rail. Note
the wire loo s fitted to the ads where normally the terminal block for the standby switch goes.
Be afraid, be very afraid...
Some of you old hands will laugh about the level of hysteria that surrounds direct mains
connection to DIY pro ects. However, this fear is more to do with our litigious society than
the real danger to the builder. Even so, I have had more than my fair share of high voltage
shocks over the years and it is not something I would want any builder to have to experience.
It has been purely luck that has saved me in several of those cases.
So I will say again – do not attempt to build a mains transformer into your modular case, or
any other pro ect, without realising that to do so carries a risk of death to either you, or to the
person who may inadvertently put their fingers into your half built construction. Furthermore,
it is up to you as the builder of such an item to make sure, that once built, the item is safe to
use and meets all current safety legislation.
This is not a complete set of instructions on how to fit a transformer into a piece of
electronic equipment. This information is offered only as a guide and should not be
considered as your only source of information. No further information, other than that
included here, will be provided by me regarding the construction of mains powered
items.
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